Ask Deandra: How Do Texas Police Prove a Drug DWI (DUID)?

Here is the longer answer: what the State has to prove in a drug DWI case, the four pieces of evidence prosecutors typically build from, where the cracks appear in each, and what the case does to your life while it is pending.

What the State Has to Prove

Texas Penal Code §49.04 makes it an offense to operate a motor vehicle in a public place while intoxicated. “Intoxicated” under §49.01(2) means either:

• Loss of normal use of mental or physical faculties due to introduction of a substance, or
• Alcohol concentration of 0.08 or more.

In a pure drug case (cannabis, prescription medications, methamphetamine, cocaine, opioids, or any other non-alcohol substance) the alcohol per-se prong is unavailable. The State has to prove the loss-of-faculties prong. That is the legal architecture every drug DWI prosecution rests on.

Importantly, Texas has no per-se drug limit. Unlike some states that set numerical thresholds for active THC or other drugs, Texas does not. The State cannot win a Texas drug DWI case by simply proving that a drug was present in the defendant’s blood at any specific concentration. The State has to prove that the drug caused the loss of the normal use of mental or physical faculties at the time of driving. That requirement creates both the structure and the soft spots of the State’s case.

What a Drug DWI Carries If the State Wins

Because a drug DWI is charged under the same statute as an alcohol DWI, it carries the same punishment range at the same level. The drug allegation does not soften anything.

• First offense. Class B misdemeanor, up to 180 days in jail, a fine of up to $2,000, driver license suspension, and exposure to the one-time state fine under Transportation Code §709.001 that attaches to DWI convictions.
• Escalation. A prior conviction, a child passenger, serious bodily injury, or a death moves the charge up exactly as it would in an alcohol case, through Class A misdemeanor and felony DWI to intoxication assault and intoxication manslaughter.

People often assume a drug case is treated as the lesser problem because there is no breath test and no number. The opposite is closer to the truth. The record, the license consequences, and the collateral damage are identical, and the case takes longer to resolve.

Piece One: Driving Behavior and Officer Observations

The first category of evidence in any drug DWI case is what the officer saw before and during the stop. The State will emphasize:

• Driving behavior. Lane drift, speeding or unusually slow driving, missed traffic signals, weaving within a lane, and accidents are all logged and described in the offense report.
• Initial contact observations. Bloodshot eyes, slow or slurred speech, dilated or constricted pupils, fumbling for documents, the odor of cannabis or other substances on the person or in the vehicle.
• Demeanor and responsiveness. Slow reactions to officer commands, inappropriate affect, agitation or unusual calm, confusion, or any indicator the officer interprets as cognitive impairment.
• Admissions about drug use. These are usually drawn out in casual conversation before any warning is given, which is exactly when people talk most freely. What you said at the roadside is evidence. Whether it is admissible evidence is a separate fight, and it is one worth having.
• Physical evidence in the vehicle. Pipes, vape pens, edible packaging, prescription bottles, syringes, residue, paraphernalia. These corroborate use but typically do not establish timing.

The defense response to this category of evidence is observational. Bloodshot eyes can come from allergies or fatigue. Slurred speech can be a natural speech pattern, anxiety, or a medical condition. Dilated pupils can be caused by stress, low light, or many medications other than the one alleged. Fumbling for documents at 2 a.m. is not unusual. Body cam and dash cam footage almost always tells a more nuanced story than the offense report.

Every indicator on the officer’s list has an innocent explanation that the officer is not trained to rule out. Elevated pulse and blood pressure come from the stress of being pulled over, from caffeine, from exertion, from hypertension or a thyroid condition. Bloodshot eyes come from contact lenses, dry air, wind, or a long day. Eyelid tremors come from fatigue. The training teaches an officer to spot indicators consistent with drug impairment. It does not teach the differential diagnosis skills needed to exclude everything else that produces the same sign.

Piece Two: Field Sobriety Test Performance

Officers routinely administer Standardized Field Sobriety Tests in suspected drug DWI cases, even though the SFSTs were validated by the National Highway Traffic Safety Administration to detect alcohol impairment but not drug impairment. The protocol mismatch is itself a defense issue:

• Horizontal gaze nystagmus is associated with central nervous system depressants such as alcohol, certain sedatives, certain prescription medications. Many drugs of abuse (cannabis, stimulants like cocaine and methamphetamine, hallucinogens) do not produce HGN. An officer who finds no HGN clues in a stimulant case has not necessarily ruled out impairment. An officer who claims HGN clues in a cannabis case has produced a finding the scientific literature largely does not support.
• Walk-and-turn and one-leg stand. Both tests measure balance, coordination, and ability to follow instructions. These are abilities that can be affected by many things, only some of which are drug impairment. Conditions of administration, age, weight, footwear, medical conditions, and roadside stress all confound the results.
• Non-standardized tests. Officers frequently use additional tests (Romberg balance, finger-to-nose, modified Romberg) that are even less validated than the standardized three. These are voluntary and the defense should attack their reliability vigorously when they were performed.

The government’s own research says the same thing. NHTSA’s laboratory work reports that HGN alone had accuracy above chance for every drug category except marijuana, which is the single most common drug category in Texas DUID arrests. A 2023 randomized clinical trial published in JAMA Psychiatry tested SFSTs against controlled Delta-9 THC dosing and concluded that the tests are useful but do not provide objective evidence of THC impairment, and that additional methods are needed. In that same trial, 59 percent of the participants who had received no THC at all still showed lack of convergence, one of the eye signs officers treat as a drug indicator.

SFSTs were never designed to be used as the State uses them in drug cases. Pointing that out, with credible expert support, can substantially undercut the State’s evidentiary foundation. Deandra Grant is a certified SFST Instructor, which means the cross-examination on administration, scoring, and the limits of each test comes from someone who teaches the tests rather than someone reading about them.

Piece Three: The Drug Recognition Evaluator

This is where drug DWI prosecutions diverge from alcohol DWI prosecutions. In suspected drug cases, after the field arrest, the defendant is often turned over to a Drug Recognition Evaluator (DRE) which is a police officer with training in identifying drug-induced impairment. The DRE conducts a twelve-step evaluation that follows a standardized protocol developed by the International Association of Chiefs of Police and NHTSA, the Drug Evaluation and Classification Program:

• (1) Breath alcohol test, to rule out or confirm alcohol involvement.
• (2) Interview of the arresting officer.
• (3) Preliminary examination of the subject — first impressions, pulse, eye condition.
• (4) Eye examinations — HGN, vertical gaze nystagmus, lack of convergence.
• (5) Divided attention psychophysical tests — walk-and-turn, one-leg stand, finger-to-nose, modified Romberg.
• (6) Vital signs examination — second pulse, blood pressure, body temperature.
• (7) Dark room examination — pupil size in three lighting conditions, ingestion examination of the mouth and nose.
• (8) Examination of muscle tone.
• (9) Examination for injection sites and a third pulse.
• (10) Subject’s statements and additional observations.
• (11) Analysis and opinions of the evaluator.
• (12) Toxicological examination — typically blood specimen.

Based on this protocol, the DRE forms an opinion as to whether the subject is impaired and, if so, by which of seven drug categories: central nervous system depressants, central nervous system stimulants, hallucinogens, dissociative anesthetics, narcotic analgesics, inhalants, or cannabis.

What the DRE Is Looking For in Each Category

Each category carries a symptom matrix, a chart of the indicators an officer is trained to associate with that class of drug. Knowing what the officer went looking for is how you judge whether they actually found it.

• CNS depressants (alcohol, benzodiazepines, barbiturates). Sluggish reactions, poor coordination, slurred speech, drowsiness, HGN present, pupils normal to dilated.
• CNS stimulants (cocaine, methamphetamine, amphetamines, MDMA). Restlessness, rapid speech, elevated pulse and blood pressure, dilated pupils, body tremors, teeth grinding, raised body temperature.
• Hallucinogens (LSD, psilocybin, mescaline). Disorientation, paranoia, hallucinations, dilated pupils, elevated pulse, unpredictable behavior.
• Dissociative anesthetics (PCP, ketamine, dextromethorphan). Blank stare, HGN present, elevated blood pressure, rigid muscle tone, reduced response to pain.
• Narcotic analgesics (heroin, fentanyl, oxycodone, hydrocodone, morphine). Droopy eyelids, constricted pupils, drowsiness, slow deliberate speech, depressed vital signs.
• Inhalants (toluene, gasoline, nitrous oxide, aerosols). Disorientation, slurred speech, possible residual odor, HGN present, flushed face, watery eyes.
• Cannabis (marijuana, THC, hashish). Reddened conjunctiva, dilated pupils in dim light, elevated pulse, impaired time estimation, eyelid tremors, possible green coating on the tongue.

Read the list and the problem announces itself. Raised pulse, dilated pupils, red eyes, and tremors show up across several categories at once, and across a long list of ordinary causes that have nothing to do with drugs. The matrix describes the typical response of someone who does not take the drug regularly. It does not describe a tolerant patient, a person with two substances on board, or someone who is simply frightened and exhausted on the side of a road at 2 a.m.

What DRE Certification Actually Requires

Juries hear the word “expert” and picture a clinician. The credential is narrower than that. The DRE curriculum is a 16-hour pre-school plus a 56-hour expert school, roughly 72 hours of combined classroom instruction, followed by about 40 hours of field certification. In the certification phase the student must complete a minimum of 15 supervised drug influence evaluations, serving as the evaluator in six of them, and reach a 75 percent toxicological confirmation rate. Certification lasts two years and is maintained by conducting four evaluations, keeping a rolling log, and attending eight hours of recertification training.

The certification is awarded by the International Association of Chiefs of Police. It is not a medical credential. A DRE has not attended medical school or nursing school, is not a paramedic, an EMT, or a lab technician, and is not certified by any medical association. They are police officers trained by other police officers to take vital signs and read a symptom chart. Training records, recertification status, rolling logs, and protocol deviations are all discoverable, and lapsed certification is common enough to be worth checking in every case.

DRE Defense Issues

The DRE protocol looks scientific. It is presented to juries as scientific. In court, the defense has to address several issues that complicate that presentation:

• Subjectivity in scoring. Many of the protocol’s elements (muscle tone, demeanor, response time, pupillary reaction) are scored subjectively. Two well-trained DREs evaluating the same subject can reach different conclusions.
• Confirmation bias. By the time a subject reaches the DRE, an arrest has been made. The DRE knows the arresting officer believed there was impairment. The DRE’s job is to identify what is causing the impairment and not to disconfirm impairment. That structural bias has been documented in studies of DRE accuracy.
• Reliability data is dated and methodologically limited. The original validation studies for the DRE protocol were conducted decades ago, with methodological choices that would not pass current peer review. Later studies have produced mixed results, particularly for cannabis recognition.
• Cannabis specifically is harder than the protocol suggests. Multiple studies have documented that DRE accuracy for cannabis recognition is lower than for some other categories, particularly when subjects are tolerant users with low to moderate blood THC concentrations.
• Training and recertification. DRE certification has training requirements, recertification cycles, and documentation that is discoverable. Training gaps, expired certifications, and protocol deviations are all defense targets.
• Combination cases. The DRE protocol is more reliable for single-substance impairment than for combination cases. Real subjects often have multiple substances on board.

What the DRE Accuracy Data Actually Shows

The government asserts that a DRE is 70 to 90 percent accurate at identifying which category of drug is causing impairment. The published research does not hold that up. Independent analysis of real Texas enforcement evaluations, measured against the toxicology that actually came back, puts the representative figure at roughly half. The laboratory validations that included placebo controls repeatedly found sober subjects called impaired. And accuracy is worst on several of the categories the protocol is most often used to call.

That is not an argument that DRE testimony is worthless. It is the reason DRE testimony belongs in a Rule 702 hearing rather than being accepted at face value. The study-by-study breakdown, including the category-level accuracy figures and the methodological problems with the validation studies prosecutors cite most often, is set out in our guide to what Drug Recognition Experts are trained to do and what they actually can do.

None of this means DRE evidence is automatically excluded. It does mean that DRE testimony is testable, contestable, and frequently weaker than the State’s direct examination suggests.

Piece Four: Blood Toxicology

In nearly every drug DWI case, the State will obtain a blood specimen by consent or by warrant. Texas Transportation Code §724.012 also lists circumstances in which an officer shall take a specimen, but those provisions no longer authorize a warrantless draw standing alone. In State v. Villarreal (Tex. Crim. App. 2014), the Court of Criminal Appeals held that the mandatory-draw statute does not create a freestanding exception to the Fourth Amendment warrant requirement. The blood is sent to a forensic laboratory for analysis, typically by liquid chromatography-tandem mass spectrometry (LC-MS/MS) for confirmatory drug testing.

The defense to blood evidence in drug cases overlaps substantially with the defense to blood evidence in alcohol cases (covered in our guide to challenging a DWI blood test), with several drug-specific additions:

• Active drug versus metabolite. This is the central forensic distinction in drug DWI cases. A positive result for a metabolite (carboxy-THC for cannabis, benzoylecgonine for cocaine) proves past use, not current impairment. The State must prove the active form was present and was producing impairment at the time of driving.
• Concentration and impairment correlation. Without a per-se limit, the State has to bridge the gap between a measured concentration and an impairment claim. That bridge often relies on generic dose-response generalizations that may not apply to the specific defendant.
• Tolerance. Chronic users develop pharmacodynamic tolerance, so the same blood concentration produces less behavioral effect in a chronic user than in a naive user. The State’s expert often does not address this. The defense expert should, and the underlying pharmacology is set out in our explanation of why a blood drug concentration does not equal impairment.
• Method validation. The forensic laboratory’s method validation for the specific analyte should document accuracy, precision, linearity, lower limit of quantitation, selectivity, and matrix effects. Method validation gaps are real and discoverable.
• Chirality issues. Some drugs exist as multiple stereoisomers with different pharmacological properties. Methamphetamine has both a pharmacologically active d-isomer and a largely inactive l-isomer found in certain over-the-counter inhalers. Laboratories that do not distinguish the isomers can produce confirmatory results that do not reflect actual drug use.
• Chain of custody. Blood evidence is only as reliable as its chain of custody. Documentation gaps, storage issues, and labeling problems are all defense targets.

The Screen Is Not the Result

Forensic drug testing normally runs in two stages. The first is an immunoassay screen, which looks for broad classes of drugs rather than a specific compound. It is sensitive, it is cheap, and it cross-reacts, which means it produces false positives. It tells the lab whether further analysis is warranted. It does not identify what is in the blood. Only confirmatory testing by GC-MS or LC-MS/MS names the compound and reports a concentration. A case that rests on a screening result alone rests on an unconfirmed result, and the distinction is printed on the lab report itself.

Make the Lab Name the Analyte

In a cannabis case this is the whole argument, and it is why a marijuana or edibles DWI so often turns on one line of the lab report. Delta-9 THC enters the bloodstream and crosses into the brain, where it produces the psychoactive effect. The liver converts it first to 11-hydroxy-THC, which is still active, and then to carboxy-THC, which is not. Active THC falls off quickly, within a few hours. Carboxy-THC is stored in fat and released slowly, so it stays detectable for days or weeks in a regular user who is stone-cold sober at the moment of the blood draw.

So a report that says only “THC metabolite detected” proves nothing about impairment. The defense should demand that the laboratory state which analyte it actually measured: parent Delta-9 THC, 11-hydroxy-THC, or carboxy-THC. If the answer is carboxy-THC, the State has proven past use and nothing more. State toxicologists have conceded on the stand that the metabolites of marijuana and cocaine have no effect on the human body. If active THC is present, the number still has to be interpreted against the person’s use pattern, the gap between driving and the draw, and tolerance.

The Same Problem Across Other Drug Classes

The gap between detection and impairment is not a cannabis quirk. It runs through most of the drug classes that show up in Texas cases.

• Benzodiazepines. Diazepam has an active metabolite, nordiazepam, with a half-life measured in days. Clonazepam runs 19 to 60 hours. A detectable benzodiazepine, or its metabolite, says nothing on its own about faculties at the time of driving.
• Opioids. Hydrocodone, oxycodone, and morphine break down into compounds that outlast the sedative effect. Codeine metabolizes into morphine, so a person taking prescribed codeine can produce a positive morphine result.
• Amphetamines. Methamphetamine persists well past the stimulant effect, and unless the lab runs a chiral analysis it cannot tell the illicit d-isomer from the l-isomer found in certain over-the-counter nasal inhalers. Texas analysts have testified plainly that their labs do not distinguish between the two isomers.
• Prescription sleep medications. Zolpidem has a short half-life but is documented to produce next-morning impairment in some people, more often at higher doses and in women, who clear it more slowly. That produces the hardest version of this case: a driver impaired by a legally prescribed medication taken exactly as directed.

Why Back-Calculation Does Not Work for Drugs

In alcohol cases the State sometimes uses retrograde extrapolation to estimate what the blood alcohol concentration was at the time of driving. Alcohol eliminates at a roughly constant rate, which is what makes the arithmetic possible at all, and even there it is contestable. Drugs do not behave that way. Most follow first-order or mixed-order kinetics, where elimination depends on concentration, liver function, the specific enzyme systems involved, the individual’s genetics for those enzymes, other medications competing for them, and more.

The practical consequence is that no one can reliably work backward from a blood draw taken an hour or two after the stop to a drug concentration at the wheel. This is not a defense theory the State disputes. Texas DPS analysts have said it under oath: back extrapolation is done routinely with blood alcohol, and with drugs it cannot be done. If a prosecution expert tries it anyway, that is an objection, not a debate.

Blood toxicology in drug cases is not the gold-standard evidence the State presents it as. It is forensic evidence that can be evaluated, challenged, and contextualized by a defense team that takes the science seriously.

Roadside drug screening is what is coming next. Oral fluid devices are already deployed in several states, Texas has been running a pilot, and not one of them measures impairment either. The devices, their published error rates, and what to demand in discovery when one appears in a case are covered in our review of roadside cannabis testing technology.

When the Drug Was Legally Prescribed

Prescription cases are the fastest-growing part of this docket, and they are the ones people misunderstand most. A valid prescription is not a defense to DWI. The statute asks whether a substance caused you to lose the normal use of your faculties, not whether you were allowed to have it. A prescribed medication can support a DWI conviction. Our guide to DWI on prescription medication works through those cases in more detail.

But the prescription still matters enormously, because it changes what the blood result means.

• Tolerance. A patient who has taken the same dose of a benzodiazepine or an opioid on a stable schedule for months or years functions at a concentration that would flatten someone taking it for the first time. The Texas DWI Manual’s own discussion of pharmacology makes the point directly: tolerance, age, gender, diet, and genetics all change what a given dose does. The DRE symptom matrix does not account for any of it. It describes the typical response of a drug-naive person.
• Therapeutic range versus impairment. For many medications the concentration that treats the condition sits inside the range the State will call impairing. A prosecutor points at the report and says an opioid was detected. The answer is that a therapeutic level in a tolerant patient is evidence of compliance with a prescription, not evidence of lost faculties. State analysts will concede on cross that a result sat inside, or even at the low end of, the published therapeutic range.
• What the records prove. Pharmacy records, the treating physician’s notes, the dosing history, the length of time on the medication, and the package insert’s driving warnings are all evidence. So is testimony from the prescriber about whether the patient was cleared to drive. A defendant who took a medication as directed, on a regimen they had tolerated for a year, with no warning against driving, is in a very different position than the report alone suggests.

Write down every substance in your system and why: the medication, the dose, when you last took it, and who prescribed it. That is not an admission. In a prescription case it is the foundation of the defense.

Suppression: Article 38.23 in a Drug Case

Texas has its own exclusionary rule, and it is broader than the federal one. Under Code of Criminal Procedure Article 38.23, evidence obtained in violation of the Constitution or the laws of Texas or the United States is inadmissible, and Texas recognizes no good-faith exception. If the stop fails, the toxicology can fall with it. Drug cases give four distinct places to push.

• The initial stop. Was there reasonable suspicion at all? Driving that reads as impairment in an offense report often reads as fatigue, distraction, or road conditions on video.
• The extension of the stop. This one is specific to drug cases. Once the breath result came back at or near zero, what articulable facts justified holding the driver longer and summoning a DRE? Moving from “there is no alcohol here” to “I am calling a drug evaluator” requires more than a hunch.
• The blood warrant. A refusal usually means the officer seeks a warrant. The affidavit has to state facts amounting to probable cause that the blood holds evidence of intoxication. Boilerplate affidavits copied from alcohol cases, with no drug-specific observations in them, are vulnerable.
• The draw and what happened after. The blood must be drawn by qualified personnel using proper equipment, collected in the correct preservative tubes, stored correctly, and documented from the draw site to the lab bench. Contamination, temperature excursions, labeling errors, and gaps in the chain are all grounds to exclude or discount the result.

Putting It Together: How the State’s Case Actually Comes Together

In a typical Texas drug DWI prosecution, the State assembles the four pieces above into a single narrative:

“The defendant was driving erratically. The officer made contact and observed bloodshot eyes, dilated pupils, and the odor of cannabis. The defendant admitted to recent cannabis use. Field sobriety tests showed multiple clues of impairment. The DRE confirmed cannabis-category impairment based on the twelve-step protocol. Blood toxicology confirmed THC and carboxy-THC at concentrations consistent with recent use. The defendant was therefore impaired by cannabis at the time of driving in violation of Penal Code §49.04.”

The defense response disassembles each piece:

• The driving behavior is consistent with fatigue, distraction, or any number of innocent explanations. Look at the actual video, not the report.
• The officer’s observations are subjective, post-hoc, and often inconsistent with what the body cam shows.
• The defendant’s admission was elicited through casual conversation in non-Mirandized circumstances and may be subject to suppression.
• The field sobriety tests were administered in conditions that confound the results, and were not validated for cannabis impairment in the first place.
• The DRE’s evaluation involves significant subjective scoring, confirmation bias, and protocol deviations and the underlying scientific reliability data for cannabis specifically is weaker than the State suggests.
• The blood result confirms cannabis use but does not prove cannabis impairment at the time of driving. Carboxy-THC reflects past use, and active THC concentration alone does not establish impairment without addressing tolerance, route of administration, and pharmacokinetic timing.

Done well, this disassembly converts what looked like an airtight case into a case the State has to fight to win. That conversion is the work of a forensic-trained defense team.

What Happens While the Case Is Pending

Getting out of jail. A first drug DWI is a misdemeanor, so release usually comes quickly, on a personal bond or a modest cash bond depending on the county and your history. Conditions commonly include no alcohol or drug use, and some courts add an ignition interlock even in a drug case. The lab work has no bearing on release. It only affects how long the case takes afterward.

The 15-day license deadline. If you gave a specimen or refused one, the Department of Public Safety can move to suspend your license through Administrative License Revocation. You have 15 days from service of the notice of suspension to request a hearing. Miss it and the suspension is automatic. Request it and you get something else worth having: a chance to put the officer under oath, months before the criminal case develops.

How long it takes. Most drug DWI cases run roughly 8 to 18 months, longer than a comparable alcohol case, because confirmatory toxicology takes time to produce and more time to analyze properly. Slower is usually better here. Waiting for the complete lab package, including the underlying data and the validation records, is what produces dismissals and reductions in these cases.

What it costs. The fee depends on the county, the substance, the complexity of the toxicology, whether an independent expert is needed, and whether the case resolves before trial. Drug cases usually require more forensic work than alcohol cases, and that work is what moves outcomes. We quote a flat fee after a free consultation so you know the number up front.

Where the case is heard. A first or second drug DWI is a misdemeanor heard in the county courts at law. Felony-level cases go to the district courts. In every one of them, getting the full lab record produced is an early fight rather than a formality, because the State’s file often contains a one-page report and nothing behind it.

What a Conviction Does Long Term

• Your record. A drug DWI conviction is a DWI conviction. It is permanent. A final conviction cannot be expunged, and only a narrow nondisclosure is available for some first offenses under conditions that many cases do not meet.
• License and insurance. The same suspension and insurance consequences apply as in an alcohol case, including the SR-22 filing requirement.
• Professional licensing. For nurses, pharmacists, physicians, teachers, commercial drivers, and anyone else holding a state license, a drug-related DWI can trigger board scrutiny that runs separately from the criminal case and sometimes cuts deeper than an alcohol case would.
• The next case. A drug DWI counts as a prior for any future DWI, alcohol or drug. That is the strongest practical reason to fight the first one rather than dispose of it quickly.

What This Means for Your Case

If you have been charged with a drug DWI in Texas:

• Hire a defense lawyer with forensic and pharmacological expertise. Drug DWI cases live or die on the science. Generic DWI defense is not enough.
• Address the ALR deadline. Even in drug cases without breath testing, refusal of a blood specimen triggers ALR consequences. The 15-day deadline runs from the date of service of the notice of suspension.
• Preserve evidence of timing. When did you consume the substance? When was the blood drawn? What does the pharmacokinetics suggest about the gap between the two?
• Preserve medical and prescription records. If the case involves prescription medications, the medical record is part of the defense.
• Write down any medical condition, injury, or stretch of missed sleep that an officer could have read as impairment, while the details are still fresh.
• Stop talking about the case. Statements you make to officers, friends, family members, social media, or anyone else can become evidence. Talk only to your lawyer.

Drug DWI FAQs

Can I be convicted if the drug was legal or prescribed?

Yes. The statute asks about impairment, not legality. A lawfully prescribed or lawfully purchased substance can support a DWI if it caused you to lose the normal use of your mental or physical faculties. The prescription is not a shield, but it is important defense evidence.

Does a positive drug test prove I was impaired?

No. It proves the substance was present. Many drugs, and nearly all inactive metabolites, remain detectable long after any effect has ended. Presence and impairment are separate questions, and the State has to prove the second one.

Can I refuse the blood test in a drug case?

You can, and the officer can then seek a warrant and draw the blood anyway. Refusal also carries ALR license consequences. What refusal does accomplish is denying the State an easy consent record and forcing the case onto evidence that is more challengeable.

What is a Drug Recognition Evaluator, and is the evaluation reliable?

A DRE is a police officer trained in a twelve-step drug influence protocol that ends in an opinion about a drug category. It is an opinion, not a measurement, and the published accuracy data is a good deal weaker than the way the opinion is presented in court. It carries real weight only when toxicology confirms a consistent substance.

How is a drug concentration in my blood interpreted?

Carefully, and usually with dispute. Tolerance, time since use, route of administration, individual metabolism, and the specific enzymes involved all change what a number means. That is why this analysis belongs to someone with pharmacology training rather than to a generalist.

Will field sobriety tests show drug impairment?

Not reliably. The standardized tests were built and validated for alcohol. They can read stimulant effects, a medical condition, or plain exhaustion as impairment, and NHTSA’s own research shows HGN performing no better than chance on marijuana.

The Bottom Line

Texas police can and do prosecute drug-based DWI cases without a blood test because they do not necessarily need one. The case is built from driving behavior, officer observations, field sobriety test performance, the Drug Recognition Evaluator protocol, and blood toxicology (if obtained). None of these pieces is unbeatable. None of them is forensically airtight. The lack of a per-se drug limit in Texas means every drug DWI rests on the loss-of-faculties prong, and that prong is contestable across every category of evidence the State will use. A drug DWI is not an automatic conviction. It is a case that requires real defense work to win. Where it sits against every other Texas DWI charge is set out on Texas DWI charges.

Drug DWI Defense at Deandra Grant Law

Deandra Grant Law defends DWI and intoxication-offense cases across North and Central Texas in Dallas, Fort Worth, Plano, McKinney, Frisco, Allen, Lewisville, Denton, Rockwall, and Waco. We handle drug DWI cases with the forensic, pharmacological, and procedural depth they require: challenging DRE protocol reliability, addressing pharmacokinetic timing, contesting metabolite-based prosecutions, and litigating the loss-of-faculties standard at the level the science demands. Our team includes an ACS-CHAL Forensic Lawyer-Scientist with a Master of Science in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology.

That training is specific. Deandra Grant’s Master of Science in Pharmaceutical Science, with a forensic science concentration, is from the University of Florida College of Pharmacy, and her Graduate Certificate in Forensic Toxicology is from the University of Florida College of Veterinary Medicine. She is also a certified SFST Instructor, and she writes the drugged-driving chapter of The Texas DWI Manual, the practice treatise Texas defense lawyers use on these cases. Partner Doug Huff holds the same ACS-CHAL Forensic Lawyer-Scientist designation, and his digital forensics training extends the defense to dash cam and body cam analysis, phone records, and GPS data, which is often where the question of whether the driving was actually impaired gets answered.

If you have been charged with a drug-based DWI in Texas (cannabis, prescription medication, or any other substance) call Deandra Grant Law at (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation. And remember: the 15-day ALR deadline runs from the date of service of the notice of suspension.

Have a DWI question you want answered in this series? Submit it at texasdwisite.com — you might see it featured in a future Ask Deandra post.