Ask Deandra: How Do Police Prove a Drug-Based DWI Without a Blood Test?

The question: How do police prove a drug-based DWI without a blood test?

The short answer: They do it through observation, blood testing, and a (theoretically) specially trained officer called a Drug Recognition Evaluator. The Intoxilyzer 9000 (the standard breath instrument used in Texas DWI cases) measures alcohol only. It does not detect cannabis, prescription medications, opioids, methamphetamine, or any other drug. So, in a drug-based DWI case, the State has to assemble its proof from a combination of officer observations of impairment, the results of a Drug Recognition Evaluator (DRE) examination, and laboratory analysis of a blood specimen for the presence of drugs. None of these elements is forensically airtight, and a real defense addresses each one.

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, and where the cracks appear in each.

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.

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 obtained through the casual-conversation interrogation techniques covered in our right-to-remain-silent post.
• 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.

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 Rhomberg) that are even less validated than the standardized three. These are voluntary and the defense should attack their reliability vigorously when they were performed.

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.

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:

• (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.

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.

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 either by consent, by warrant, or under the mandatory testing provisions of Texas Transportation Code §724.012. 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 blood-test post), 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.
• 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.
• 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.

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.

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 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.
• 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.

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.

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.

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.