What Is a Texas Marijuana DWI, and Why Is There No Per Se THC Limit?
A Texas marijuana DWI is a criminal charge under Tex. Penal Code § 49.04 based on impairment of mental or physical faculties — not a positive blood test, and this page focuses specifically on the THC science, testing methodology, and marijuana-specific defenses that distinguish it from other drug DWI cases. Texas has no per se THC blood concentration limit. No nanogram threshold triggers automatic guilt. The state must prove actual impairment at the time of driving. THC metabolites remain detectable for days to weeks after last use. A positive blood test for carboxy-THC does not prove impairment occurred at the wheel. Delta-9-THC — the active compound — peaks within 30 to 60 minutes of consumption and declines rapidly. Chronic users retain elevated baseline THC levels with no functional impairment at those levels. The National Highway Traffic Safety Administration acknowledges no validated relationship exists between THC blood levels and driving impairment. This scientific uncertainty is the foundation of every effective marijuana DWI defense in Harris County courts. A marijuana DWI lawyer Houston who understands this science can turn the state's own evidence into reasonable doubt.
Critical fact: THC metabolites (carboxy-THC) remain detectable in blood for days to weeks after last use. A positive blood test does not prove impairment at the time of driving. The state must prove actual impairment occurred at the wheel — not merely that THC was present in your bloodstream at some point.
How Does the State Build a Marijuana DWI Case in Texas?
A Texas marijuana DWI prosecution is built on three evidentiary pillars: officer observations, a Drug Recognition Evaluator (DRE) evaluation, and a blood test for THC. Each pillar can be challenged independently. Officers allege red eyes, marijuana odor, slow speech, and poor field sobriety test performance. Many of these observations describe normal human variation, fatigue, or medical conditions — not impairment. A DRE is an officer trained in a 12-step evaluation to classify drug impairment by category. DRE opinions are contested scientifically. Studies show significant false-positive rates for cannabis. Blood tests measure both delta-9-THC and carboxy-THC metabolites. Only active delta-9-THC correlates with recent consumption and potential impairment. Carboxy-THC is an inactive metabolite that persists for weeks. A warrant is required for a blood draw in most marijuana DWI cases. A defective warrant suppresses all blood evidence obtained from that draw. A marijuana DWI lawyer Houston challenges each of these three pillars independently rather than accepting the state's narrative as a whole.
Police reports in marijuana DWI cases typically allege red/bloodshot eyes, odor of marijuana, slow speech, and poor performance on Standard Field Sobriety Tests (SFSTs). Herman cross-examines these observations — many are consistent with fatigue, allergies, medical conditions, or normal human variation, not impairment.
A DRE is an officer trained in a 12-step evaluation protocol to identify drug impairment. DRE evidence is contested scientifically — studies show DRE evaluations have significant false-positive rates, particularly for marijuana. Herman retains expert witnesses to challenge DRE methodology and the officer's qualifications.
If blood was drawn, the lab will report levels of active THC (delta-9-THC) and inactive metabolites (carboxy-THC, or THC-COOH). Active THC does not correlate reliably to impairment — individual tolerance, frequency of use, and metabolism all affect impairment at any given THC level. Herman challenges the chain of custody, the lab analysis methodology, and the toxicologist's impairment conclusions.
Unlike alcohol breath tests, marijuana blood draws typically require a warrant. If police drew blood without a warrant and without your consent, the blood evidence may be suppressed entirely. Herman reviews the warrant application and draws to ensure they were constitutionally obtained. A marijuana DWI lawyer Houston reviews every warrant application line by line looking for exactly this kind of defect.
What Does the Science Say About THC Blood Levels and Driving Impairment?
The science of marijuana impairment and driving is unsettled — and that uncertainty directly benefits defendants charged under § 49.04 in Texas courts. THC metabolites are fat-soluble. They accumulate in fatty tissue and release slowly into blood over days. A regular user may test positive for THC days after last use with no functional impairment. Active delta-9-THC drops rapidly after consumption. The impairing effect dissipates before blood is typically drawn in an arrest scenario. Individual tolerance creates further complexity. A chronic user may function normally at blood THC levels that would impair an infrequent user significantly. The NHTSA acknowledges this variation and has declined to endorse any per se THC impairment threshold. No forensic toxicologist can reliably state that a specific THC blood level caused specific driving impairment on a specific evening. That scientific gap is the centerpiece of every Herman Martinez marijuana DWI defense. A marijuana DWI lawyer Houston builds this scientific uncertainty into the core of every defense strategy.
- THC metabolites are fat-soluble and can remain in blood for days to weeks after a person's last use — especially in frequent users.
- Active delta-9-THC peaks within 30–60 minutes of consumption and drops rapidly, but chronic users may have elevated baseline levels without acute impairment.
- The National Highway Traffic Safety Administration (NHTSA) has acknowledged that THC blood levels are not reliable indicators of driving impairment.
- Unlike alcohol, there is no validated scientific relationship between nanograms of THC per milliliter of blood and actual driving impairment.
- Tolerance plays a massive role — a chronic user may function normally at blood THC levels that would impair a novice user significantly.
Blood test challenges are the most powerful defense tool in marijuana DWI cases — see the breath and blood test challenges page for chain of custody issues, lab methodology defects, and how Herman attacks test results. In many marijuana DWI arrests, law enforcement also discovers marijuana in the vehicle and adds a possession charge — see the drug crimes defense page for how Herman defends the accompanying possession or delivery charge. A marijuana DWI lawyer Houston coordinates the DWI defense with any accompanying possession charge as a single unified strategy. This page focuses specifically on the THC science and marijuana-specific defense tactics; for how Herman challenges the Drug Recognition Evaluation protocol, opioid and prescription-medication DUID cases, and blood draws for other controlled substances, see the broader drug DWI defense overview.
Page last reviewed for accuracy against current Texas Penal Code, NHTSA guidance, and Texas hemp law provisions: August 2026.
Marijuana DWI Questions
Yes, the state can charge you — but the case is significantly weaker when active THC levels are low or undetectable. Prior-day marijuana use does not produce active impairment while driving the following day. Active delta-9-THC peaks and dissipates within hours of consumption. If your blood shows only carboxy-THC metabolites without active THC, the state cannot credibly prove you were impaired at the time of driving. Herman challenges these cases directly on the impairment evidence. A marijuana DWI lawyer Houston can request lab retesting to distinguish active THC from inactive metabolites in exactly this situation.
The odor of marijuana alone does not establish DWI — it may support probable cause for a search, but not proof of impairment. Since hemp is now legal in Texas and smells identical to marijuana, courts are scrutinizing marijuana-odor stops more carefully. Odor does not prove impairment occurred. Herman raises the hemp-versus-marijuana distinction in every stop where odor was the primary basis for detention, particularly since Texas legalized hemp in 2019.
Refusing a blood test triggers a 180-day license suspension for a first refusal under Texas ALR law. Police cannot obtain breath test evidence for marijuana — they must obtain a warrant for blood. A defective warrant results in suppression of all blood evidence. Herman evaluates every blood draw warrant for probable cause deficiencies and procedural compliance. A successful suppression motion in a marijuana DWI frequently causes the entire charge to collapse. A marijuana DWI lawyer Houston scrutinizes every warrant for probable cause before the state ever gets to trial.
No — Texas has no per se THC blood concentration limit for driving. The state must prove actual impairment under § 49.04, not merely detect THC in blood. This distinguishes marijuana DWI from alcohol DWI, where a .08 BAC creates a legal presumption of intoxication. The prosecution must rely on subjective officer observations and DRE evaluations — evidence Herman challenges more effectively than a chemical number alone.
A DRE is a law enforcement officer trained in a 12-step protocol to identify drug impairment by category. DRE testimony is contested on scientific grounds in courts across Texas. The 12-step protocol was not developed through randomized clinical drug trials. DRE opinions rest on interpretation, not objective measurement. Herman challenges DRE conclusions through cross-examination targeting the officer's training record, protocol deviations, and peer-reviewed studies undermining DRE reliability for cannabis classification specifically. A marijuana DWI lawyer Houston with access to toxicology experts can dismantle a DRE opinion piece by piece.
Yes — Texas does not recognize medical marijuana use as a defense to DWI charges. The Compassionate Use Program covers only specific conditions with low-THC cannabis. An out-of-state medical card provides no protection under Texas law. However, documented medical use establishes context for why THC metabolites appear in blood. Herman uses medical use history to support the argument that detected metabolites represent prior therapeutic use — not active impairment at the time of driving. A marijuana DWI lawyer Houston uses this medical history strategically rather than treating it as irrelevant.
Blood tests measure both delta-9-THC and carboxy-THC metabolites using gas or liquid chromatography methods. Delta-9-THC indicates recent consumption. Carboxy-THC indicates prior use without active impairment. Labs frequently report both values without distinguishing which is impairing. Chain of custody errors, improper sample storage, and instrument calibration failures can each invalidate results. Herman retains forensic toxicologists to review the full lab file — methodology, instrument logs, analyst qualifications, and sample handling — in every marijuana DWI case.
Standard NHTSA field sobriety tests — HGN, Walk-and-Turn, and One-Leg Stand — were developed for alcohol impairment and are not scientifically validated for marijuana detection. Officers also use the Modified Romberg Balance and Lack of Convergence tests in drug DWI cases. None were validated through marijuana-specific clinical studies. Herman challenges every FST result in marijuana DWI cases and presents expert testimony on their documented limitations for cannabis impairment detection.
Yes — marijuana DWI cases are among the most dismissible DWI charges in Texas. An unlawful stop, a defective blood warrant, or an unreliable DRE evaluation can each independently collapse the case. The state must prove impairment at the specific moment of driving — a high bar when blood evidence reflects metabolites from prior-day use rather than active intoxication. Herman attacks the state's evidence at each evidentiary layer to pursue dismissal in every case. A marijuana DWI lawyer Houston pursues dismissal as the primary goal in every case, not a fallback position.
No — the ALR process is identical. You have 15 days from arrest to request a hearing to contest the administrative suspension. Suspension periods mirror those for alcohol DWI. Herman files the ALR hearing request immediately upon retention for every marijuana DWI client. He challenges the lawfulness of the stop and the basis for the chemical test request at the hearing — applying the same urgency and rigor used in alcohol DWI cases.