Texas is one of the strictest states in the country when it comes to marijuana. Recreational use remains illegal. Medical marijuana exists in extremely limited form under the Compassionate Use Program, available only for specific conditions with a very restricted registry of approved physicians. And yet, with neighboring states having legalized cannabis and the reality that millions of Texans use marijuana — legally elsewhere, illegally here — DWI cases involving THC are a significant and growing part of criminal courts in Harris County.
Marijuana DWI cases are fundamentally different from alcohol DWI cases. The science is different. The legal framework is different. The defenses available are different. And the potential for injustice — for someone to be convicted of impaired driving when they weren’t impaired — is arguably greater than in any other type of DWI case. Understanding those differences is essential if you’re facing a marijuana DWI charge in Texas.
No Legal Limit for THC in Texas
This is the first and most important thing to understand: unlike alcohol, where Texas law establishes .08 as the per se legal limit, there is no per se THC concentration limit in Texas law. There is no number — no nanograms per milliliter of THC in blood — that automatically makes you guilty of DWI.
Texas Penal Code § 49.01 defines intoxication for DWI purposes in two ways: having an alcohol concentration of .08 or above (the per se standard for alcohol), or not having the normal use of mental or physical faculties by reason of the introduction of alcohol, a controlled substance, a drug, or a combination of two or more substances into the body.
For marijuana cases, only the second definition applies. The prosecution must prove that you lacked the normal use of your mental or physical faculties because of marijuana. They cannot simply point to a THC number on a lab report and say you were per se intoxicated. They must prove actual impairment at the time you were driving.
This is simultaneously good news and complicated news for people charged with marijuana DWI in Texas.
How Texas Tests for Marijuana: Blood Draws and What They Find
When a Texas officer suspects marijuana impairment during a DWI investigation, the primary evidence-gathering method is a blood draw. Unlike alcohol breath testing, there is no roadside device that reliably measures THC impairment. Saliva testing devices exist but are not yet widely deployed in Texas and have significant reliability questions. The standard approach is to obtain a blood sample — either through a warrant or consent — and submit it to a laboratory for toxicological analysis.
Blood tests for marijuana can detect multiple compounds:
- Delta-9-THC: The primary psychoactive compound in marijuana. This is the compound that produces the “high.” Its presence in blood indicates recent use.
- 11-Hydroxy-THC (11-OH-THC): An active metabolite produced during THC metabolism. Also psychoactive. Present in blood relatively briefly after use.
- 11-Nor-9-Carboxy-THC (THC-COOH): An inactive metabolite — a breakdown product with no psychoactive effects. This is what stays in the bloodstream for days to weeks after use. Its presence tells you someone used marijuana at some point. It tells you nothing about whether they were impaired when driving.
Here is the critical issue: many laboratories report all three compounds, and many prosecutors present all three numbers to juries without clearly distinguishing between the active compounds (THC and 11-OH-THC, which have some relationship to impairment) and the inactive metabolite (THC-COOH, which tells you nothing about impairment at the time of driving). This conflation of compounds is a major source of injustice in marijuana DWI cases and a key area of defense.
The “Per Se” vs. “Impaired at the Time of Driving” Problem
Many states that have legalized marijuana have adopted per se THC limits — typically 5 nanograms per milliliter of delta-9-THC in blood. The science behind these limits is contested, but at least they create a defined threshold. Texas has no such limit. As described above, Texas marijuana DWI cases require proof of actual impairment.
The fundamental challenge for prosecutors is this: they need to prove impairment at the time of driving, but they only have evidence collected at the time of the blood draw — which may have been an hour or more after the traffic stop, and hours or days after the person last consumed marijuana.
For alcohol, forensic toxicologists use retrograde extrapolation to estimate BAC at the time of driving based on the test result. The math is reasonably reliable because alcohol elimination from the blood follows a predictable pattern. For THC, the equivalent calculation is far less reliable. THC doesn’t leave the bloodstream in a predictable linear pattern. It is fat-soluble, meaning it is stored in fatty tissue and released back into the bloodstream at varying rates depending on body composition, frequency of use, and metabolic factors. A regular marijuana user may have substantial THC in their blood even when they haven’t used in 24 hours and are not impaired. A casual user may clear THC quickly. There is no reliable formula.
This unpredictability is both the central problem for prosecutors and the central opportunity for defense attorneys.
THC Stays in Blood for Days or Weeks After Impairment: The Key Defense
The scientific literature is clear on a point that has enormous implications for marijuana DWI prosecution: THC persists in the blood of regular marijuana users at detectable levels for days to weeks after last use. Regular, heavy users can test positive for detectable levels of delta-9-THC for 24-48 hours after last use — or longer — even without any acute impairment.
This means that a person can be completely sober, not impaired in any way, and still have blood test results that show positive THC concentrations. If there is no legal limit, and the prosecution must prove impairment at the time of driving, then a positive THC blood test alone — even with a substantial number — cannot be sufficient to convict.
Herman Martinez builds marijuana DWI defense around this science. The questions that become central to the defense include: When did the client last use marijuana? Are they a regular user with high tolerance? What is their pattern of use? How long was the window between driving and blood draw? What specific compounds were detected, and in what concentrations? What physical observations did the officer actually make, and are those observations consistent with acute THC impairment or with something else?
Field Sobriety Tests for Drug Impairment: The DRE Protocol
Standard field sobriety tests (the HGN, Walk and Turn, and One Leg Stand) are validated for alcohol impairment. They are not validated for marijuana impairment. The NHTSA manual itself acknowledges that the HGN test — which looks for involuntary eye jerking — is not reliably caused by marijuana. Cannabis use causes a different set of physiological responses.
When officers suspect marijuana impairment, they may call for a Drug Recognition Expert (DRE) — an officer with specialized training in identifying drug impairment through a 12-step evaluation protocol. The DRE protocol includes checks of pulse rate, blood pressure, body temperature, muscle tone, pupil size, and other physiological indicators, along with the standard field sobriety tests and a structured interview.
There are several important things to know about DRE evidence. First, not every Texas agency has DREs available, and not every marijuana DWI arrest involves a DRE evaluation. Second, the scientific validity of DRE opinions — particularly the ability to identify the specific category of drug causing impairment — is contested in the scientific literature. Third, DRE protocols create a paper record of observations that can be systematically challenged. Herman knows the DRE protocol in detail and cross-examines DRE officers on their observations, their methodology, and the scientific basis for their conclusions.
Our detailed guide to field sobriety test limitations covers the broader issues with SFST reliability, and those issues are amplified when applied outside the alcohol context for which they were designed.
Marijuana Impairment vs. Alcohol Impairment: Observable Differences
Trained officers and jurors learn to associate certain signs with alcohol impairment: slurred speech, difficulty maintaining balance, strong odor, bloodshot eyes, slow reaction time in an obvious way. Marijuana impairment has a different profile — and that difference matters for defense purposes.
Cannabis-impaired drivers typically show: red, bloodshot eyes (conjunctival injection), elevated heart rate, possible eyelid tremors, body tremors in some cases, possible slowed reaction time, and a particular pattern on the Walk and Turn and One Leg Stand that may differ from the alcohol pattern. The odor of marijuana may be detectable. Speech may be less obviously affected than with alcohol.
The key question in every marijuana DWI case is whether the observations the officer documented are actually consistent with acute THC impairment — or whether they’re consistent with a sober person who recently used marijuana and has detectable metabolites but no functional impairment. These are not the same thing, and expert testimony can help juries understand the distinction.
Texas Medical Marijuana and Interstate Travel
Texas has an extremely limited medical marijuana program called the Compassionate Use Program (CUP), which permits low-THC cannabis (no more than 1% delta-9-THC) for specific conditions including epilepsy, terminal cancer, PTSD, ALS, autism spectrum disorder, and others. The number of registered physicians and dispensaries is extremely small compared to states with broader medical marijuana programs.
Texas does not recognize medical marijuana authorizations from other states. If you are a legal medical marijuana patient in California, Colorado, or any other state, that authorization provides no legal protection in Texas. Using cannabis in your home state and then traveling to Texas — even if no additional consumption occurred here — can result in prosecution if you’re arrested for DWI and test positive for THC. You had no right to possess or use it in Texas, and your out-of-state authorization is not a defense to a Texas DWI charge.
This is a situation that affects tourists, business travelers, and people relocating from legal states who don’t realize that their legal behavior at home has no protection once they cross the Texas state line. If you’re in this situation, Herman’s defense strategy must address both the residual THC issue and the question of actual impairment at the time of driving.
Prescription Medications and Synthetic THC
Some prescription medications can cause positive THC results on testing. Dronabinol (Marinol) is FDA-approved synthetic THC used for nausea and appetite stimulation in cancer patients. Nabilone is another synthetic cannabinoid prescription drug. These medications are legal when prescribed but can produce positive THC test results. Defense strategy in these cases involves establishing the prescription, the prescribing physician’s guidance, and evidence of whether the use was consistent with the prescription and whether impairment occurred.
What Herman Does Differently in Marijuana DWI Cases
Marijuana DWI cases require expertise that goes beyond standard DWI defense. They require understanding of forensic toxicology, the specific pharmacokinetics of cannabis in the human body, and the scientific literature on THC, impairment, and blood testing. They require knowing how to cross-examine DRE officers effectively and how to retain and use toxicology expert witnesses. They require educating juries on science they’ve likely never considered.
Herman Martinez handles marijuana DWI cases in Houston with the depth of analysis these cases require. He examines every aspect of the blood test — what was tested, how it was collected, how it was stored, what specific compounds were found, and what those findings actually mean for impairment at the time of driving. He challenges the DRE evaluation where present. And he builds a defense that centers on the fundamental point: a positive THC test is not the same as proof of impairment.
If you’ve been charged with a marijuana DWI in Houston or Harris County, the case is not hopeless just because a blood test came back positive for THC. The science doesn’t support automatic guilt. The law doesn’t either. But you need an attorney who understands both.
Talk to Herman About Your Marijuana DWI Case
Call Herman Martinez at (713) 242-1779 for a free, confidential case evaluation. Or use the online evaluation form to start the conversation. Marijuana DWI cases require a defense attorney who understands the science, knows the law, and can communicate both effectively to prosecutors and juries in Harris County. That’s exactly what Herman provides.
