Texas DWI law doesn't just cover alcohol. The statute defines intoxication as the loss of normal mental or physical faculties due to "alcohol, a controlled substance, a drug, a dangerous drug, a combination of two or more of those substances, or any other substance" — one of the broadest definitions in the country. If it impairs you, driving on it can be a DWI.
No legal limit means no easy line
Unlike alcohol's 0.08, Texas has no per-se limit for THC or any drug. The state must prove actual impairment, which makes drug DWI cases hinge on three things:
- The Drug Recognition Expert (DRE) evaluation — a 12-step protocol (pulse, blood pressure, pupil size, muscle tone, psychophysical tests) performed by a specially certified officer. Each step has protocols, and deviations matter.
- The blood test — usually the only way to detect drugs. Presence isn't impairment: THC metabolites linger for days or weeks after use, long after any impairing effect ends. What the lab found, at what concentration, and what that concentration actually proves about driving-time impairment are all contestable. See blood test defense.
- The driving and the video — without a number to lean on, prosecutors lean harder on driving behavior and body-cam footage. Normal driving and a normal-looking video are powerful defense evidence.
Prescription medication is not a shield
"I had a prescription" is not a defense to DWI in Texas — the statute reaches impairment by legally obtained drugs: benzodiazepines, sleep aids, opioid painkillers, muscle relaxants, even some over-the-counter medications in combination. But it is powerful mitigation and context: jurors respond differently to a patient following doctor's orders than to illicit use, and the warning labels, dosage timing, and prescribing records become defense evidence.
Common drug DWI scenarios in DFW
- Marijuana (legal status elsewhere is irrelevant — impairment while driving in Texas is the charge)
- Prescription sedatives and sleep medications taken before an "I'll be fine" drive
- Poly-drug combinations — a glass of wine plus a prescribed medication amplifying each other
- Post-accident blood draws where hospital records become the state's evidence
Content last reviewed: