Recreational cannabis is legal in Missouri. Driving after using it is not. How the state proves a marijuana DWI, and where the case has weaknesses.
Short answer: yes. Missouri's DWI statute doesn't care whether you were drinking, smoking, or taking your prescriptions. RSMo 577.010 covers driving while impaired by 'any drug or combination of drugs.' Marijuana is a drug under the statute, regardless of whether you bought it legally at a dispensary.
Long answer: the state has a much harder time proving a marijuana DWI than an alcohol DWI. Knowing why is the difference between rolling over and putting up a real defense.
No 'legal limit' for weed in Missouri
An alcohol DWI in Missouri has a per-se threshold: blood-alcohol content of 0.08% and you're presumed impaired, full stop. Marijuana has no equivalent number. There is no 'three nanograms of THC and you're guilty' rule under Missouri law. The state has to prove actual impairment using the totality of the evidence.
That cuts both ways. There's no automatic conviction at any THC level. But it also means prosecutors lean heavily on subjective evidence. What the officer says, what the field sobriety tests showed, what the Drug Recognition Expert (DRE) observed.
What the state actually uses to prove it
- Officer observations: bloodshot eyes, slow speech, smell of cannabis, admissions ('I smoked an hour ago').
- Driving behavior: how you were operating the vehicle before the stop.
- Field sobriety tests (FSTs): the walk-and-turn, one-leg stand, horizontal gaze nystagmus. These were validated for alcohol, not cannabis. There's real research showing they don't reliably detect THC impairment.
- Drug Recognition Expert protocol: a 12-step evaluation done at the station by a specially-trained officer. Eye exams, vital signs, balance tests, an interview, and a chemical sample.
- Blood, urine, or oral fluid testing showing THC presence.
Why THC levels are misleading
Alcohol metabolizes in hours. THC doesn't. A regular cannabis user can show measurable THC in blood for days, sometimes weeks after their last use. Long after any impairment has worn off. A urine test can show metabolites for over a month for heavy users.
That means a positive THC test does not prove the driver was impaired at the moment of the stop. It just proves they used cannabis at some point in the recent past. Expert testimony challenging the impairment inference is a real defense angle.
License consequences are still real
A marijuana DWI in Missouri triggers the same administrative license action as an alcohol DWI. You have 15 days from arrest to request a hearing with the Department of Revenue. Miss that window and the suspension proceeds automatically, regardless of how the criminal case ends.
Common defenses
- Challenge the stop. Was there reasonable suspicion to pull you over?
- Challenge the FSTs. Were they administered correctly, and are they even valid for cannabis impairment?
- Challenge the DRE evaluation. DRE testimony can be excluded as junk science under Frye in Missouri courts.
- Challenge the chemical test. Collection procedures, chain of custody, lab analysis.
- Expert testimony on THC metabolism. Separating 'recent use' from 'current impairment.'
Recreational cannabis being legal does not change the DWI analysis. Plan for the legal-system response the same way you'd plan for an alcohol DWI: 15-day administrative deadline, criminal court date, and an attorney before the first chemical-test result is in.
This is general information, not legal advice. Specific defenses depend on the facts of your case and applicable law. Past results do not guarantee similar outcomes.