Recreational marijuana is legal in Missouri. Driving while impaired by it is not, and arrests for marijuana DWI have become a regular part of the docket. But a marijuana DWI is a very different case from an alcohol DWI, and in many of them the State’s evidence is much thinner than the arrest report suggests.
The reason is simple. Almost every tool police use to investigate impaired driving was built for alcohol.
The law
Is there a legal limit for marijuana when driving in Missouri?
No. Missouri has a 0.08 limit for alcohol, but there is no equivalent number for THC.
The DWI statute, RSMo 577.010, makes it a crime to drive in an “intoxicated condition,” and Missouri law defines that to include being under the influence of a controlled substance or drug. So the State can charge a marijuana DWI, but it has to prove actual impairment. There is no level that proves the case automatically.
That matters because THC can show up in a blood or urine test long after any effect has worn off, especially for regular users. A positive test shows that marijuana was used at some point. It does not, by itself, show that the driver was impaired at the time of driving.
The field tests
Do field sobriety tests detect marijuana impairment?
They were not designed to. The three standardized field sobriety tests were developed and validated to detect alcohol impairment.
What the tests measure
The eye test (horizontal gaze nystagmus), the walk-and-turn, and the one-leg stand were studied against blood alcohol levels. Their scoring criteria are tied to alcohol. Applying the same scoring to marijuana asks the tests to do something they were never validated to do.
When the tests are skipped
In many DWI arrests, the officer goes from a smell to an arrest without giving the standardized tests at all, or gives them in a way the training does not support. When the body camera shows that, the question becomes whether the officer had probable cause to arrest in the first place.
The training
What is a Drug Recognition Expert, and did one evaluate you?
Officers have different levels of training in drug impairment, and the difference matters in court.
- Standard DWI training covers the field sobriety tests, which are built around alcohol.
- ARIDE (Advanced Roadside Impaired Driving Enforcement) is a short additional course on recognizing drug impairment. Most patrol officers do not have it, and it does not make an officer a drug recognition expert.
- A Drug Recognition Expert (DRE) has completed a longer certification and follows a multi-step evaluation, usually at the station, that ends in an opinion about which category of drug caused impairment.
Many departments in our area have few DREs or none, so many marijuana DWI arrests are made by an officer with no drug-specific certification at all. And when a DRE evaluation was done, its reliability is disputed. We challenge it in every case where one was performed.
The defense
How do you fight a marijuana DWI in Missouri?
Probable cause for the arrest
An odor, a statement about using earlier, or red eyes may justify investigating further. Whether they justify an arrest for impaired driving is a separate question, and it is often the strongest issue in the case.
The driving itself
Many marijuana DWI stops start with a minor equipment or speed violation, not with impaired driving. The video of how the person actually drove, stopped, and spoke is often the best evidence for the defense.
The officer’s qualifications
If the officer is not a DRE and has no ARIDE training, the State has to rely on observations from tests designed for alcohol. That gap is worth putting in front of a prosecutor and, if needed, a jury.
The test result
A blood result showing THC proves use, not impairment. Without a per se limit, the State still has to connect the result to how the person was driving.
Prosecutors know these cases are hard to prove. A weak marijuana DWI is one of the situations where a reduction to a lesser offense is realistic. Every case depends on its own facts and evidence.
Answers
Marijuana DWI questions we hear
Can I get a DWI for marijuana in Missouri now that it is legal?
Yes. Missouri’s DWI law covers driving while under the influence of a controlled substance or drug, and legalization did not change that. The State has to prove you were actually impaired while driving.
What is the legal THC limit for driving in Missouri?
There is none. Missouri has a per se limit for alcohol but not for THC, so a test result alone does not establish impairment.
Can a blood test prove I was high while driving?
A blood test can show THC was present. Because THC can remain detectable well after its effects fade, the result does not by itself prove impairment at the time of driving.
Do field sobriety tests work for marijuana?
The standardized field sobriety tests were developed and validated for alcohol impairment. Their use to show marijuana impairment is a legitimate point of challenge.
Will my license be suspended for a marijuana DWI?
The automatic administrative suspension in Missouri is tied to an alcohol reading of 0.08 or more. Refusing a chemical test after arrest triggers a one-year revocation regardless of the substance. A conviction carries its own license consequences.
Have an old marijuana conviction? See what to do if it was not expunged automatically.
See also refusing a breath test in Missouri and our Missouri DWI defense page, or call 573-638-8529.
Talk to us
A marijuana DWI is worth a close look.
The video, the officer’s training, and the test result all matter. The sooner we review them, the more options there are.
Attorney advertising. The choice of a lawyer is an important decision and should not be based solely upon advertisements. This article is general information about Missouri law, not legal advice, and it does not create an attorney-client relationship. Outcomes depend on the facts and evidence in each case.
© 2026 Vindex Law, PC · 235 East High Street, Suite 102, Jefferson City, MO 65101