Short answer

Yes. Driving with THC in your system can be illegal in all 50 states, and the charge does not depend on whether you bought the cannabis legally. Two factors decide the outcome: whether the drug impaired your driving, and whether your state sets a chemical threshold for THC in blood. In states with a per se limit, a lab result above that number can support a DUI conviction even if you felt sober and drove normally. In states with a zero tolerance law, any detectable amount of THC or its metabolites may be enough to charge you.

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How states define driving with THC

State DUI statutes fall into three groups. Which group you live in determines how much proof a prosecutor needs.

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Impairment based laws

The state must show that cannabis affected your ability to drive. Evidence includes lane drifting, speed, field sobriety tests, and a drug recognition expert opinion. A positive blood test alone will not convict you, but it supports the case.

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Per se limits

The statute names a number. Colorado, Montana, Pennsylvania, Utah, and Washington use 5 nanograms of delta-9 THC per milliliter of whole blood. Nevada and Ohio use 2 ng/mL. Rhode Island uses 5 ng/mL. At or above the limit, the prosecution does not have to prove impairment beyond the lab result.

What Happens If You Get Pulled Over for a Cannabis DUI

Zero tolerance laws

Arizona, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Michigan, Oklahoma, and several other states prohibit any detectable amount of THC or its breakdown products behind the wheel. Michigan courts require evidence beyond the inactive metabolite, so the practical reach of that state's law is narrower than the statute reads.

The metabolite problem

Carboxy-THC is a non-impairing metabolite. It can stay detectable in blood or urine for days to weeks after you last used cannabis. Arizona courts have upheld DUI convictions based on carboxy-THC alone, with no evidence of impairment. Michigan's Supreme Court went the other way in 2023, ruling that the inactive metabolite by itself does not prove a driver was under the influence. Two drivers with the same habits can face different charges based on which state line they cross.

Medical cards and legal sales are not a defense

Adult-use legalization regulates sale and possession. It does not change driving law. A medical marijuana card does not make it legal to drive impaired, and most states reject the card as a defense at trial. A small number of states give registered patients a limited affirmative defense if they can show they were not impaired, but you carry the burden of proving it.

Commercial drivers and federal land

CDL holders fall under FMCSA rules. Marijuana remains a Schedule I drug, and a medical recommendation is not an acceptable explanation for a positive DOT test. A positive result can pull you out of safety sensitive duty. Driving on federal land, including national parks, military bases, and some federal highways, puts you under federal DUI law no matter what your state allows.

What happens during a traffic stop

  1. An officer who suspects cannabis impairment will look for bloodshot eyes, the odor of burnt marijuana, slurred speech, and slow reactions.
  2. You will be asked to step out and perform field sobriety tests such as the walk and turn or the one leg stand.
  3. A drug recognition expert may be called to run a 12-step evaluation and record pupil size, pulse, and blood pressure.
  4. There is no breath test for THC. If the officer wants a chemical sample, you will be asked for blood, and in many states a warrant can be obtained if you refuse.
  5. Refusing a chemical test triggers implied consent penalties, usually an automatic license suspension, separate from any DUI charge.

How to lower your risk

  1. Do not drive after using cannabis. Set a ride, call a taxi, or stay where you are.
  2. Treat edibles with more caution than smoked flower. Onset takes 30 to 120 minutes, and impairment can last six hours or longer.
  3. Do not keep open cannabis containers in the passenger area, since that can create a separate charge in many states.
  4. Understand that no calculator tells you when your blood THC drops below your state's limit. Levels fall fast after smoking and rise unpredictably with edibles.
  5. Talk to a DUI attorney in your state before assuming a medical card or a legal purchase protects you.