No US state permits driving while impaired by cannabis. No federal THC limit exists. Each state writes its own rule. The rules fall into three groups: per se limits, zero tolerance, and impairment-based prosecution.
States with per se THC limits
A per se limit is a number. At or above that number, the driver commits an offense. Prosecutors do not need other evidence of impairment.
Medical Marijuana Patients Driving Laws: A Comprehensive Guide
- Pennsylvania: 1 ng/mL THC in blood. Medical cannabis patients have an affirmative defense under Act 16.
- Ohio: 2 ng/mL THC in blood, 10 ng/mL in urine. Ohio Revised Code 4511.19.
- Nevada: 2 ng/mL in blood, 5 ng/mL in urine. Nevada Revised Statutes 484C.110.
- Colorado: 5 ng/mL of active THC in whole blood. Colorado Revised Statutes 42-4-1301.
- Illinois: 5 ng/mL in whole blood, 10 ng/mL in urine or other body fluid. 625 ILCS 5/11-501.2.
- Washington: 5 ng/mL active THC in whole blood. RCW 46.61.502.
- Montana: 5 ng/mL THC in blood.
Some states let a defendant argue that the number does not prove impairment. Colorado and Washington allow that rebuttal. Pennsylvania allows it for registered medical patients.
Zero tolerance states
A zero tolerance state treats any detectable amount of THC or its metabolite as a violation. Maine bars driving with any amount of a scheduled drug in the body. Arizona and Georgia use similar language for a drug or its metabolite. A medical card does not create an exemption in these states.
cannabis driving laws by state
States with no per se THC limit
California, Michigan, Oregon, Massachusetts, Alaska, and New York publish no THC number. An officer must build a case from driving behavior, field sobriety tests, and a blood draw. The state then argues the driver was impaired. A positive THC test alone does not close the case.
Why THC limits differ from BAC limits
Alcohol limits rest on decades of data. A 0.08% BAC matches a measurable level of impairment for most drivers. THC does not work the same way. THC leaves blood and enters fat tissue within hours. A frequent user can carry active THC in blood after no recent use. NHTSA-funded research found no blood THC level that matches a degree of impairment across drivers. States that publish 5 ng/mL numbers use them as legal thresholds, not as medical cutoffs.
What happens at a traffic stop
An officer who suspects cannabis impairment looks for odor, red eyes, speech, and lane drift. Field sobriety tests follow. A blood draw needs a warrant or consent after Birchfield v. North Dakota (2016). Refusal leads to a license suspension under implied consent laws in every state. A first cannabis DUI can carry license suspension, fines, jail time, and classes. The range varies by state.
Other rules
Cannabis legal status does not change DUI law. Some states, including Michigan and Ohio, bar open cannabis containers in the passenger area of a vehicle. Cities and counties add local rules. State legislatures change these numbers between sessions. Confirm the current figure in the state code before you rely on it.