No US state lets you drive while impaired by cannabis. Retail sales are legal in 24 states and the District of Columbia. Driving after use is a separate charge in all 50 states. A store receipt is not a defense in court.
The short answer
Legal purchase does not equal legal driving. Police in legal states write cannabis DUI tickets at the same rate as police in states with no retail market. Impairment is the standard. Officers look at driving behavior, not at what you bought.
States with a THC blood limit
Six states write a number into law. A driver at or above the limit can be charged without more proof of bad driving.
- Colorado: 5 nanograms of THC per milliliter of whole blood.
- Washington: 5 ng/mL.
- Montana: 5 ng/mL.
- Pennsylvania: 5 ng/mL.
- Illinois: 5 ng/mL of THC, or 10 ng/mL of carboxy-THC, the metabolite.
- Nevada: 2 ng/mL in blood, 5 ng/mL in urine.
States without a number
Most states set no numeric limit. A prosecutor must show impairment through lane drift, field sobriety tests, and a drug recognition expert evaluation. A few states use zero tolerance wording. In those states any detectable THC can support a charge. Blood THC stays detectable for days in daily users. Detection is not the same as impairment, and courts have narrowed how far those laws reach.
Timing after use
- Smoking and vaping: effects start in 10 minutes. Peak impairment runs 30 minutes to 1 hour. Most users return to baseline inside 3 hours.
- Edibles: effects start in 30 to 120 minutes. Peak impairment runs 2 to 4 hours. Effects can last 6 to 8 hours.
- Dose, tolerance, body weight, and method of use change every number above.
No chart gives a safe hour count. Safety agencies advise against driving for several hours after any use, and against driving for the rest of the day after edibles.
The bag in your car
Legal states have transport rules. Keep cannabis in the sealed child-resistant package from the store. If the package is open, put it in the trunk or a locked container. Keep it out of the driver's reach. An open container of cannabis in the cabin can bring a fine with no use at all.
Federal roads, borders, and CDL
Possession is illegal on federal land, including national parks, national forests, and Bureau of Land Management acreage. Driving impaired there is a federal charge. Carrying cannabis across a state line violates the Controlled Substances Act, even between two legal states. Commercial driver's license holders cannot use cannabis at all under FMCSA rules. A state medical card gives no protection.
What happens at a stop
An officer watches for lane drift, slow reactions, and odor. Tests include the walk and turn and the one leg stand. A drug recognition expert then checks eye motion, pulse, and muscle tone. A blood draw usually follows. Implied consent laws mean a refusal triggers a license suspension in most states.
Penalties
Penalties vary by state. A first offense brings fines, license suspension, and possible jail time. Repeat offenses add higher fines and longer suspensions. A cannabis DUI stays on a driving record and can raise insurance rates for years.
What a budtender cannot tell you
Store staff can explain products, dose, and onset time. They cannot test your THC level or clear you to drive. No roadside device measures cannabis impairment the way a breath test measures alcohol. The decision to drive is yours, and so is the charge.