Can You Drive the Day After Smoking Cannabis?

Short answer

No US state sets a fixed wait time for cannabis. Police can arrest a driver in all 50 states if they observe impairment. THC stays in blood and urine for hours to days, so a positive test one day later is possible. In most states, a positive test alone does not prove impairment.

How long THC stays in the body

Blood: THC peaks within 10 minutes of smoking, then falls. Occasional users drop under 5 ng/mL within 3 to 4 hours. Daily users can stay above 5 ng/mL for 24 hours or more.
Saliva: detectable 1 to 24 hours.
Urine: about 3 days after one use, up to 30 days with heavy daily use.
Hair: months.

Blood THC level does not track impairment. Two people at the same ng/mL can perform differently on a road test.

What research shows about next-day driving

A 2016 AAA Foundation for Traffic Safety study dosed drivers and ran them through a driving simulator. At 1 hour, impairment matched a blood alcohol level of 0.05 to 0.08. At 3 hours the effect was gone in occasional users. Heavy users, defined as 4 or more times a week, still showed effects at 4 hours on some measures.

What Happens If You Get Pulled Over for a Cannabis DUI

A 2017 Colorado Department of Transportation study of more than 3,000 drivers found no link between crash risk and blood THC below 5 ng/mL.

Meta-analyses by Rogeberg and Elvik put crash odds for cannabis-positive drivers at roughly 1.2 to 1.4 times baseline. Alcohol is higher.

Data on the 8 to 24 hour window is thin. Studies are small. No lab value maps to a specific safe hour.

US law

Every state bans driving under the influence of cannabis. The states differ on how they prove it.

Impairment states: officer observation, field sobriety tests, drug recognition expert exam.
Per se states: a set blood THC limit, often 5 ng/mL. Colorado, Montana, Nevada, Ohio, Pennsylvania, Washington.
Zero tolerance states: any detectable metabolite. Arizona, Georgia, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, Utah, Wisconsin.

Arizona and some other states charge on carboxy-THC, an inactive metabolite that can stay in blood for weeks. A medical cannabis card does not block a DUI charge in any state.

Factors that change the answer

Dose and potency. Flower runs 15 to 25 percent THC. Concentrates run 70 to 90 percent.
Route. Edibles peak at 2 to 4 hours and last 6 to 8 hours. Next-morning impairment is possible.
Use frequency. Daily users carry more residual THC.
Alcohol, sleep loss, and other drugs.

Practical steps

Wait at least 8 hours after smoking before driving. Wait longer after edibles.
Do not mix cannabis with alcohol.
Check your state limit and your state court rules.
Keep product out of the car. Open container laws cover cannabis in many states.
Field sobriety tests can be refused in many states. A chemical blood test refusal triggers license penalties under implied consent laws. Check your state.

Bottom line

Blood THC can test positive 24 hours after smoking. Occasional users clear most of the drug within 4 hours. Heavy users and edible users can still be impaired the next morning. Do not drive if you feel foggy, slowed, or unsteady.