Yes, driving high is dangerous: THC slows reaction time, weakens lane control and divided attention, and raises crash risk, and driving while impaired by cannabis is a DUI in every state. The added risk is smaller than for drunk driving but real, and it jumps sharply when cannabis is combined with alcohol. Research suggests waiting at least 5 hours after smoking or vaping and longer after edibles.
This page began as a short news item on research from the Pacific Institute for Research and Evaluation, which found that marijuana, cocaine and amphetamines were common in drivers killed in crashes. That finding still stands, and since then researchers have learned much more about how cannabis affects driving, how long impairment lasts and why blood tests make cannabis DUI cases complicated. This guide covers the evidence and the law in Washington and Oregon as of October 2026.
Driving High: Evidence at a Glance
| Claim | Evidence level | Source |
|---|---|---|
| Cannabis use increases the risk of motor vehicle crashes | Substantial | National Academies 2017 |
| THC impairs lane control, tracking, reaction time and divided attention | Strong (lab and driving studies) | Hartman and Huestis 2013 |
| Size of the crash risk increase | Modest; about 1.3 to 2 times, depending on study design | Rogeberg 2019 |
| Most impairment fades within about 5 to 7 hours after inhaling | Moderate (meta-analysis) | McCartney 2021 |
| Cannabis plus alcohol is worse than alcohol alone | Moderate | Ramaekers 2011 |
| CBD-dominant cannabis impairs driving | Not in one controlled trial | Arkell 2020 |
| Blood THC level reliably shows impairment | No; daily users can carry THC for weeks | Bergamaschi 2013 |
How Common Is Cannabis in Serious Crashes?
Very common, though finding THC does not prove it caused a crash.
- A 2013 Pacific Institute for Research and Evaluation study in Addiction looked at 16,942 drivers killed in single-vehicle crashes from 1998 to 2010. About 45% tested positive for alcohol. Among drug classes, stimulants and cannabinoids were the most common, each at about 7%. The authors stressed that drug-involved crashes were less common than alcohol-involved ones and that the focus on drunk driving should continue.
- A 2022 NHTSA study of 7,279 seriously or fatally injured road users at seven trauma centers found active THC in 25.1% and alcohol in 23.1%, making cannabis the most frequently detected substance. Nearly 20% tested positive for two or more drug categories.
Both studies share the caveat the original researchers made: THC can linger in the blood of regular users long after the high wears off, so a positive test shows use, not necessarily impairment at the time of the crash.
How Cannabis Affects Driving
A 2013 review by NIDA scientists pulled together decades of lab, simulator and on-road studies. THC impaired:
- Lane position. Drivers weave more within the lane.
- Tracking, the ability to keep the car steady through small corrections.
- Reaction time to sudden events.
- Divided attention, such as watching the road while checking mirrors or responding to a signal.
Drivers who are high often slow down and leave more space, which partly offsets the risk. That compensation breaks down in complex or unexpected situations, which is when crashes happen. The review found complex tasks stayed impaired even in regular users who had built tolerance. The CDC says cannabis slows reaction time and decision-making and that the safest choice is not to drive after using.
A controlled on-road trial in the Netherlands put numbers on it. Occasional users who vaporized THC-dominant cannabis weaved noticeably more 40 to 100 minutes later than after placebo, while CBD-dominant cannabis caused no measurable impairment. By 4 to 5 hours, the THC difference was no longer significant at that modest 13.75 mg dose. The authors cautioned that the doses may not match typical use and that the trial could not fully rule out impairment from CBD-dominant cannabis.
How Much Does Driving High Raise Crash Risk?
Estimates vary with study design. The NIDA review estimated roughly double the crash risk after recent smoking. A 2019 meta-analysis of 13 culpability studies, which compare drivers who caused crashes with those who did not, estimated a smaller average increase of about 1.28 times. For comparison, alcohol’s crash risk rises steeply with blood alcohol level and is much larger at legal-limit levels. Cannabis is less dangerous behind the wheel than alcohol, but it is not safe, and the risk is highest for occasional users and with higher doses. Our motor function research guide covers the lab findings in more detail.
Weed and Alcohol Together
This is the most dangerous combination. In a 2011 Dutch study of heavy cannabis users, THC alone caused little measurable impairment on lab tests, but THC combined with alcohol impaired divided attention more than alcohol alone, and the users had no tolerance to alcohol’s effects. Even a modest drink on top of cannabis can push a driver into dangerous territory. See our comparison of weed vs. alcohol.
How Long After Smoking Weed Can You Drive?
The best estimate comes from a 2021 University of Sydney meta-analysis of 80 studies. After inhaling a moderate 20 mg dose of THC, most driving-related skills recovered within about 5 hours and nearly all within about 7. The authors recommended waiting at least 5 hours after inhaled cannabis before driving or other safety-sensitive tasks. Higher doses take longer.
| How you used | Suggested minimum wait before driving |
|---|---|
| Smoked or vaped, moderate dose | At least 5 hours; longer after heavy use |
| Edibles, drinks or capsules | Longer; plan for 8 hours or more, or wait until the next day |
| Any amount plus alcohol | Do not drive; arrange a ride |
| CBD-only product with no THC | No impairment found in one trial, but check labels; many contain THC |
The edibles row is a cautious estimate: the Sydney review found oral THC impairment may take longer to fade, but did not set a number. Feeling sober is not a reliable guide, and even after impairment fades, regular users may still test above legal limits. Our dosage guide explains why edibles hit later and last longer.
Cannabis DUI Laws in Washington and Oregon
| Rule | Washington | Oregon |
|---|---|---|
| THC blood limit (21+) | 5 ng/mL of active THC within 2 hours of driving is a per se DUI (RCW 46.61.502) | No numeric limit; DUII is based on being under the influence of an intoxicant (ORS 813.010) |
| Drivers under 21 | Any THC above 0.00 is an offense (RCW 46.61.503) | Impairment-based |
| Open container | Cannabis must be in the trunk or a sealed package; using in a vehicle is an infraction (RCW 46.61.745) | Using cannabis while driving is illegal |
| First-offense class | Gross misdemeanor | Class A misdemeanor, minimum $1,000 fine |
Washington’s 5 ng/mL rule is controversial because blood THC does not track impairment well. In a NIDA study of 30 daily smokers held on a locked unit, some still had low but measurable THC in their blood weeks after their last use, while studies show occasional users can be impaired at levels below 5 ng/mL. Officers also use drug recognition experts and field sobriety tests. Full details are in our Washington and Oregon law guides.
Risks and Side Effects
- Slower reactions, weaving and poor judgment of distance and time
- Drowsiness, especially late at night or after edibles
- Much higher risk when combined with alcohol or sedating medicines
- Delayed, stronger effects from edibles that can hit while already driving
- Legal risk: DUI arrest, license loss, fines and insurance costs, even for medical patients
Talk to Your Doctor
If you use cannabis for a medical condition, ask your doctor how your dose and product affect your ability to drive, and whether other medicines you take add to drowsiness. A medical card does not protect you from a DUI charge. If you find you regularly drive after using, or cannot plan around it, that may be a sign of a bigger problem; see our marijuana addiction guide.
Driving High FAQ
Is it illegal to drive high?
Yes, in every state. Washington sets a 5 ng/mL blood THC limit for adults and zero tolerance for drivers under 21; Oregon and many other states prosecute based on impairment.
Is driving high safer than driving drunk?
Neither is safe. Research shows a smaller crash risk increase for cannabis than for alcohol at the legal limit, but THC still impairs key driving skills, and combining the two is worse than either.
How long should you wait to drive after smoking weed?
At least 5 hours after a moderate inhaled dose, based on a large meta-analysis, and longer after heavy use or edibles.
Can you get a DUI the next day after smoking weed?
It is possible. Blood THC usually drops below 5 ng/mL within hours, but heavy users can carry residual THC longer, and a test over Washington’s limit is a DUI even if you feel fine. Drivers under 21 in Washington can be charged for any detectable THC.
Can police test for weed while driving?
Officers use field sobriety tests and drug recognition experts and can get a blood test with consent or a warrant. Some states also use roadside saliva screens.
Does CBD affect driving?
CBD-dominant cannabis did not impair driving in a controlled trial, but many CBD products contain some THC, which can impair you and show up in a blood test.
More research in our cannabis research section.
For adults 21+ in legal states. General information, not medical advice. Talk to a doctor before using cannabis for a health condition.
