From the Guidelines
There is no safe level of THC for driving, and any detectable amount of THC in your system can impair driving ability and is generally considered unsafe. Unlike alcohol, which has established legal limits (typically 0.08% blood alcohol concentration in most U.S. states), THC affects individuals differently and can impair coordination, reaction time, judgment, and perception even at low levels. Most jurisdictions have zero-tolerance policies for driving with any measurable amount of THC in the bloodstream. The psychoactive effects of cannabis can last several hours after use, but THC can remain detectable in your system for days or weeks, even when you're no longer experiencing impairment. For safety, you should avoid driving completely for at least 6-8 hours after using cannabis, though waiting 24 hours is more prudent. The safest approach is to separate cannabis use entirely from driving by arranging alternative transportation if you plan to consume cannabis. This recommendation applies to both recreational and medical cannabis users, as impairment occurs regardless of why the substance is being used. According to a study by the American College of Physicians 1, public health information campaigns related to driving under the influence of cannabis may be particularly important, and the National Highway Traffic Safety Administration has used strategies from its anti–drunk driving campaigns to promote “If you feel different, you drive different” messaging to educate the public on impairment due to cannabis consumption.
Some key points to consider:
- THC can impair driving ability even at low levels
- Most jurisdictions have zero-tolerance policies for driving with any measurable amount of THC in the bloodstream
- The psychoactive effects of cannabis can last several hours after use
- THC can remain detectable in your system for days or weeks after use
- Public health information campaigns are important for educating the public on the risks of driving under the influence of cannabis, as noted in a study by the American College of Physicians 1.
It's also worth noting that the American College of Physicians has called for more research on the effects of cannabis on driving ability, as well as the development of more effective public health campaigns to educate the public on the risks of driving under the influence of cannabis 1.
From the Research
THC Levels and Driving Impairment
- The level of Tetrahydrocannabinol (THC) considered safe for driving is not clearly defined, as it can vary depending on several factors, including the individual's tolerance and the method of consumption 2.
- Studies have shown that THC can impair driving performance and increase crash risk, with effects lasting up to eight hours after oral THC consumption 2.
- There is no evidence that cannabidiol (CBD) impairs driving, but patients using THC-containing products should avoid driving and other safety-sensitive tasks, particularly during initiation of treatment and in the hours immediately following each dose 2.
Detection of THC in Blood and Oral Fluid
- THC can be detected in the blood of chronic daily cannabis smokers for up to 30 days after last use, with median concentrations decreasing gradually over time 3.
- A study validated a method for quantifying THC in oral fluid collected with on-site collectors, with detection limits ranging from 8 to 922 ng/mL in neat oral fluid 4.
- Combining toxicology testing with field sobriety tests can improve the accuracy of identifying cannabis-impaired drivers, with a 2 ng/mL THC cutoff in oral fluid reducing false positives 5.
Correlates of Marijuana Drugged Driving
- A survey of marijuana users in Colorado and Washington found that 43.6% of respondents reported driving while high in the past year, and 23.9% reported driving within 1 hour of using marijuana at least 5 times in the past month 6.
- Increased perception that driving high is unsafe was associated with lower odds of marijuana DUI, while increased knowledge of marijuana DUI laws was also associated with lower odds, but to a lesser extent 6.
- Interventions targeting safety perceptions related to marijuana DUI may be more effective in reducing the incidence of marijuana DUI than those targeting knowledge of DUI laws 6.