Cannabis Impairment and Driving

Short answer

Cannabis can impair driving. Research on how much it raises crash risk gives mixed sizes, and no one has found a reliable "safe" wait time that fits everyone. Health Canada says not to drive high. [1] This page is education, not legal or medical advice.

How to read the evidence

Labels used here: established (many studies agree), mixed (studies disagree on size), insufficient evidence, and evidence of harm (an association seen in studies, which does not prove cause). "Odds ratio" is a way of comparing chances; 1.0 means no difference.

What cannabis does to driving skills

NIDA says cannabis can alter time perception and impair thinking, memory and body movement. [2] In laboratory, simulator and on-road studies, a 2022 meta-analysis (57 studies, 1,725 participants) found that cannabis alone was linked to poorer control of lane position and slower driving speed. [3] The same review found that cannabis combined with alcohol was linked to larger driving problems than either one alone. [3] Label: impairment of driving skills is established in experimental studies.

Crash risk

A 2012 meta-analysis of nine observational studies found that driving after cannabis was associated with a higher risk of collision (odds ratio 1.92), especially for fatal collisions, with large differences between studies. [4] NIDA says cannabis is the drug most often found in the blood of drivers in crashes, and that several analyses found higher crash risk after use. It also notes that a National Highway Traffic Safety Administration study found no significant increased crash risk attributable to cannabis. [2] Presence in blood does not prove a driver was impaired. Label: mixed on the size of the risk.

How long can impairment last?

There is no simple answer. Justice Canada says the existing evidence does not yet provide general guidance on how much cannabis can be consumed before driving is unsafe, or how long a driver should wait. [5]

One 2021 systematic review (80 publications) modeled recovery after a study amount of 20 mg of inhaled THC. It predicted that most driving-related thinking skills would recover within about 5 hours and almost all within about 7 hours. It found regular users showed less impairment than occasional users, and that impairment after swallowed THC may take longer to fade. [6] The authors concluded people should wait at least 5 hours after inhaled use before safety-sensitive tasks. [6] This is a model built from study amounts, not a personal guide. Real products vary and amounts are not standard.

Eaten cannabis acts differently. Health Canada says effects of cannabis that is eaten or drunk can take up to two hours to begin and up to four hours to develop fully. [1] NIDA warns that because edibles are slower, people may use more, raising the chance of serious harm. [2]

CBD and medicines

The approved CBD medicine's label says to watch for sleepiness and advises patients not to drive or operate machinery until they have enough experience with it. [7] The FDA says CBD with alcohol or other drugs that slow brain activity raises the risk of sedation and drowsiness, which can lead to injuries. [8] See CBD and medication interactions.

Law: two examples read this session

Laws change and differ by place. Two examples:

• Canada (Justice Canada page, read 2026-10-22). The Criminal Code makes it an offence to have a prohibited level of THC in the blood within two hours of driving: 2 ng or more but under 5 ng per ml of blood for the lower offence, and 5 ng or more per ml for the more serious one. [5] The page says the law does not include a medical exemption. [5]
• United Kingdom (GOV.UK, read 2026-10-22). It is illegal to drive if you are unfit because of legal or illegal drugs, or if you have certain levels of illegal drugs in your blood even if driving is not affected. The page says the law does not cover Northern Ireland, though a driver there can still be arrested for being unfit. [9]

Other countries and US states were not verified for this page. See cannabis laws by country and how we verify legal information.

Related

For other risks, see cannabis adverse effects, when to seek urgent help after cannabis and THC.