Cannabis impairs the cognitive and motor abilities necessary to operate a motor vehicle and increases the risk of crash involvement.

Effects of cannabis on driving can be less visible than those associated with alcohol, but they are every bit as impairing in their own way. Some of these effects include reduced ability to divide attention, poor time and space management, and a reduced ability to allocate concentration.

Research has found that driving within-several hours of smoking marijuana increases the risk of a crash that can result in injury or death. Cannabis is the most common drug found in drivers aged 16–19 (60.8%). This age group is more likely to have cannabis in their system than any other age group (Canadian Centre on Substance Use and Addiction).

Some people believe that using cannabis makes them better drivers, but evidence clearly shows that it impairs driving ability. These misperceptions can result in driving decisions that put the health and safety of everyone at risk.

Telltale signs of cannabis use include:

  • Distinct odour of cannabis
  • Dilated pupils
  • Eyelid and leg tremors
  • Lapses of attention and concentration
  • Red eyes
  • Impaired motor function: coordination, balance, judgement and information processing (reaction time)

When cannabis is smoked, tetrahydrocannabinol (THC) is rapidly transferred into the blood from the lungs, reaching a peak within minutes of smoking and dissipating slowly over several hours. THC blood levels fall as the THC is distributed into the fatty tissues of the body.

THC blood levels depend on the amount ingested, the concentration of THC in the cannabis, the amount of body fat, the extent of experience with cannabis and the manner in which the drug is used (inhaled, applied to the body or orally ingested).

Oral ingestion of cannabis delays the absorption of THC and results in a lower peak THC concentration.

It is important to know that combining alcohol and cannabis impairs a driver even more than consuming cannabis or alcohol alone. There is a significant combination effect when cannabis is consumed with alcohol. When combined, even at low levels, a greater level of intoxication occurs and the risk of collision increases significantly. Just a small amount of alcohol mixed with cannabis considerably increases the negative effects on driving skills.


Whether impaired by drugs or alcohol, the same rules apply. Find out more about impaired driving laws.

Talking to youth about cannabis? Check out this discussion facilitation guide from the Canadian Centre on Substance Use and Addiction.