Impaired driving is impaired driving. Studies suggest driving while high increases your chance of being in a collision.

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. The most common drug found in drivers aged 16–19 is cannabis (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.


Drivers, especially young drivers, need to be aware that other causes of impairment (illicit drugs, prescription drugs, recreational drugs, over-the-counter drugs and fatigue) can also impair one’s ability to operate a vehicle safely. Effects can include reduced ability to divide attention, poor time and space management, and reduced ability to allocate concentration. These effects can increase crash risk by up to eight times, with some crashes resulting in death (Canadian Centre on Substance Use and Addiction).

Drivers with a medical condition may be more susceptible to the side effects of prescription drugs because they often use multiple medications in combination, and they are more likely to have pre-existing conditions that can increase both the frequency and severity of adverse effects.

Just because the prescription drug was prescribed to you by your doctor does not necessarily mean it is okay to drive after use. This includes a prescription for medical cannabis. Cannabis is an impairing substance and should not be used before driving. Medical cannabis users are not permitted to drive while impaired. Medical users who have consumed cannabis have to make other plans (e.g. taxi, ride-share, designated driver) to get to their destination.

The same is true for other prescription drugs. Given the properties of some prescription drugs, it might not be safe to operate a vehicle after consumption. Be sure to read the label and speak to your doctor or pharmacist. The effects of some prescription drugs can last for several hours. While many prescription and over-the-counter drugs do not necessarily affect driving abilities, many of these drugs can have side effects severe enough to impair driving, even at safely prescribed doses. According to the American Automobile Association, the following drugs have the potential to impair driving:

  • Tranquilizers
  • Narcotic pain pills
  • Sleep medicines
  • Some antidepressants
  • Cough medicines
  • Antihistamines and
  • Decongestants

Impairment by some drugs, such as sedatives, might not be obvious and the effects of some sleep medications can linger into the next morning. Effects include slowed reaction time, sleepiness, poor psychomotor performance, impaired coordination, reduced ability to divide attention, increased errors and difficulty following instructions.