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
  • reduced ability to allocate concentration

Research has found that driving within several hours of consuming cannabis increases the risk of a crash that can result in injury or death. Cannabis is the most common drug found in drivers aged 16 to 19 (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.

Alcohol and cannabis

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.

Signs of use

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 functions, such as:
    • coordination
    • balance
    • judgement
    • reaction time

Effects of cannabis on the body

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
  • concentration of THC in the cannabis
  • amount of body fat an individual has
  • extent of experience with cannabis
  • 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.

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