How does cannabis affect driving?
Cannabis is a depressant drug, which means it slows down messages travelling between your brain and body. When large doses of cannabis are taken, it may also produce hallucinogenic effects. The main active chemical in cannabis is THC (delta-9 tetrahydrocannabinol).
Research shows that after recent use of THC the risk of being killed in a fatal crash is similar to a driver with a BAC of up to approximately 0.151.
The effects of any drug (including cannabis) vary from person to person. How cannabis affects a person depends on many things including their size, weight and health, also whether the person is used to taking it. The effects of cannabis, as with any drug, also depend on the amount taken.
There is no safe level of cannabis use. Use of any drug always carries some risk—even medications can produce unwanted side-effects. It is important to be careful when taking any type of drug.1
These factors make it difficult to predict exactly, in what way, and for how long, cannabis will affect driving ability.
As a general guide, cannabis can cause:
A person who has been using cannabis may think that, if they are especially careful, they will be able to drive safely. However, the cannabis may have affected their view and experience of reality, and their judgement. Their actions and responses may be quite different to what is actually needed, but they may not be aware of how much their driving skills have been affected.
Even after a small amount of cannabis you should not drive for at least 5 hours.
Tips for driving safely
If you have used cannabis, the safest option is not to drive.
Especially avoid driving late at night or early in the morning when natural tiredness may increase the sedating effects of cannabis.
1. VicRoads. (n.d.) Cannabis & road safety.
Last updated: 21 May 2014