Hello, this is a very important question.

In Australia we have worked very hard to reduce the road toll from alcohol but only in the last decade have we been focusing on reducing the road toll from drug use. Cannabis affects driving by altering reaction times, alertness, concentration and coordination, making driving more dangerous, in a similar way to someone driving intoxicated from alcohol. Because of this, the police have introduced harsh penalties for people detected with cannabis in their system. Also, drivers found to be under the influence of drugs may find they are not covered by insurance if they have an accident and alcohol or other drugs are detected.

Cannabis has an active ingredient which is called tetrahydrocannibidol (THC) and this ingredient is absorbed into your organs very quickly. Traces of THC can be detected by a variety of methods (saliva, blood, urine) for days or weeks, depending on how heavily you smoke.

A saliva test is the least sensitive of the methods and will show positive for cannabis if you used in the last 12–24 hours. As with alcohol, the likelihood of testing positive will depend on you (size, how much you smoke) and the substance (how strong it was). For example, if you have only one or two puffs you might be clear in 6 hours but if you smoke a lot you might still be positive in 14 hours. Results varies greatly so it is hard to give exact advice as to a safe window.

I would suggest a 12–24 hour minimum for anyone before driving after using cannabis, but of course the safest action is to use minimally and give the car a miss.