Cannabis types have different effects on driving

Cannabis users should not drive for at least four hours after inhaling the drug. Research at Maastricht University shows that within this time frame the driver is very likely to drift from side to side on the road. After four hours, cannabis products have little or no observable effect on driving, whether they contain Δ9-tetrahydrocannabinol (THC), or combinations of THC and cannabidiol (CBD), the two main active substances. However, cannabis that only contains CBD has little or no noticeable effect on driving behaviour even in the hours immediately after use. The results of the study were published today in the scientific journal JAMA.

Test car

Participants in the study drove eight journeys of around 100 kilometres in a special test car in the Maastricht area. Sometimes they were first given cannabis using a vaporizer, while on other occasions they were given a placebo in the same way. The types of cannabis which the participants inhaled also varied. The study shows that THC in particular influences driving behaviour. Drivers who had used cannabis with THC as the main constituent drifted from side to side in a way that is comparable with people who have a blood alcohol level of more than 0.05 percent, the legal alcohol limit for drivers in the Netherlands. Only four hours after use the influence of the cannabis was minimal again. Cannabis that mainly contains the active ingredient CBD, on the other hand, appeared to have little or no effect on driving ability in the hours immediately after use.


The study involved 26 healthy, regular cannabis users. Of the 188 test journeys, 16 had to be stopped early for safety reasons. All participants received the same dose, which for both the active ingredient THC and CBD was 13.75 milligrams, enough to produce a normal cannabis high in participants. ‘These results are highly relevant in view of international legislation regarding the use of cannabis while driving,’ says research leader Dr Jan Ramaekers, Professor of Psychopharmacology in Maastricht. ‘In many countries, drivers who test positive for cannabis are committing an offence irrespective of the time of use and the type of cannabis, while these factors produce very different outcomes with regard to road safety.’

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