People who drive within three hours of smoking marijuana are at nearly twice the risk of being in an accident that leads to serious injury or death, compared with sober drivers, according to a new review of the research.
About 4% of the American adult population — nearly 11 million people — report having driven under the influence of a drug other than alcohol, overwhelmingly marijuana.
Prior research on the risks of stoned driving has been mixed, with about half of the studies finding that marijuana raises the chance of crashing and the rest showing either no effect or a slight decrease in risk. Driving simulation studies with experienced marijuana users suggest that when people have consumed high doses of the drug, there’s an increased risk of accident, but that, unlike with alcohol, users are aware of their impairment and tend to drive more cautiously, rather than with greater recklessness.
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For the new review, published in BMJ, Canadian researchers pooled the results of nine well-designed, high-quality studies that included nearly 50,000 drivers involved in crashes in multiple countries. They found that recent marijuana use was associated with a 92% increased risk of fatal or near-fatal accidents. The better the quality of the study, the more likely it was to show an increase in marijuana-related risk.
The risk for minor collisions, however, was not raised significantly — possibly reflecting the fact that many stoned drivers attempt to be more cautious and can therefore compensate for some risks. That compensation may fail, however, when quick reflexes are most needed.
While driving stoned is clearly risky — and combining marijuana with alcohol is even more dangerous — drunk driving remains a bigger hazard on the road. Driving with a blood alcohol concentration of .08 — the legal limit — nearly triples the risk of crashing; a blood alcohol concentration of .10 almost quintuples it. Although drunk driving deaths have dropped by more than half since 1982, they still represent about one-third of all auto fatalities and kill about 11,000 people annually.
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Determining the precise role of marijuana in traffic deaths is more complicated than studying alcohol because blood levels of marijuana’s active ingredient, THC, are not always directly linked with levels of impairment the way blood alcohol levels are.
Interestingly, researchers have also found that states that legalize medical marijuana have fewer fatal car crashes, largely because of a decline in drunk driving. In other words, people may be substituting marijuana for alcohol — and while it’s not advisable to drive under the influence of either — the net result, when it comes to traffic deaths, could be a reduction in harm because smoking pot raises the crash risk less than drinking does.
Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.