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WEDNESDAY, Oct. 12 (HealthDay News) -- Drivers who get behind the wheel after using marijuana run more than twice the risk of crashing compared to others, a new study finds.

The risk rises even higher if the driver has also been drinking alcohol.

The authors of a study published online Oct. 4 in Epidemiologic Reviews believe the findings are especially relevant in light of recent moves to legalize medical marijuana in many states.

"As more and more states consider medical use of marijuana, there could be health implications," said study senior author Dr. Guohua Li.

Even as alcohol use has decreased over the past four decades, illicit use of non-alcoholic drugs, such as prescription medications and marijuana, has increased, said Li, a professor of epidemiology at Columbia University Mailman School of Public Health in New York City.

A large U.S. survey in 2009 estimated that more than 10 million people aged 12 and over had driven while under the influence of illicit drugs in the previous year. And testing has revealed that 28 percent of drivers who die from a crash and more than 11 percent of drivers in general test positive for drugs other than alcohol. Marijuana is the most commonly detected drug in drivers after alcohol.

In their study, Li and his co-authors assessed information from nine prior studies in six countries looking at marijuana use and motor vehicle accidents.

The studies looked at different time frames, with some assessing marijuana use as little as one hour before driving and others looking at one year or more. According to one study cited, driving skills are acutely affected for three to four hours after use.

All but one study found a higher risk of crashes in drivers who use marijuana, and that study was a small one, conducted in Thailand, where marijuana use is relatively low.

Overall, the risk of a crash was almost 2.7 times higher among marijuana users than non-users, the authors found. And the response was dose-specific, the authors said. That is, the more marijuana smoked -- in terms of frequency and potency -- the greater the likelihood of a crash.

Marijuana may interfere with reaction times and coordination, among other things, experts say. The authors of the new study said it is critical to determine the excess crash risk related to marijuana in different doses, strengths, and administration methods, such as smoking versus vaporization.

None of the studies in this grouping looked directly at medical marijuana, which is now legal in 16 states plus the District of Columbia in the United States.

However, one expert cautioned against inferring too much from this study, which was not designed to capture cause and effect.

"We can't really say yet that marijuana increases the risk by two or three times," said Chuck Farmer, director of statistics at the Insurance Institute for Highway Safety in Arlington, Va. "Most of their studies pointed to a very strong bad effect of marijuana on driving, but there are other studies out there that actually go the other way."

But other experts expressed some alarm at the findings. "At [its] annual meeting in late September, the Governors Highway Safety Association strengthened its drugged driving policy," said Jonathan Adkins, a spokesperson for the association.

"We see this as a national priority and are seeking a range of actions to address the problem comprehensively," Adkins said.

More information

The U.S. National Highway Safety Administration has statistics on drug-impaired driving.

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