CVNW February 2013

Comparing Alcohol and Marijuana The effects of alcohol can't be compared, apples to apples, with the effects of marijuana, says Lenny Frieling, an attorney and chairman of the Colorado chapter of the National Organization for the Reform of Marijuana Laws (NORML).

"Unlike alcohol, the correlation between active THC in your blood and impairment in driving is poorly correlated," he says.

Paul Armentano, deputy director of NORML nationally, agrees. "There is much greater variability about how marijuana influences behavior [compared to alcohol]," he says. The same amount of marijuana, for instance, may greatly affect a new smoker but not an experienced or chronic smoker, Armentano says.

Body weight can play a role, too, in one's reaction, he says.

Maximum blood levels of THC occur before the onset of impairment, he says. "As the levels go down, impairment goes up."

"The body processes marijuana in a fundamentally different manner than it does alcohol," Armentano says. For that reason, blood tests for marijuana are not impairment tests, he says, but simply detection tests that don't reflect the degree of impairment.

More study is needed, he says, to figure out how to gauge if someone's driving is affected by his marijuana use. It's a matter of identifying the best tests to detect actual impairment from marijuana use, he says.

While the effect of marijuana are variable, he says, "generally users perform most poorly 20 to 40 minutes after inhalation, but after 60 minutes their performance often begins to return to what it was before smoking."

Having a medical marijuana prescription has no impact on whether a driver will get a DUI, Frieling says, although many motorists think otherwise. If they are under the influence, they will likely be charged, he says.

Research on Alcohol, Marijuana and Driving Marijuana and alcohol do affect drivers differently, as Armentano says and several medical studies suggest.

In one study, published in 2009, Yale researchers looked at the effects of marijuana and alcohol on the ability to drive, reviewing published studies.

Both substances impaired driving skills, they found.

However, they found that the effects of marijuana varied more greatly among people than did alcohol effects.

They also found that marijuana smokers tend to compensate while driving by a variety of strategies, such as driving more slowly or passing less often.

However, combining alcohol with marijuana eliminates the ability to use the strategies, they found.

In a more recent study, published in 2012 in the journal Clinical Chemistry, researchers who reviewed published evidence found that blood THC concentrations of 2-5 nanograms per milliliter ''are associated with substantial driving impairment, particularly in occasional smokers."

They found that higher blood THC concentration is linked with increased crash risk, although other experts disagree. But studies don't show a direct correlation between impaired driving and THC concentration, the researchers say.