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LINDA WERTHEIMER, HOST:

This is MORNING EDITION, from NPR News. I'm Linda Wertheimer.

DAVID GREENE, HOST:

And I'm David Greene. Good morning.

Today in Your Health: legal marijuana. By this summer, adults in Washington State will be able to walk into a store and buy it. Colorado opened its first pot stores in January. Many other states are considering legalizing or decriminalizing marijuana. But there's not a lot of data on possible health effects of the drug. In a moment, we'll hear about marijuana and the teenage brain.

But first, NPR's Richard Knox looks at other health concerns.

RICHARD KNOX, BYLINE: Jacquelene Cohen smokes pot once or twice a month. She calls it a simple pleasure.

JACQUELENE COHEN: It's pouring a nice bourbon for a friend, or it's passing a pipe with marijuana in it. It's just something nice to do when you're sitting around conversationally.

KNOX: Cohen, a 29-year-old publicity director for a Seattle publishing company, is your average marijuana user.

MARK KLEIMAN: Most cannabis users are moderate users. They use a joint a week or less. So the typical pattern of cannabis use in this country is very occasional use.

KNOX: That's Mark Kleiman of UCLA. He's an authority on marijuana who knows the evidence as well as anyone. He doesn't think the drug poses big health risks, not enough to keep it a crime to possess, use or sell it. But he does worry about users who become hooked on marijuana.

KLEIMAN: The main risk of cannabis is becoming habituated to cannabis and spending your whole life stoned.

KNOX: Heavy users are a small minority. But Jacquelene Cohen says they're not hard to find.

COHEN: I have friends that I've never seen not stoned. They are always stoned.

KNOX: She says her friends don't seem the worse for it.

COHEN: My friends that smoke a lot of marijuana are productive and creative and sociable and are able to form good relationships and make good life choices.

KNOX: That's a problem with marijuana: People have widely different, even contradictory perceptions about its harmfulness. But most people might agree it's not good to have a sizable chunk of the population dependent on a mind-altering drug - that is, have a habit they find hard to shake that interferes with their daily life. Something like one in nine cannabis users fits this definition.

KLEIMAN: Not a very high-risk, but it's a high risk if it's you or your child or your parent or your sibling. And so people who say: Oh, cannabis isn't abusable, cannabis isn't addictive, it seems to me, just aren't looking at the data.

KNOX: Kleiman - who's advised Washington State on legalization - worries that more young users will get hooked on cannabis if prices fall and the marijuana industry starts advertising heavily. So does Dr. Herbert Kleber, a drug-abuse researcher at Columbia University. He definitely does not support legalization.

HERBERT KLEBER: I think it's a bad idea. I don't think we really know what we're getting into.

KNOX: Kleber says it's been very hard to study the effects of cannabis.

KLEBER: We should have been doing a lot more research to find out just how useful it is, how it affects the brain, et cetera, et cetera. So we need to have much more research also on how to get off it.

KNOX: That lack of research is the reason why the evidence isn't very clear when it comes to adverse health effects other than dependence. Take lung damage. Marijuana smoke contains many of the same toxic chemicals as cigarette smoke. But even heavy marijuana smokers don't seem to have more lung cancer or emphysema. Many worry that marijuana might increase the risk of schizophrenia.

Here's Mark Kleiman again.

KLEIMAN: I think the evidence is plenty strong that somebody who's had a schizophrenic episode should stay away from cannabis, and that somebody with a family history of schizophrenia should stay away from it.

KNOX: Still, as millions of people have used the drug in recent years, schizophrenia incidence hasn't increased. And then there's driving under the influence. Kleiman says one study allowed heavy marijuana users to smoke as much as they wanted before testing them on a driving simulator.

KLEIMAN: The maximum impairment you get to when you let people get as stoned as they want to be is about the same as the impairment at .08 of alcohol - so just the drunk-driving limit.

KNOX: That's not entirely reassuring. But it suggests marijuana isn't nearly as dangerous as alcohol. But here's an important point: Kleiman says marijuana users are driving-impaired for hours after they think their high has worn off. And that's something they need to know. And, Kleiman says, even a little alcohol combined with marijuana is dangerous while driving.

Richard Knox, NPR News.

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