We used to fear Mary Jane, then we laughed about her, and now many of us think she's downright wholesome. Marijuana's public image has undergone a stunning transformation since the scare-mongering of Reefer Madness and the dope comedies of Cheech and Chong, but many doctors believe that weed's rehabilitation as a virtual wonder drug may be distracting us from its real health dangers.

It seems that plenty of people have bought the idea that marijuana is a harmless herb, or better. Stories proclaiming the benefits of "medical marijuana" – for ailments as varied as arthritis, MS, glaucoma and Alzheimer's – abound in mainstream media like International Business Times, and at patient support sites such as Livestrong.org. Voters in Washington and Colorado recently approved measures to begin legalizing pot, and a reinvigorated movement in B.C. is pushing for similar changes. A poll in the summer showed that two-thirds of Canadians are okay with decriminalizing weed for personal use.

Pot supporters promote its supposed benefits at big trade shows like the Treating Yourself Expo, which celebrated its third annual edition in May in Toronto. Doctors aren't nearly so well mobilized on the issue, but many say the health risks of smoking marijuana are more extensive and better understood than ever before.

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"There's a pretty potent lobby that makes claims about the medical benefits of cannabis, and anybody who disputes them is labelled part of the war on drugs," says Dr. Meldon Kahan, medical director of the Substance Use Service at Women's College Hospital in Toronto. "But there's no role, or hardly any role, for smoked cannabis in the treatment of chronic pain. There are safer alternatives, such as cannabis in pill form or inhalers. There are toxins in cannabis smoke that are carcinogens, and that accelerate heart disease. Smoked cannabis is addicting, unsafe during pregnancy and especially dangerous for young people, in terms of triggering psychosis, depression and mood disorders."

According to Health Canada, addiction is a complex phenomenon that includes psychological cravings, difficulties in controlling use, symptoms of withdrawal, and persistence in the addictive activity even when it is obviously damaging one's health, relationships and day-to-day functioning. All can apply to heavy cannabis smokers, says Kahan.

Teens are still developing neurologically, he says, which makes them more vulnerable than adults to the adverse effects of marijuana, especially of the powerful strains for which B.C. is famous. A recent report in the Proceedings of the National Academy of Sciences about a long-term study of more than 1,000 pot-using teens in New Zealand said that those who continued smoking into their 30s suffered significant cognitive deficits related to memory, reasoning and ability to process information.

According to a 2007 report by Health Canada, 8.2 per cent of young people use cannabis on a daily basis. Many teens smoke weed to cope with the anxieties of adolescence, and find it very tough to quit.

"People who take cannabis regularly get a mood-leveling effect," Kahan says. "When they stop suddenly, there's a tremendous rebound anxiety that can go on for days or weeks, and that makes them want to take it again."

Many young users smoke it with tobacco, a combo that researchers are finding to be much more addicting than marijuana alone. Dr. Bernard Le Foll, a leading researcher at Toronto's Centre for Addiction and Mental Health, says his research shows that when nicotine and tetrahydrocannabinol (or THC, the element in pot that delivers the "high") are received together, they magnify each other's effects on brain chemistry.

A 2009 study at the University of B.C. led by Dr. Wan Tan found "a significant synergistic effect between marijuana smoking and tobacco smoking" that increases risk for chronic obstructive pulmonary disease. COPD (which includes emphysema and chronic bronchitis) can aggravate the risk of pneumonia, heart problems, glaucoma and lung cancer.

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John Westland, a social worker at the Hospital for Sick Children's Adolescent Substance Abuse Outreach Program, says many of his teenaged patients combine weed and tobacco in the quick-hit form known as "poppers" (not to be confused with the amyl nitrites inhaled at clubs and raves). They use a modified water pipe that delivers a head rush they don't get from smoking straight marijuana.

"From my experience with my patients, I would say the addiction potential is higher for sure," Westland says. The relative cheapness of poppers is also a draw, as is the societal notion that pot is pretty much okay, and that a few cigarettes won't hurt you. "As their perception of risk goes down, use goes up," Westland says. Withdrawal is an ugly process that deprives jittery patients of sleep and appetite, he says, and can drag on through cycles of relapse and repeated efforts.

So why is weed regarded as relatively benign? How can any kind of smoking seem okay in 2012?

Pot's current reputation has certainly benefited from growing skepticism about established medicine. Marijuana is seen as whole and natural, not a refined pharmaceutical produced by a big corporation. Weed as a painkiller or treatment for nausea may appeal to the same people who seek out herbal equivalents of pharmaceuticals such as Valium, whose effects can be mimicked by valerian root.

Pot also benefits from current demographics. In contrast to seniors of, say 20 years ago, aging boomers today have fond memories of sharing a joint in their college dorm, and may not be aware that today's marijuana is probably much more potent than what they smoked in '68.

The widespread feeling that prohibition has failed both users and society as a whole has also helped. Surely a little pot smoking can't be more harmful, say weed activists, than a harsh legal regime that has cost us so much in money and damaged lives.

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"Whatever people think the harms of cannabis are, those are best reduced by a legally regulated system," says Dana Larsen, leader of a Sensible B.C. initiative to decriminalize weed in B.C. In any case, he says, "The use of cannabis since the 1960s has pretty much steadily risen in Canada, as has the severity of the laws, so the idea that prohibition is doing anything to keep cannabis out of the hands of youth is totally wrong."

No doubt. What does work is information. A 2011 survey report from the U.S. National Institute on Drug Abuse found that while cannabis use is increasing among American high school students, tobacco smoking has dropped by half since 1997. In those 15 years, tremendous societal scorn has come down on smoking, the ill effects of which are pictured on every cigarette pack. Marijuana, meanwhile, has acquired a public image almost as benign as wheatgrass. Perhaps our long-running relationship with Mary Jane is due for another change.

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Pills vs. joints

Health Canada permits people to take marijuana to relieve "severe pain" or "debilitating symptoms" of several illnesses, "if a specialist confirms the diagnosis and that conventional treatments have failed or [are] judged inappropriate." But there's fierce debate about whether smoked cannabis is a good way to deliver the pain and nausea-suppressing effects of THC.

"There's no other medicine that people would even think of smoking," says Meldon Kahan, medical director of the Substance Use Service at Women's College Hospital in Toronto. Cannabinoid pills and inhalers are safer, he says, and deliver medication over a longer period. Smoked marijuana delivers a surge of THC that is uncontrolled, intoxicates the patient and wears off sooner.

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But Dana Larsen, a marijuana activist who runs the Medical Cannabis Dispensary in Vancouver, says that smoked weed does negligible harm to patients. Pills, he said, would be a worse option.

"The idea that people should turn away from a natural herb that's been used for thousands of years, and instead use synthetic pharmaceutical products, is a failed argument," Larsen says. "Those products are going to cost a lot more, they're not going to have the same therapeutic benefits, and I would say they often present more harms."

In any case, Larsen says, many of his customers opt for non-psychoactive extracts of cannabis, including tinctures, capsules and lotions.