Pot less of a cancer risk than tobacco, study suggests

October 17, 2005

Andre Picard, Globe and Mail

Marijuana smokers are less likely to contract cancer than cigarette smokers, new research suggests.

While cannabis and tobacco smoke are chemically similar, the key difference is that cigarettes contain nicotine, which appears to bolster the cancer-causing properties of tobacco, while cannabis contains tetrahydrocannabinol (THC, the active ingredient in pot), which may actually reduce the carcinogenic properties of some chemicals.

"Current knowledge does not suggest that cannabis smoke will have a carcinogenic potential comparable to that resulting from exposure to tobacco smoke," said Robert Melamede, chairman of the department of biology at the University of Colorado at Colorado Springs.

The new study, published in today's edition of the medical journal Harm Reduction, is a review and analysis of research that has already been published.

The research has important political implications in the ongoing debate about medical marijuana.

One of the principal reasons public-health officials and medical experts oppose the use of marijuana as a prescription drug is the belief that the risks outweigh the benefits, and the fear that endorsing medical marijuana undermines anti-smoking campaigns.

Marijuana contains about four times the level of tar found in cigarettes, and is believed to place smokers at risk of lung cancer and other cancers related to smoking.

But Dr. Melamede said there is no solid evidence that cannabis smoking increases the risk of lung cancer or other cancers related to tobacco smoking such as breast, colon and rectal cancer.

He said there is evidence from studies done on laboratory rats that the THC in cannabis smoke "exerts a protective effect" against potential carcinogens and evidence that nicotine found in cigarettes activates the growth of tumours.

"While both tobacco and cannabis smoke have similar properties chemically, their pharmacological activities differ greatly," Dr. Melamede said.

But Roberta Ferrence, director of the Ontario Tobacco Research Unit and professor of public health at the University of Toronto, expressed grave doubts about the research, likening it to splitting hairs.

"It may be that cannabis is slightly less carcinogenic but tobacco smoke is extremely carcinogenic so that doesn't tell us very much," she said.

Dr. Ferrence said that most carcinogens are a byproduct of combustion, so "anything you burn and inhale is going to be carcinogenic -- including tobacco and cannabis. There is no way, based on this research, that you can say that smoking cannabis is safe."

She also noted that many people who smoke marijuana mix it with tobacco, and that makes the chemical distinctions moot. "From a public-health perspective, smoking is smoking," Dr. Ferrence said.

Tobacco smoke contains more than 4,000 chemical compounds, dozens of which are known carcinogens.

An estimated 5.1 million Canadians, or 20 per cent of the population 15 and older, report smoking cigarettes regularly, according to the Tobacco Use Monitoring Survey. More men (22 per cent) smoke than women (17 per cent).

By contrast, an estimated three million people, or 12.2 per cent of those 15 or older, reported that they smoked marijuana at least once in the past year, according to Statistics Canada.

Nearly half (47 per cent) of those who had used cannabis in the previous year smoked less than once a month, 10 per cent reported weekly use, and another 10 per cent said they smoked pot daily.

Canada has had a medicinal marijuana program since 2001. Since then, Health Canada has issued about 750 licences for people to smoke marijuana for the treatment of chronic pain and other ailments.



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