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Last year, Health Canada kicked off an anti-marijuana ad campaign — repeated shortly before the start of the election campaign — that said the drug was responsible for lower IQs, a statement derived from two separate studies whose conclusions have since been challenged.

The Conservatives also often link marijuana use to increased risks of mental health issues, such as psychosis and schizophrenia, but medical research on that is divided as well.

Harper likened what the government is trying to do with marijuana to its tobacco control strategy.

“We’ve spent a couple of generations trying to reduce the use of tobacco in Canada with a lot of success,” he said.

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“Tobacco is a product that does a lot of damage — marijuana is infinitely worse and is something we do not want to encourage.”

A government survey of tobacco use found that the overall smoking rate among Canadians over the age of 15 declined from 25 per cent in 1999 to 16 per cent in 2012.

By contrast, Statistics Canada has reported that the prevalence of marijuana use among Canadians over the age of 15 has remained relatively stable, with 12 per cent reporting they used the drug in the last year both in 2002 and 2012.

However, the Canadian Cancer Society notes that while 85 per cent of lung cancers can be directly linked to smoking, more evidence is needed to know whether there’s a similar cancer risk posed by smoking marijuana.

While some studies suggest there is an increased risk, the quality of the research is not as strong as the evidence on tobacco and cancer, the society says on its website.