The remarks come the morning after the federal leaders' French-language debate, in which Harper's clash with Liberal Leader Justin Trudeau over the issue of legalization was among the evening's more memorable exchanges.
The Liberals support legalization; Trudeau argued during the debate that if pot were legal and regulated, young people would be less able to easily procure the drug than they are currently.
The Conservatives are vehemently opposed to the idea, with Harper saying that regulating its sale in the same way as cigarettes or alcohol would do nothing to keep it out of the hands of kids.
When asked Saturday how the Conservatives square that position with the fact medicinal marijuana is currently used by thousands of Canadians to treat a variety of causes, Harper said there's overwhelming evidence about the drug's long-term effects.
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.
"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.
Social issues were a major theme during Friday's debate, among them the divisive nature of the current controversy surrounding a Conservative ban on wearing Islamic face coverings during citizenship ceremonies — a popular policy in Quebec.
Both Trudeau and Mulcair have accused Harper of using the niqab issue as a political wedge and making people uneasy. But Harper has insisted it is the Liberal and NDP leaders who are out of step on the issue — and he repeated that position Saturday.
"We've taken policies on this matter, a policy that is supported by the overwhelming majority of Canadians of all backgrounds," he said. "The other parties have created a difficulty for themselves by taking positions that are simply out of step with the values of Canadians."
While government and private polls suggest most Canadians do not think niqabs ought to be worn at citizenship ceremonies, that hasn't stopped critics from accusing Harper of politicizing the issue to score political points.
He's faced the same accusations over Friday's Conservative promise to set up a tip line for people to report so-called "barbaric cultural practices" like forced marriage.
Asked if he considered domestic violence at large to be one such practice, Harper didn't directly answer the question.
He said the tip line was designed to help bring more cases of forced marriage to light.
"We want to make sure these things are brought out of the shadows and the rights of women to their physical safety is protected in this country," he said.
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