Teenagers in Canada use cannabis more than any other developed country, according to a new study released by Unicef.

The report released last week shows that 28 per cent of 15-year-olds admitted to having used cannabis in the past year. The figure comes from a World Health Organization (WHO) study conducted in 2009, which surveyed teenagers across 29 developed nations, including more than 15,000 in Canada.

This is the second time in a row that the WHO study has ranked Canadian teenagers as the highest cannabis users, though the percentage of teens itself has dropped. In 2002, the same survey showed that 37.5 per cent of 15-year-olds in Canada had used cannabis in the past year.

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"They feel it's not a drug, that it's not harmful, it's not going to hurt me," said John Westland, a social worker in the adolescent substance abuse program at Sick Kids Hospital in Toronto. "Part of it is probably because it's smoked and not injected – they don't see it as a chemical since it's not a pill," he said. "They see it as natural, not man-made."

He added that, from what he's heard from the teenagers he speaks with in Toronto, ease of access may be another reason why Canada is ranked so high. "It doesn't matter which school you go to. You can go to a public school, a private school, a religious school – I've seen patients from all those," he said.

"They can access it from school, they can get it from their older siblings. It doesn't have to be a stranger on the corner."

Still, the 28-per-cent figure from the WHO sounds high, he said. He questioned the report's methodology, saying self-reported surveys such as this one can be misleading, especially when comparing different countries where teenagers face differing levels of repercussions for admitted drug use. He cited a 2011 survey by the Centre for Addiction and Mental Health that showed the number of teenagers from grade 7 through grade 12 in Ontario who had used cannabis is slightly lower, at 22 per cent.

Unicef Canada's president David Morley said he was "shocked" by Canada's ranking of cannabis use, especially given that the same survey showed that Canadian teens had a relatively low smoking rate (4 per cent). "Here we are amongst the best at smoking and exercise and healthy eating and stuff," he said. "But we're at the bottom for this?"

Michel Perron, CEO of the Canadian Centre on Substance Abuse, sees Canada's ranking as a potential symptom of "mixed messages" over cannabis in recent years. He said that discussions over medical marijuana and decriminalization "gives rise to confusion among young people – 'is this drug good or bad, is it legal, is it illegal?"

He added that the high number may also speak to a generational shift, with the parents of young people today possibly having been cannabis users in their own youth. "There's a different relationship than, say, if you rewind 20 years ago between parents and the youth," he said.

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Cannabis use is only allowed in Canada for medical purposes and with a doctor's approval. According to Health Canada, cannabis can be addictive, and long-term use can lead to breathing problems, cognitive impairment and an increased risk of psychosis or schizophrenia.