Health Canada's latest anti-marijuana ad makes a "scary" claim about lowered IQ that has been widely challenged by researchers, but the campaign went ahead anyway after that message tested well with focus groups.

An ominous 30-second ad now on YouTube and TV warns that smoking too many joints can seriously harm a teen's developing brain, with the words "Decreased IQ" crossing the screen.

The spot was chosen after that message got the strongest reaction from focus groups of parents who were privately shown a similar ad and several alternatives in cities across Canada in June.

The parents, described by the interviewers as "generally uninformed regarding marijuana health risks," reacted with alarm when told marijuana can trigger psychosis, schizophrenia and a drop in IQ in young, still-developing brains.

The information on the harmful effects of cannabis on mental functioning was "surprising and scary" to them, says a newly released report by Harris Decima, commissioned by Health Canada at a cost of $95,000.

A previous TV ad making the claim about reduced mental ability "was seen as the most hard-hitting" of the five ads shown to parents, Harris Decima said in its June 27 report to Health Canada. None of the five spots was Health Canada productions, but were shown to the groups to assess the impact of each type of message so the department could craft a new one.

"These were all ads chosen to see what kinds of ads the focus group would respond to," said Sean Upton of Health Canada.

Link to lower IQ challenged

The assertion about lowered IQ is derived in part from a 2012 Duke University study of 38 heavy users of cannabis, but has since been challenged, notably by a British study of 2,612 young people which could find no such link.

A spokesperson for Health Canada, Eric Morrissette, cited other research published in 2012, known as the Dunedin Study, as the basis for the ad's assertion about reduced IQ.

But the House of Commons health committee reported in October that there are doubts about the Dunedin Study's conclusion. A key witness — Zach Walsh, a psychology professor at the University of British Columbia — told the committee in May that "substantial concerns have been raised in various publications" about the IQ claim.

Another committee witness warned MPs against using "scare tactics" in ads, which must rather focus on "accurate information," says the report of the committee, which recommended the government launch a public-awareness campaign.

Health Canada began its current $7.5 million ad campaign the day before the report appeared, using two 30-second spots, one on marijuana, the other on prescription drug abuse.

[Pot] was relegated by some as the "least of the evils" with which their child may experiment - Harris Decima focus group findings

The marijuana ad makes no specific reference to psychosis or schizophrenia, referring instead to "loss of memory," "learning problems," and "decreased IQ," stated as fact.

The ad was posted in October on YouTube where, to date, it has attracted almost 750,000 views and 1,800 comments, most of them negative and some focusing on the controversial claim about lowered IQ.

There are also about 7,800 thumbs-down ratings for the spot on YouTube, eight times as many as thumbs-up. An official has said the department welcomes such public debate, regardless of their views on Health Canada's position.

Harris Decima said parents it consulted generally felt marijuana smoking should be discouraged, but did not believe there were any dire health consequences.

'Comfortable with marijuana'

"It was relegated by some as the 'least of the evils' with which their child may experiment," says the focus-group report. "Parents in Kelowna [B.C.], Vancouver and Halifax appeared to be somewhat more 'comfortable' with marijuana."

The other focus-group communities were Calgary, Kitchener and Mississauga in Ontario and Quebec City. Spurred by court decisions, the federal government has created a regulated medical marijuana industry that some predict will soon be worth $1 billion. (Associated Press file photo)

"Messaging focusing on cognitive effects such as mental functioning as well as memory and IQ were the strongest messages for many parents," Harris Decima reported.

The marijuana issue has been fraught politically in the run-up to the 2015 federal election, with the Conservative party running ads attacking Liberal Leader Justin Trudeau's proposal to legalize recreational marijuana, saying consumption among vulnerable kids would only increase.

At the same time, the Conservative government has presided over the creation of a commercial medical-marijuana industry, under pressure from the courts, with sales expected to top $1 billion in a decade.

Last summer, Health Canada tried to secure the endorsement of several key organizations, including the Canadian Medical Association, to its forthcoming anti-drug campaign.

The three groups balked, saying the marijuana issue had been too politicized, and they pulled out in August.

The president of the Canadian Medical Association, Dr. Chris Simpson, said Health Canada had also asked the association to sign a confidentiality agreement "that would have limited our ability to comment freely on the campaign and the issue itself."

Morrissette said the Health Canada ad campaign, running Oct. 20 to Dec. 28, includes seven weeks of TV ads and 10 weeks of ads on the web and in social media.

Follow @DeanBeeby on Twitter