Drug advertisements aimed at consumers may not be having the effect on sales that opponents and proponents of the practice assume they do, a new study suggests.

The analysis, by researchers from Harvard Medical School and the University of Alberta, looked at Canadian sales data for three drugs that were heavily advertised in the United States, ads which Canadians watching U.S. television would have seen.

The researchers found no evidence of a spike in sales for two of the drugs after the TV ads started to run. There was a marked increase in sales for a third drug but the effect was short-lived.

"I think that we've shown that the effects are pretty unimpressive for the three drugs we've looked at," said Harvard Prof. Stephen Soumerai, the senior author. "Two out of three there isn't an ounce of effect."

The study was published Tuesday in the British Medical Journal's online edition. The authors say it is the first to actually test for evidence of an impact of drug ads by using what's called a control group.

In science, evidence generated by comparing a group that received an intervention — in this case, watched TV drug ads — to a group that did not is considered to be a stronger level of proof than evidence gathered by simply observing what happens after an intervention.

For this study, the researchers took advantage of Canada's linguistic divide, comparing sales for the three drugs in most of anglophone Canada to sales in Quebec.

The authors assumed Quebecers watch much less U.S. television than other Canadians and therefore would see fewer drug ads.

CanWest challenging ban

Direct-to-consumer drug advertising is illegal in most countries around the world, including Canada. In Canada, media giant CanWest Global Communications is fighting a court challenge of the ban, saying it violates the right to free expression enshrined in the Charter of Rights and Freedoms.

Currently, Health Canada allows drug makers to advertise the name of a drug without stipulating what it is for, or advertise about a medical condition, while urging sufferers to seek medical help. Health Canada does not require cable or satellite TV operators to block drug ads on U.S. channels available to Canadians.

Only two countries — New Zealand and the United States — allow direct-to-consumer drug advertising. In the latter, it's a multibillion-dollar enterprise; it is estimated the U.S. pharmaceutical industry spends about $5 billion a year advertising prescription medications to the public through TV, radio and print ads.