The Canadian labels led to a 2.9 to 4.7 percentage point drop in smoking rates - which would mean 5.3 to 8.6 million fewer smokers in the U.S. if the same result were obtained, according to researchers at the University of Illinois at Chicago and the University of Waterloo.
The researchers used statistical methods to compare smoking rates in the U.S. and Canada for a nine-year period before and after the graphic warning labels were introduced in Canada. The price of cigarettes were factored into the analyses.
The study found the "regulatory impact analysis" used by the U.S. Food and Drug Administration to gauge the effectiveness of Canada's graphic warning labels to be inaccurate.
The FDA had estimated only a 0.088 percentage point reduction in smoking rates after graphic warning labels were mandated in Canada.
Jidong Huang, research specialist at UIC's Institute for Health Research and Policy and lead author of the new study, said when he and his collaborators corrected the FDA's methodological flaws and took into account the purchase prices paid by smokers, they found that graphic warning labels reduce cigarette-smoking prevalence at much higher rates.
Graphic warning labels on cigarette packages have been implemented in more than 40 countries, but not in the US.
In 2009, the Family Smoking Prevention and Tobacco Control Act gave the FDA authority to regulate the manufacture, distribution and marketing of tobacco products, including requiring prominent warning labels for cigarettes and smokeless tobacco products.
However, the tobacco industry challenged the FDA's requirement for graphic warning labels, and a US Court of Appeals ruled that the federal agency lacked evidence that graphic warning images would reduce the number of Americans who smoke.
Studies had shown that such labels increase knowledge of the harmful effects of tobacco products, motivate smokers to attempt quitting, and decrease relapse rates among smokers who have quit, but not whether they reduce overall smoking rates.
The UIC and University of Waterloo researchers hope the new study will provide support for a revised FDA proposal to require graphic warnings.
The findings are published online in the journal Tobacco Control.