In recent years, researchers have suggested that smokers can boost their lifespans by cutting down instead of quitting.
But now, a new Scottish study that followed people for decades suggests that many smokers won't gain extra years by limiting -- but not eliminating -- their bad habit.
"You may be fooling yourself if you think that reducing the number of daily cigarettes will protect you from the health risk of smoking," said Dr. Steve Schroeder, a professor of medicine at the University of California, San Francisco, who studies how to convince smokers to quit.
However, the study isn't definitive, and another tobacco-use researcher said it has several potential weaknesses.
"It is not possible to know the detailed smoking history of every subject, and there's a myriad of subtle differences in consumption. There is little doubt that there is a difference in risk between one cigarette per day and 20, but it is not possible to measure all gradations in between," said Brad Rodu, a professor of medicine at the University of Louisville. "The other problem is reliability. Smokers who report fewer cigarettes might be underreporting, or they might be compensating by smoking more intensely."
Why does this kind of research matter? Because "there is an emerging trend for smokers to smoke fewer cigarettes, and some don't even smoke daily," Schroeder said.
Earlier this year, Great Britain's National Institute for Health and Clinical Excellence made news by urging some smokers to cut down -- with the help of nicotine gum and other alternatives -- instead of giving up. But there's controversy because people who smoke fewer cigarettes may smoke them more intensely.
The authors of the new study say medical officials shouldn't promote cutting down unless they understand its potential benefits. Previous research has been mixed, with some studies showing that smoking fewer cigarettes didn't affect lifespan.
In the new study, researchers tracked two groups of Scottish smokers. One included 1,524 working people who were tracked from the 1970s (when they were in their 40s, 50s and 60s) to 2010. The other group included 3,730 people from the general population who were tracked for about the same period of time.
The researchers found no difference overall in death rates between those who smoked and those who only cut down. In one of the studies, they found a lifespan benefit for those who cut down, but only among those who smoked 21 or more cigarettes per day. In the United States, that would be about a pack-a-day habit.
"These inconclusive results support the view that reducing cigarette consumption should not be promoted as a means of reducing mortality, although it may have a valuable role as a step toward smoking cessation," the researchers wrote.
The lead author of the study, public-health researcher Carole Hart at the University of Glasgow, in Scotland, declined to comment on the study. The two other study authors did not respond to a request for comment.
It remains clear that quitting itself is a good idea. The study found that smokers who quit were 25 percent to 34 percent less likely to die over the period of the research.
"Quit between ages 25 and 34 and save 10 years of life; between 35 and 44, save nine years; quit between 45 and 54, gain six years; quit between 55 and 64, gain four years," UCSF's Schroeder said.
If that's the case, why might cutting down not help much, if it all?
"Many of the immediate effects come from reduced cardiovascular disease, which is sensitive to very low smoking exposure," he said. "The benefit from cancer risk reduction is not apparent for many years."
Schroeder cautioned, however, that cutting down still has benefits. "The main, and possibly only, benefit from reducing cigarette intake -- as opposed to stopping -- is that it makes it easier to quit subsequently," he said. "It also reduces the exposure of others to secondhand smoke."
The study was published online in the American Journal of Epidemiology.