How much weight will you gain after you stop smoking?

(Time.com) -- Your heart and lungs will thank you for kicking your cigarette habit, but your waistline may not.

A new study finds that ex-smokers may end up packing on a few more pounds than they expected.

On average, say French and British researchers, people gain about 10 pounds after kicking the habit. That's substantially higher than the roughly 6 pounds often quoted in smoking cessation literature, and double the 5 pounds that many female smokers report being willing to tolerate before attempting to quit, according to the authors.

Weight gain has long been associated with quitting -- not surprisingly, considering that nicotine is an appetite suppressant and a stimulant -- and many people who don't quit cite fear of ballooning weight as a reason.

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But the new findings back up what any doctor will tell you: you're still better off quitting.

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For the new study, published in the British Medical Journal, the research team closely examined data from 62 previous randomized controlled trials of smoking cessation programs involving people who were motivated to quit.

All of the studies assessed weight changes among participants, and the researchers separated out those who used quitting aids like nicotine replacement therapy or the drugs buproprion (Zyban) or varenicline (Chantix) from those who quit without treatment.

"We kept these groups separate because these pharmacotherapies could have a small effect on weight gain in the short term," the authors write.

The authors looked at weight gain in participants who had succeeded in quitting smoking for at least 12 months. On average, quitters who didn't rely on drugs or nicotine replacement to kick cigarettes, had gained 2.5 pounds one month after quitting, 5 pounds at two months, 6.3 pounds at three months, 9.3 pounds at six months, and 10.3 pounds at 12 months.

But these numbers aren't set in stone, the researchers say.

The study found great variability in the amount of weight people gained. Some people even lost weight. For instance, says study author Henri-Jean Aubin, professor of psychiatry and addiction medicine at Hôpital Paul Brousse, in France, a quarter of ex-smokers gained less than 2 pounds or lost weight after quitting, while an equal number gained more than 17 pounds.

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"Although our study has confirmed that there is substantial weight gain on average during the first year of continuous abstinence, a prediction of average weight gain will be wrong for most individual smokers," says Aubin, adding, "The good news is that after the first [three months], weight gain is decelerating substantially. Nearly 20% of the smokers actually lose weight after one year of continuous abstinence."

So, while it's true that some quitters will gain a significant amount of weight, a great many will actually lose extra fat — an added health bonus on top of putting out cigarettes for good.

In an editorial accompanying the new study, Esteve Fernández, an associate professor of epidemiology at the University of Barcelona, and Simon Chapman, a professor of public health at the University of Sydney, argue that many real-world quitters fare better than those included in the review.

The data in the study include only those smokers who volunteered for clinical trials and attended smoking-cessation clinics, a "self-selecting minority of smokers who may differ in important respects from those who quit without professional assistance," the authors write.

Those who decide they need help to stop smoking tend to lack self-efficacy. They might have similar problems with the dietary and physical activity behaviors important in weight control. So these results may not be generalizable to all smokers who quit because two-thirds to three-quarters of ex-smokers stop smoking without professional help or interventions.

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Fernández and Chapman urge potential quitters not to be put off by the new findings, noting further that previous studies have found that ex-smokers may gain weight in the short term after quitting, but not in the long term.

"Modest weight gain does not increase the risk of death," they write. "Smoking does."

Aubin says physicians should stress the long-term benefits of quitting to their patients and encourage them to start exercising, which not only helps reduce weight gain, but also may help them stay non-smokers.

"Quitting smoking at age 40 increases life expectancy by nine years, even taking into account the possible post-cessation weight gain. If their smoking patients do not take steps now to quit smoking despite the risk of weight gain, when will they do it?" he says.