Smokers who manage to quit gain on average nine to 11 pounds in the first year - significantly more than what was previously thought, according to a new study. 

A team of French and British researchers at the Universite Paris-Sud and the University of Birmingham in England, analyzed results from 62 previous studies and found that the average weight gain of people who’ve quit smoking was “substantially higher” than the six pound increase often cited in smoking cessation leaflets, and nearly double the amount the average female smoker is willing to tolerate.

Researchers said that most of the pounds are gained in the first three months of quitting, and they noted that they health benefits of quitting smoking far outweigh the weight gained and therefore should not discourage people from trying to quit.

"Smoking cessation is associated with a mean increase of 4-5kg in bodyweight after 12 months of abstinence, and most weight gain occurs within three months of quitting," the authors wrote in the study, published in the British Medical Journal.

Researchers found that while 16 percent of people had actually lost weight compared to when they smoked, 37 percent of people had gained up to 11 pounds, 34 percent of participants had gained between 11 pounds to 22 pounds and 13 percent were at least 22 pounds heavier than when they were smoking.

A recent study done at Yale University found that nicotine, the addictive substance in cigarettes, suppresses the appetite by linking onto certain receptors in the brain that increase the activity of neurons that influence food desire. 

Researchers noted that while most ex-smokers in the reviewed studies gained weight, the fact that some smokers lost weight and others gained more than 22 pounds meant that there are a wide variety of outcomes among individuals.

“Some people are either destined or able to prevent weight gain without intervention, whereas others seem likely to gain enough weight that puts them at risk of diabetes, among other complications,” the authors wrote, noting that the varied effect of giving up smoking and weight “is rarely described or discussed in the literature”.

The U.S. Centers for Disease Control and Prevention estimates that smoking kills 443,000 and sickens 8.6 million each year in the U.S., and is the leading cause of preventable death in the developed world.

Researchers said that because weight gain limits some of the health benefits associated with quitting smoking and linked to an increased risk of certain medical conditions like diabetes, health professionals should “usefully give patients a range of expected weight gain” and intervene early to keep ex-smokers from gaining too much weight.

"Although obesity is positively associated with an increased risk of all cause mortality, cohort studies indicate that modest weight gain does not increase the risk of death; smoking does," they concluded.