Obviously, smoking is bad for you and stopping is a good thing. But probably the major downside to stopping smoking is people often gain weight.
Research has shown that while the majority of people who quit smoking gain weight there is considerable variability in the amount of weight gain. One of the biggest predictors of how much weight you gain depends on how much you smoked, to begin with, so those who smoke more cigarettes will probably put on more weight when they stop. Some studies show that the average weight gain is about 2.6kg or 5.7 lbs, with about 10% of men and 13% of women gaining 10 kg or greater after quitting, and often those who did put on the most weight were the most overweight, to begin with.
It is not 100% clear why this happens but there appears to be a variety of reasons. Nicotine in cigarettes is a known appetite suppressant so when you stop smoking your appetite for food comes back and in particular for sweet sugary foods. Furthermore, some people find smoking helps with stress so when you stop smoking and don’t fund other ways to help your stress you might eat more to manage your emotions.
To offset this there are a few things that have some evidence to help. These include personalised weight management programs, like this course with a health coach or a therapist can help you and increase your physical activity. Whilst I cannot find any evidence for it I also find seeing a psychologist for those who use smoking to help with stress is also important. And in particular, I recommend you start these changes before you quit, so the wheels are in motion by the time you actually stop.