What negative side effects to quitting smoking are there?

Weight gain: The one and only negative side effect when you quit smoking is weight gain due to increse in appetite.

They're temporary: There's very little down side to stopping tobacco, and huge benefits that last forever. "Side effects" are highly variable, meaning some people have none, and some have temporary discomfort and other symptoms of withdrawal. Such as being irritable, sleepy, etc. These go away. Some folks crave certain foods, and may gain weight. Drink water. More info at http://smokefree.gov/steps-to-prepare