The drugs may postpone weight gain as you quit smoking.
Medical researchers usually toss out 10 pounds as the magic number for weight gain when you quit smoking. If you use Zyban or nicotine replacement, you probably won't gain weight while you're quitting -- but you'll gain it later.
One study showed that after a year, women who quit smoking by using Zyban gained as much as 13 pounds less than women who quit while using a placebo. After two years, the group had gained 8 pounds less. The men who took Zyban showed substantially smaller differences than women, with an average weight gain of 3 pounds less [source: Doctor's Guide].
Further research has shown that in the long run, that weight gain will most likely occur. That's because nicotine affects metabolism, and when you lose the boost, your body moves toward its natural weight. But it's a whole lot less demoralizing to put off the worry about the pounds you might gain while you're focused on losing your friend the cigarette.