The problem with antidepressants and all drugs really, is that they affect each of differently, or can. So the answer truly is, both. Some people gain, some people lose and some are not affected at all.   The only way you will know is to begin taking the drug, keeping yourself to your normal diet and watching for weight gain, if any, closely. Nothing says you cannot stop taking a drug in consultation with your doctor, if you desire. Maybe you'll be one the lucky ones and lose weight! I took for awhile and nothing happened, well, I felt better, but the side effect I had made me have to stop.    Merely Me, the community leader here, and writer, wrote an article on her Prozac experience last year, you can read that here.