Here is a little piece of an article. You might want to read the whole thing because it gives other reasons why prozac might act this way.
Taking Prozac to Stay Thin?
If you gain weight when you're depressed, and you get a prescription for antidepressant drugs, remind the doctor who prescribed the medication that you want one that doesn't cause more weight gain. While every patient is different, Serzone (nefazodone), Wellbutrin (bupropion), and Effexor (venlafaxine) usually don't cause much weight gain. Prozac-type medications may cause short-term weight loss but long-term weight gain in some people.
http://my.webmd.com/content/article/...tselectedguid={5FE84E90-BC77-4056-A91C-9531713CA348}