Peritol is an anti-histamine prescribed usually for allergic reactions. However, it also stimulates appetite and for that reason it is sometimes prescribed to anorexics. It is believed it does this by reducing the amount of serotonin that reaches the appetite center of the hypothalamus.
The claim is that Peritol restores normal appetite in those who, because of anorexia or prolonged illness, have lost it. However, people without such problems have reported weight gain on the drug. If taking it, it is probably advisable to monitor how much you eat if you don't wish to gain weight.