Apetamin may promote weight gain because it contains cyproheptadine hydrochloride, a powerful antihistamine whose side effects include increased appetite. Though it’s unclear how this substance increases appetite, several theories exist. First, cyproheptadine hydrochloride appears to increase levels of insulin-like growth factor (IGF-1) in underweight children. IGF-1 is a type of hormone linked to weight gain . In addition, it seems to act on the hypothalamus, a small section of your brain that regulates appetite, food intake, hormones, and many other biological functions .