Why would you rather be injected with drugs instead of eating more to gain weight?   Vicki is right, you really need to talk to your doctor or a certified nutritionist about gaining weight, if that's your goal.   Just so you know, Periactin is an antihistamine and is used for seasonal and year-round allergy symptoms such as watery/itchy eyes, runny nose, and sneezing. It is also used to relieve itching and hives caused by some skin conditions.   Yes, side effects of Periactin include increased appetite and weight gain, but they also include drowsiness, dizziness, blurred vision, loss of coordination, upset stomach, constipation, and dry mouth, throat and nose.   I wouldn't try taking a medication -- any medication -- for anything other than its intended purpose without talking to a doctor first.   Keep in mind, it's much more difficult to lose weight than to gain it, so work with a doctor or nutrionist to make sure you stay healthy no matter what.