Hello .... i understand your concern.... it is not important to just gain weight quickly . its more important to gain weight rightly at the right places. a regular cardio exercise would do great help. good exercise increases apetite. eat three large meals a day. however make sure your diet is a balanced one with good amount of proteins. if you are a non vegitarian you could consume lean meat like chicken, egg whites. Anabolic drugs initially give good results but are known for long term harmful effects. so try to gain weight the right way... exercise, eat healthy, have a sound sleep as the body grows during sleep.