For most people, weight gain supplements are the last thing on their list. However, for many athletes, upping the muscle mass can provide a competitive advantage for sports in which speed, power, and size are necessary. The key to weight gain is to ensure that you acquire lean body tissue, not fat.
How much weight gain you can expect depends on a number of factors, including prior resistance-training experience, genetics, body mass, gender, diet, training program, and of course motivation. Young 18-25 male body builders frequently achieve increases of up to 20% during the first year of heavy resistance training. However, initial progress greatly exceeds subsequent results, and after a few years, increases typically drop to around 1-3% per year.