Am a 25 year old man. I like to gain my weight. Is their essential pills there to increse weight with out any side effects?

Probably not pills. If one is truly underweight (based on height, family background, and body type), a dr. Can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while also doing weight-training to increase muscle. Proteins: egg white, chicken breast, salmon, ... Better fats: olive oil, peanuts, almonds, tofu, ....