Hi im dr.hema physiotherapist. Yes you can go for weight gain supplements that claim to be free from any side effects if taken in the correct dose . But, they may be harmful if taken for a prolonged time.So, you can continue your protein supplements once consulting your physician.in mean time follow a good diet. Also your physic is determined by gene, diet, health illness.If it is genetic we can t change. Try changing your diet (before any capsules) and follow for 3 to 6 months. Consume more of natural diet like fresh fruits, green vegetables, whole milk, eggs, wheat,nuts, banana (less water content fruit with more carbohydrate), dry grapes, yogurt,Olive oil in salads, cereals for breakfast. Try to eat six times a day, with each meal (or at least three of them) containing protein, starch, vegetables and nuts. Your diet should be rich in protein, you can go for protein suppliments (as you mentioned) or protein shakes. A sample meal would include a turkey sandwich on whole-grain bread with mayonnaise and tomatoes and a fruit smoothie. Eat often and choose wisely, and you can expect to gain an average of half a pound to 1 pound per week. Take proper rest.avoid any type of stress. You can continue with gym concentrate on strengthening exercises. Still you feel difficulty check your thyroid, blood count,worms after consulting your dr. Hope the information helps.