Hello dear, Most of the weight gain supplements/ mass gainers claim to be free from any side effects if taken in the optimum dose. But, they may be harmful if taken for a prolonged time, such as: 1. They contain small amounts of steroids, and can lead to a number of bone-related problems. 2. High amount of protein can affect kidney & liver function. 3. High sugar levels can result in diabetes with long term use. 4. Hormonal imbalances, reduced sexual desire, depression, etc. So, before choosing any such health supplement, it is advisable to consult your family physician once. Moreover, you should not compromise on routine healthy food such as: 1. Consume more of natural diet like fresh fruits, green vegetables, milk, eggs, wheat, etc. 2. Avoid oily and junk food. 3. Stay away from smoking & alcohol. Despite these measures, if you experience difficulty, it will be better to rule other causes of underweight. Investigations like complete blood count, thyroid profile, estimation of blood sugar level will be helpful to rule out any chronic infection, hormonal and metabolic causes. Wishing you a healthy life. Take care.