I'm 20 years old, I'm been doing weight training since 8 months. My weight gradually increased from 57 to 63 kg, at this point it stopped increasing. I still look lean for my height (181 cm), I have doubled my diet and done overeating for one month. Still there is no gain in weight. My gym trainer and few gym friend's are  suggesting me to take mass gainer for one and half month. I'm confused now weather to take it or not, my health in long term in my priority, they say there is no side effects. So whats your suggestion and is there any other way to gain mass?