Hi, welcome to healthcare magic forum.
I will suggest you to adopt natural way of weight gain. Do regular half an hour exercise,
yoga ,
meditation with balanced diet including more protein,
vitamins, green leafy vegetables, milk, egg, fruits and juices.All these will take quite long time to so the result but this is the best way. There will be no side effects or any harm in this way of gaining weight.If you opt for medication not only they are harmful, also they have lot of unwanted side effects.Lastly increase your protein intake,like egg, milk,fish,chicken,cheese,nuts,soyabeans,black gram,.etc.
Please don't take any medicines to increase weight.
Wish you Good health.
Thanks.