Is it unlawful to use drugs to get rid of thinness or body weakness, and what are the regulations for that?

A Muslim's body is his mount in life, so he must preserve its health and strength to be able to fulfill his duties. There is no religious impediment to using medicinal drugs to increase the weight of a thin or weak body until it reaches the normal weight, and this falls under the category of seeking treatment. The permissibility of this is conditioned upon it not resulting in harm or excessive weight gain that hinders the performance of duties, as the Prophet, peace and blessings be upon him, said: "There should be no harm nor reciprocal harming."