What is the ruling on injecting a mentally ill brother with an injection to stop his sexual desire, in order to prevent harm to him and his family?

The fundamental principle is to safeguard the sanctity of a person who is insane or mentally deficient, and it is not permissible to assault them. The optimal treatment for their sexual urges, if possible, is to marry them off, in order to avert the harm of desire, protect them from immorality, and ensure their care. This should be done through their guardian.

If it is impossible to marry off the insane person, and there is fear of them harming household members, or if they commit reprehensible acts in front of them, then they should be isolated in a place where their harm can be contained. If this is not possible, it is permissible to give them medicine that weakens their sexual urges if their harm is thereby mitigated. If this medicine does not work, it is permissible to give them medicine that completely eliminates their sexual urges if their harm and annoyance cannot be repelled except by that, due to the prevention of evils and corruptions therein.