What is the ruling on harming oneself; is it forbidden (haram) or disliked (makrooh)?

Harm is absolutely forbidden. Harming oneself is even more forbidden, especially if it leads to one's demise, based on the Prophet's (PBUH) saying: "There shall be no harm or reciprocating harm," and Allah's Almighty saying: "And do not throw [yourselves] with your [own] hands into destruction," and "And do not kill yourselves. Indeed, Allah is ever Merciful to you." The Prophet (PBUH) also warned that whoever kills himself by any means will abide eternally in the Fire of Hell.