Analgesics or pain killers are medications used to eliminate pain. They can be taken orally, intravenously, subcutaneously, transdermally, or intramuscularly. There are 3 levels of analgesics whose analgesic effect rises as the value of the adverse effect rises. Morphine derivatives are the most powerful, fast-acting, and long-lasting. Other medications have pain-killing benefits and are turned away from their original use: antiepileptics, antidepressants, or neuroleptics. Analgesics can have side-effects such as nausea, constipation, or respiratory distress. Their efficiency depends on the nature of the pain.