One of a group of drugs derived from opium, an extract of the poppy plant Papaver somniferum that depresses brain function (a narcotic action). Opiates include morphine and its synthetic derivatives, such as heroin and codeine. They are used in medicine chiefly to relieve pain, but the use of morphine and heroin is strictly controlled since they can cause drug dependence and tolerance. Opiates exert their effect by binding to cell receptors that bind endorphins, the body’s endogenous opioids, or natural painkillers.