All drugs in the opiate class act on opiate receptors in the brain. They mediate relief from pain and produce feelings of euphoria. Opiates, which are referred to as narcotics by scientists and medical practitioners, are highly addictive and can either be natural, semisynthetic, or synthetic. Natural opiates such as opium are derived from the opium poppy. The active ingredients of opium are morphine and codeine. The most common semisynthetic opiate is heroin, which is five to ten times more potent than morphine. Examples of synthetic opiates include methadone and the prescription pain medication Demerol.

Excerpt and image from Psychoactive Drugs

Opiate drugs are derived from or chemically similar to those derived from, the opium poppy. Familiar opiates include morphine, heroin, and methadone. Because heroin enters the brain faster than morphine, it produces a bigger rush of effects and is more strongly addictive. Opiates relax people, decrease their attention to real-world problems, and decrease their sensitivity to pain. Although opiates are frequently addictive, people who take them as painkillers under medical supervision almost never abuse them. Addiction depends on the person, the reasons for taking the drug, the dose, and the social setting.

Excerpt from Kalat's Biological Psychology, (2012, p. 74).