(sometimes called GOM, an acronym for "God's own medicine") is a narcotic drug which is obtained from the unripe seed pods of the opium poppy (Papaver somniferum L. or the synonym paeoniflorum). To harvest opium, the skin of the ripening pods is scored by a sharp blade. The slashes exude a white, milky latex, which dries to a sticky brown resin that is scraped off the pods as raw opium.

Opium resin contains two groups of alkaloids: phenanthrenes (including morphine and codeine) and benzylisoquinolines (including papaverine). Morphine is by far the most prevalent and important alkaloid in opium, consisting of 10%-16% of the total. It binds to and activates opioid receptors in the brain, spinal cord and gut. Regular use, even for a few days, invariably leads to physical tolerance and dependence, and a characteristic and highly unpleasant withdrawal syndrome occurs when the dosage is suddenly reduced. Various degrees of psychological addiction can also occur, though this is relatively rare when opioids are properly used to treat pain as opposed for their euphoric effects. Strong pain is so stimulating itself that dependence when treating strong pain is rare. These mechanisms result from changes in nervous system receptors in response to the drug. In response to the drug, the brain creates new receptors for opiates. These receptors are "pseudo" receptors and do not work. When the opiates are out of the body, the brain has more receptors than before the use of the drug, but only the same amount of endogenus opiates (endorphines) to fill these receptors. This is why opiate addiction is different from other addictions and also the reason for the fast building tolerance for the drug. (The brain needs more and more drugs to achieve the same effects.)

Opium smokers in an "opium den" in the East End of London, 1874.The image of the narcotic poppy capsule, an entheogen, was an attribute of deities, long before opium was extracted from its milky latex. At the Metropolitan Museum's Assyrian relief gallery, a winged deity in a bas-relief from the palace of Ashurnasirpal II at Nimrud, dedicated in 879 BCE, bears a bouquet of poppy capsules (prudishly described by the museum as pomegranates).

Opium has been a major item of trade for centuries, and has long been used as a painkiller and sedative. It was well known to the ancient Greeks, who named it opion ("poppy juice"), from which the present name - a Latinisation - is derived. Many patent medicines of the 19th century were based around laudanum (known as "tincture of opium", a solution of opium in alcohol). Opium can also be smoked, sometimes in combination with tobacco. Opium smoking was often associated with immigrant Chinese communities around the world, with "opium dens" becoming notorious fixtures of many Chinatowns.

In the 19th century, the smuggling of opium to China from India, particularly by the British, was the cause of the Opium Wars. It led to Britain seizing Hong Kong and to what the Chinese term the "century of shame". This illegal trade became one of the world's most valuable single commodity trades and was described by the eminent Harvard historian J.K Fairbank as "the most long continued and systematic international crime of modern times."

There were no legal restrictions on the importation or use of opium in the United States until the Harrison Narcotics Tax Act of 1914. Medicines often contained opium without any warning label. Today, there are numerous national and international laws governing the production and distribution of narcotic substances. Its pharmaceutical use is strictly controlled worldwide and non-pharmaceutical uses are generally prohibited.

Although opium is used in the form of paregoric to treat diarrhea, most opium imported into the United States is broken down into its alkaloid constituents. These alkaloids are divided into two distinct chemical classes, phenanthrenes and isoquinolines. The principal phenanthrenes are morphine, codeine, and thebaine, while the isoquinolines have no significant central nervous system effects and are not regulated under the Controlled Substances Act. Opium is also processed into heroin, and most current drug abuse occurs with processed derivatives rather than with raw opium.