narcotic drug that is obtained from the unripe seedpods of the opium poppy (Papaver somniferum), a plant of the family Papaveraceae. (See poppy.) Opium is obtained by slightly incising the seed capsules of the poppy after the plant's flower petals have fallen. The slit seedpods exude a milky latex that coagulates and changes colour, turning into a gumlike brown mass upon exposure to air. This raw opium may be ground into a powder, sold as lumps, cakes, or bricks, or treated further to obtain such derivatives as morphine, codeine, and heroin. Opium and the drugs obtained from it are called opiates.

The pharmacologically active principles of opium reside in its alkaloids (alkaloid), the most important of which, morphine, constitutes about 10 percent by weight of raw opium. Other active alkaloids such as papaverine and codeine are present in smaller proportions. Opium alkaloids are of two types, depending on chemical structure and action. Morphine, codeine, and thebaine, which represent one type, act upon the central nervous system and are analgesic, narcotic, and potentially addicting compounds. Papaverine, noscapine (formerly called narcotine), and most of the other opium alkaloids act only to relax involuntary (smooth) muscles.

Opiates exert their main effects on the brain and spinal cord. Their principal action is to relieve or suppress pain. The drugs also alleviate anxiety; induce relaxation, drowsiness, and sedation; and may impart a state of euphoria or other enhanced mood. Opiates also have important physiological effects; they slow respiration and heartbeat, suppress the cough reflex, and relax the smooth muscles of the gastrointestinal tract. Opiates are addictive drugs—i.e., they produce a physical dependence (and withdrawal symptoms) that can only be assuaged by continued use of the drug. With chronic use, however, the body develops a tolerance to opiates, so that progressively larger doses are needed to achieve the same effect. The higher opiates—heroin and morphine—are more addictive than opium or codeine. Opiates are classified as narcotics because they relieve pain, induce stupor and sleep, and produce addiction. The habitual use of opium produces physical and mental deterioration and shortens life. An acute overdose of opium causes respiratory depression which can be fatal.

Opium was for many centuries the principal painkiller known to medicine and was used in various forms and under various names. Laudanum, for example, was an alcoholic tincture (dilute solution) of opium that was used in European medical practice as an analgesic and sedative. Physicians relied on paregoric, a camphorated solution of opium, to treat diarrhea by relaxing the gastrointestinal tract. The narcotic effects of opium are mainly attributable to morphine, which was first isolated about 1804. In 1898 it was discovered that treating morphine with acetic anhydride yields heroin, which is four to eight times as potent as morphine in both its pain-killing properties and its addictive potential. The other alkaloids naturally present in opium are much weaker; codeine, for example, is only one-sixth as potent as morphine and is used mainly for cough relief. Since the late 1930s, various synthetic drugs have been developed that possess the analgesic properties of morphine and heroin. These drugs, which include meperidine (Demerol), methadone, levorphonal, and many others, are known as synthetic opioids. They have largely replaced morphine and heroin in the treatment of severe pain.

Opiates achieve their effect on the brain because their structure closely resembles that of certain molecules called endorphins (endorphin), which are naturally produced in the body. Endorphins suppress pain and enhance mood by occupying certain receptor sites on specific neurons (nerve cells) that are involved in the transmission of nervous impulses. Opiate alkaloids are able to occupy the same receptor sites, thereby mimicking the effects of endorphins in suppressing the transmission of pain impulses within the nervous system.

The opium poppy was native to what is now Turkey. Ancient Assyrian herb lists and medical texts refer to both the opium poppy plant and opium, and in the 1st century AD, the Greek physician Dioscorides described opium in his treatise De Materia Medica, which was the leading Western text on pharmacology for centuries. The growth of poppies for their opium content spread slowly eastward from Mesopotamia and Greece. Apparently opium was unknown in either India or China in ancient times, and knowledge of the opium poppy first reached China about the 7th century AD. At first, opium was taken in the form of pills or was added to beverages. The oral intake of raw opium as a medicine does not appear to have produced widespread addictions in ancient Asian societies.

Opium smoking (drug abuse) began only after the early Europeans in North America discovered the Indian practice of smoking tobacco in pipes. Some smokers began to mix opium with tobacco in their pipes, and smoking gradually became the preferred method of taking opium. Opium smoking was introduced into China from Java in the 17th century and spread rapidly. The Chinese authorities reacted by prohibiting the sale of opium, but these edicts were largely ignored. During the 18th century European traders found in China an expanding and profitable market for the drug, and the opium trade enabled them to acquire Chinese goods such as silk and tea without having to spend precious gold and silver. Opium addiction became widespread in China, and the Chinese government's attempts to prohibit the import of opium from British-ruled India brought it into direct conflict with the British government. As a result of their defeat in the Opium Wars, the Chinese were compelled to legalize the importation of opium in 1858. Opium addiction remained a problem in Chinese society until the Communists came to power in 1949 and eradicated the practice.

In the West, opium came into wide use as a painkiller in the 18th century, and opium, laudanum, and paregoric were active ingredients in many patent medicines. These drugs were freely available without legal or medical restrictions, but the many cases of addiction they caused did not arouse undue social concern. Morphine was first isolated from opium about 1804, and the hypodermic syringe was invented at mid-century. Their use in combination on hundreds of thousands of sick or wounded American soldiers in the Civil War produced unprecedented numbers of addicts. Heroin, which was first synthesized in 1898, proved even more addictive than morphine, and by the early decades of the 20th century the legal use of opiates of any kind had been curtailed. The traffic in such drugs then went underground, leading to a vast illicit trade in heroin by the late 20th century. In the meantime, the smoking of opium had greatly declined in the 20th century, partly because opium had been supplanted by more potent derivatives, and partly because of determined legal efforts in China and other developing nations to eradicate it.

Though opium smoking has declined, opium remains the starting product for heroin, which has millions of addicts worldwide. The cultivation of opium, which was formerly centred in India, Turkey, and China and was sometimes carried on legally, shifted to other countries and became wholly illicit in the second half of the 20th century. By the century's end, the nations of Myanmar (Burma), Afghanistan, Laos, Iran, and Pakistan had become the leading producers and exporters of opium.