Opium and its various constituents exert effects upon the body ranging from analgesia, or insensitivity to pain, to narcosis, or depressed physiological activity leading to stupor. Opium users describe experiencing a feeling of calm and well-being. Opium addicts in otherwise good physical and mental health whose drug needs are met are thought to experience no debilitating physiological effects from their addiction, although there is some evidence that immune function is compromised. However, their preoccupation with the drug and its acquisition can lead to malnutrition and general poor self-care and an increased risk of disease.

Opium was commonly used as an analgesic until the development of morphine. Morphine continues to be prescribed for relief of severe pain, but fears of its addictive potential have limited its use. Laudanum was used in the 1800s to promote sleep and alleviate pain; codeine suppresses coughing; paregoric stops diarrhea. Medicinal opiates were freely available in the United States and Europe in the 19th cent., and the number of addicted people surged as a result.

The medicinal properties of opium have been known from the earliest times, and it was used as a narcotic in Sumerian and European cultures at least as early as 4000 B.C. The drug was introduced into India by the Muslims and its use spread to China. Early in the 19th cent., against Chinese prohibitions, British merchants began smuggling opium into China in order to balance their purchases of tea for export to Britain, an act that set the stage for the Opium Wars. Chinese emigrants to the United States, who were employed to build the transcontinental railroad, brought the opium-smoking habit to the West Coast.

During the 19th cent. opium was grown in the United States as well as imported. Besides indiscriminate medical use, opiates were available in the United States in myriad tonics and patent medicines, and smoking in opium dens was unhindered, resulting in an epidemic of opiate addiction by the late 1800s. The generous use of morphine in treating wounded soldiers during the Civil War also produced many addicts.

Importation of opium by Chinese nationals was prohibited in 1887; in 1906 the Pure Food and Drug Act required accurate labeling of patent medicines. The Harrison Narcotics Act of 1914 taxed and regulated the sale of narcotics and prohibited giving maintenance doses to addicts who made no effort to recover, leading to the arrest of some physicians and the closing of maintenance-treatment clinics. Since then, numerous laws attempting to regulate importation, availability, use, and treatment have been passed, and the concern with opium addiction per se has largely been replaced by concern with heroin, cocaine, marijuana, and other illegal drugs.

Large quantities of opium are still grown, some for legitimate use, on opium poppy farms in Southwest Asia (primarily Afghanistan and Pakistan), Southeast Asia (the Golden Triangle, primarily in Myanmar), and Latin America (primarily Colombia); the vast majority of the world's opium is currently produced in Afghanistan. The opium gum may be crudely refined and smoked (e.g., brown sugar) or converted to morphine and heroin. Growers usually make more for opium than for other crops, and the cultivation and refining employ hundreds of thousands of people, but the real profits go to the drug traffickers. It is estimated that the street price for heroin is 153 to 183 times that of the opium bought from the farmer. Despite laws and agreements to control its use, a worldwide illicit opium traffic persists.

See also drug addiction and drug abuse.