Drug Classes

Drug Dependence, psychological and sometimes physical state characterized by a compulsion to take a drug in order to experience its psychological effects. Psychological dependence, or habituation, is present when the compulsion to take a drug is strong, even in the absence of physical withdrawal symptoms.
The drugs that are commonly abused, besides substances such as alcohol and tobacco, can be grouped into six classes: the opioids, sedative-hypnotics, stimulants, hallucinogens, cannabis, and inhalants.
The class of opioids includes drugs derived from opium (such as morphine and heroin) and its synthetic substitutes (such as methadone). Medically, morphine is a potent pain reliever; indeed, it is the standard by which other pain-relieving drugs are measured. It and other opium derivatives also suppress coughing, reduce movements of the intestine (providing relief from diarrhoea), and induce a state of psychological indifference. Heroin, a preparation synthesized from morphine, was introduced in 1898 as a cough suppressant and nonaddicting substitute for morphine. The addictive potential of heroin was soon recognized, however, and its use was prohibited in many countries, even in medical practice. Users report that heroin produces a “rush” or a “high” immediately after it is taken. It also produces a state of profound indifference and may increase energy.
Opioids produce different effects under different circumstances. The drug taker's past experience and expectations have some influence, as does the method of administering the drug (by injection, ingestion, or inhalation). Symptoms of withdrawal include kicking movements in the legs, anxiety, insomnia, nausea, sweating, cramps, vomiting, diarrhoea, and fever.
During the 1970s, when scientists isolated substances called enkephalins, naturally occurring opiates in the brain, they discovered what many believe to be the reason behind physical dependence on opioids—that is, the drugs are thought to mimic the action of enkephalins. If true, this hypothesis suggests that physical dependence on the opioids may develop...

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