The narcotic analgesics (or narcotics) belong to the class of opioid drugs. Opiates (e.g., opium, morphine, and codeine) are derived from the opium poppy, whereas the synthetic derivatives (e.g., heroin) and purely synthetic drugs (e.g., Demerol and methadone) are called opioids. Heroin crosses the blood-brain barrier faster than morphine, giving a larger and faster ''high,'' but it is converted to morphine once in the brain. These drugs are sedative analgesics, because they have pain-relieving properties as well as being central nervous system depressants. All of the drugs also have abuse potential because they result in euphoria. The sedative effects of opioids result from opiate receptors in the mesencephalic reticular formation, whereas the reinforcing effects are due to stimulation of the mesolimbic dopamine system. The psychoactive effects of opium have been known for centuries. After the opiate receptor was identified, the search began for the endogenous chemical that bound to this receptor. The endorphines (the name is derived from ''endogenous morphine'') were identified. En-dorphines are released with stress and exercise; they appear to function as natural painkillers. It is the release of endorphines that is responsible for ''runner's high,'' the feeling of euphoria and sedation that follows strenuous exercise.

Central nervous system depressants are similar to opiates but without the analgesic effect. They include barbiturates (e.g., pentobarbital, secobarbital, amo-barbital, and phenobarbital), sedative hypnotics [e.g., scopolamine and methaqualone (quaaludes)], and anxiolytics (e.g., diazepam). Barbiturates are used as sleeping pills and there is cross tolerance and cross dependence with alcohol, which has similar depressant effects on cognitive function. Alcohol is a central nervous system depressant, which has effects on membrane excitability.

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