The term narcotic is believed to have been coined by Galen to refer to agents that benumb or deaden, causing loss of feeling or paralysis. The term is based on the Greek word (narcosis), the term used by Hippocrates for the process of benumbing or the benumbed state. Galen listed mandrake root, altercus (eclata) seeds, and poppy juice (i.e. opium) as the chief examples.

In U.S.legal context, narcotic refers to opium, opium derivatives, and their semi-synthetic or fully synthetic substitutes "as well as cocaine and coca leaves," which although classified as "narcotics" in the U.S. Controlled Substances Act (CSA), are chemically not narcotics. Contrary to popular belief, marijuana is not a narcotic. Neither are LSD and other psychedelic drugs.

Many law enforcement officials in the United States inaccurately use the word "narcotic" to refer to any illegal drug or any unlawfully possessed drug. An example is referring to cannabis as a narcotic. Because the term is often used broadly, inaccurately or pejoratively outside medical contexts, most medical professionals prefer the more precise term opioid, which refers to natural, semi-synthetic and synthetic substances that behave pharmacologically like morphine, the primary active constituent of natural opium poppy.

Drug effects depend heavily on the dose, route of administration, previous exposure to the drug, and the expectation of the user. Aside from their clinical use in the treatment of pain, cough suppression and acute diarrhea, narcotics produce a general sense of well-being, known as euphoria, and reduce tension, anxiety, and aggression. These effects are helpful in a therapeutic setting and contribute to their popularity as recreational drugs, as well as helping to produce dependency.

Narcotic use is associated with a variety of side effects, including drowsiness, itching, sleeplessness, inability to concentrate, apathy, lessened physical activity, constriction of the pupils, dilation of the subcutaneous blood vessels causing flushing of the face and neck, constipation, nausea, vomiting and, most significantly, respiratory depression. As the dose is increased, the subjective, analgesic, and toxic effects become more pronounced. Except in cases of acute intoxication, there is no loss of motor coordination or slurred speech, as occurs with many depressants such as alcohol or barbiturates.

With repeated use of narcotics, tolerance and dependence develop. The development of tolerance is characterized by a shortened duration and a decreased intensity of analgesia, euphoria and sedation, which creates the need to administer progressively larger doses to attain the desired effect. Tolerance does not develop uniformly for all actions of these drugs, giving rise to a number of toxic effects. Although the lethal dose is increased significantly in tolerant users, there is always a dose at which death can occur from respiratory depression. It is clear, however, that tolerance and dependence, both part of the conventional idea of addiction, are insufficient to explain in totality what addiction is. Addiction is a broader behavioural phenomenon that also encapsulates nonsubstance based activity (such as excessive and compulsive gambling, excessive and compulsive eating, and a range of other excessive and compulsive behaviours) that has many of the same characteristics that substance based dependency displays. Moreover, it isn't always the case that those with a physical dependency to opiates find it too difficult to get over their "addiction," because so-called medical addicts (those that become physically dependent on opiates given for pain relief after treatment) only have to "give-up" the physical symptoms - they don't also have the all important psychological and life-style attachment to the drug which goes to make up the all-encompassing "addiction."

Physical dependence refers to an alteration of normal body functions that necessitates the continued presence of a drug in order to prevent the withdrawal or abstinence syndrome. The intensity and character of the physical symptoms experienced during withdrawal are directly related to the particular drug in use, the total daily dose, the interval between doses, the duration of use and the health and personality of the user. In general, narcotics with shorter durations of action tend to produce shorter, more intense withdrawal symptoms, while drugs that produce longer narcotic effects have prolonged symptoms that tend to be less severe.