An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract. The receptors in these organ systems mediate both the beneficial effects and the side effects of opioids.

Opioids are among the world's oldest known drugs; the use of the opium poppy for its therapeutic benefits predates recorded history. The analgesic (painkiller) effects of opioids are due to decreased perception of pain, decreased reaction to pain as well as increased pain tolerance. The side effects of opioids include sedation, respiratory depression, constipation, and a strong sense of euphoria. Opioids can cause cough suppression, which can be both an indication for opioid administration or an unintended side effect. Opioid dependence can develop with ongoing administration, leading to a withdrawal syndrome with abrupt discontinuation. Opioids are well known for their ability to produce a feeling of euphoria, motivating some to recreationally use opioids.

Although the term opiate is often used as a synonym for opioid, the term opiate is properly limited to the natural alkaloids found in the resin of the opium poppy (Papaver somniferum). In some definitions, the semi-synthetic substances that are directly derived from the opium poppy are considered to be opiates as well, while in other classification systems these substances are simply referred to as semi-synthetic opioids.

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