An opioid is any chemical that resembles morphine or other opiates in its pharmacological effects. Opioids work 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 the beneficial effects as well as the psychoactive and the side effects of 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 Papaver somniferum (opium poppy), while opioid refers to both opiates and synthetic substances, as well as to opioid peptides. Opioids are among the world’s oldest known drugs; the therapeutic use of the opium poppy 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. Morphine-like opioids are not only well known for their addictive properties, but also for their ability to produce a feeling of euphoria, motivating some to use opioids recreationally.