An opioid is a chemical that works by binding to opioid receptors, which are found principally in the central nervous system and the gastrointestinal tract. The receptors in these two organ systems mediate both the beneficial effects and the side effects of opioids. Although the term opiate is often used as a synonym for opioid, it is more properly limited to the natural alkaloids found in the resin of the opium poppy and, more loosely, the semi-synthetic opioids derived from them.