An opioid is a chemical that works by binding to opioid receptors, which are found in the central nervous system and the stomach and intestines. Common opioids include OxyContin, Vicodin, and fentanyl. The pain relieving effects of opioids are due to decreased perception of pain, decreased pain reaction, and increased pain tolerance. Physical dependence can develop with continued opioid use, leading to a withdrawal syndrome with abrupt discontinuation. Opioids can produce a feeling of euphoria, which can lead to recreational use of opioid drugs.