Drugs that bind to opioid receptors in the nervous system or gastrointestinal tract and produce pharmacological effects including pain relief, sedation, constipation and respiratory depression. Opioids include opiates (drugs that contain chemical compounds directly from the opium poppy), semi-synthetic opioids (drugs synthetically derived from chemical compounds found in the opium poppy) and other fully synthetic drugs designed to have similar chemical properties and pain relieving effects as opiates. Examples of opioids include heroin, oxycodone (OxyContin®), hydrocodone, codeine, methadone, morphine and fentanyl.