- A class of drug used to reduce moderate to severe pain. Opioids can be made from the opium poppy plant or in the laboratory. They block pain signals by binding to opioid receptors on nerve cells in the brain, spinal cord, gastrointestinal tract, and other organs in the body. Some opioids may also be used to treat cough and diarrhea. Opioids used for pain relief are usually safe when taken for a short time and as prescribed by a health care provider, but they can be highly addictive and as a result have often been misused or abused.