An opioid is a drug that can suppress the perception of pain and calm a person's emotional response to pain. It does this by reducing the effect of pain signals sent by the nervous system. Opioids produce a feeling of well-being (euphoria). They also cause mood changes, cloudy thinking, and deep sleep. Side effects of opioids may include constipation and nausea. Opioids are also called opiates or narcotics.

Examples of prescription opioids include fentanyl, hydromorphone, and morphine. Heroin is an example of an illegal opioid.

Frequent use of opioids may make a person physically dependent on them. Stopping use or using less may lead to withdrawal. Withdrawal symptoms can include nausea, sweating, chills, diarrhea, stomach cramps, and muscle aches.

Some people who use opioids may develop opioid use disorder. They keep using opioids even though it causes harm to them or others. This can lead to an accidental overdose and death. Moderate to severe opioid use disorder is sometimes called addiction.