The United States surgeon general issued a rare national advisory on Thursday urging more Americans to carry naloxone, a drug used to revive people overdosing on opioids.

The last time a surgeon general issued such an urgent warning to the country was in 2005, when Richard H. Carmona advised women not to drink alcohol when pregnant.

What is naloxone?

Naloxone is a medication designed to immediately reverse an opioid overdose. It blocks the brain’s opioid receptors and restores normal breathing in people who have overdosed on fentanyl, heroin or prescription painkillers. Its effects last for 30 to 90 minutes, which ideally buys enough time to get medical attention.

Who should carry the drug?

“Active drug users, people who live with or love drug users, and people on methadone or buprenorphine, who are often coming out of treatment and know people at high risk of overdose,” said Robert Childs, executive director of the North Carolina Harm Reduction Coalition in Wilmington, N.C. The surgeon general also listed patients who take high doses of prescribed opioids.

People who are coming out of prison or detox programs should carry the drug, because detox lowers tolerance, Mr. Childs said. Those who work in places where there are public bathrooms or where drug users congregate, such as shopping center parking lots, should consider getting trained to use naloxone and keep it on hand.