The United States has not seen two years of declining life expectancy since 1962 and 1963, when influenza caused an inordinate number of deaths. In 1993, there was a one-year drop during the worst of the AIDS epidemic.

“I think we should take it very seriously,” said Bob Anderson, chief of the Mortality Statistics Branch at the National Center for Health Statistics, which is part of the CDC. “If you look at the other developed countries in the world, they’re not seeing this kind of thing. Life expectancy is going up.”

The development is a dismal sign for the United States, which boasts some of the world’s highest spending on medical care, and more evidence of the toll the nation’s opioid crisis is exacting on younger and middle-aged Americans, experts said.