The opioid epidemic may be a major reason for recent declines in Americans' life expectancy, a new study says.

A second study found rising death rates among Americans ages 25 to 64, but cited a number of factors as potential causes.

In the first study, researchers looked at 18 wealthy nations and found that most of them had declines in life expectancy in 2015. It's the first time in recent decades that many of these countries had simultaneous declines in life expectancy for both men and women, and the declines were larger than in the past.

In countries other than the United States, the declines in life expectancy were mostly among people 65 and older. The main causes were influenza and pneumonia, respiratory disease, heart disease, Alzheimer's disease, and other mental and nervous system disorders.

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