The life expectancy of Americans has decreased for the past two years, and opioid use is one reason cited by the Centers for Disease Control and Prevention in Atlanta.

Now, a new study by the CDC shows the largest percentage change in drug overdose deaths for the past 20 years was in an unexpected demographic: middle-age women.

"Among women aged 30–64 years, the unadjusted drug overdose death rate increased 260%, from 6.7 deaths per 100,000 population (4,314 total drug overdose deaths) in 1999 to 24.3 (18,110) in 2017," the CDC reported.

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The rate of drug overdose deaths involving any opioid increased 492 percent, from 2.6 per 100,000 population in 1999 to 15.5 in 2017, the report continued.

"Specific groups of Americans are exquisitely vulnerable to the catastrophic consequences of the opioid epidemic," Dr. Harshal D. Kirane, director of addiction services at Staten Island University Hospital in New York, told CBS News. "Sadly, we are continuing to identify new groups to add to the list."

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Increases were seen in the number and rate of deaths involving antidepressants, benzodiazepines, cocaine, heroin and synthetic opioids, the CDC said. Deaths related to prescription opioids also increased, with the largest rise in women 55-64 years old.

By using National Vital Statistics System mortality data from 1999 to 2017, the CDC also found the age at which women are dying from drug overdoses increased in every category but one, synthetic opioids.

For example, in 1999, the average age of death caused by antidepressants was 44.8. In 2017, however, it had risen to 48.9. The largest increase was seen in cocaine deaths, rising from 40.4 years old in 1999 to 45.1 in 2017.

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The CDC report calls these numbers “unacceptably high” and concludes that targeted efforts are needed to reduce them. These efforts include considering the “the unique biopsychosocial needs of women”; implementing CDC guidelines for prescribing opioids; and examining if Medicaid programs are following the guidelines.