Overall life expectancy in the United States just took a dip for the second time in as many years. The change, which was a relatively small drop from 78.7 years to 78.6 years for a baby born in 2016, is still significant because it marks the first time the national average has fallen in two consecutive years since the early 1960s. However, unlike some years where a complicated mix of factors contributes to an overall decline, this year’s movement seems to have a seemingly obvious cause.

The National Center for Health Statistics (NCHS) — the organization that handles health data and comes up with the averages — notes that the overall mortality rate for the US actually declined year-over-year in 2016. But if a greater percentage of the population died in 2015 than in 2016, why would the life expectancy average decrease? The answer seems to be drugs, and not the good kind.

Along with tracking the number of deaths, the group also keeps a tally on the causes, and this most recent batch of data shows a sharp jump in drug deaths. In fact, deaths via drug overdose climbed a whopping 21%. Unintentional overdoses have skewed the numbers such that the averages dipped despite less people actually dying.

The NCHS says that over 63,000 people died of drug overdoses in 2016. Since drug overdoses are categorized as “unintentional injuries” for statistical purposes, it caused that category to see a significant year-over-year rise, making it the third overall most common cause of death behind heart disease and cancer. Rates of death by opioid overdose rose for both men and women, but the mortality rate is still significantly higher for men.

It’s important to remember that the figures are also based on forward-looking projections. The average life expectancy is only valid for babies born in the corresponding years, and that average takes into account a variety of trends, and the ongoing opioid crisis is most definitely weighing things down.

“This was the first time life expectancy in the U.S. has declined two years in a row since declines in 1962 and 1963,” the group says of the latest data. “The new report shows the decline in life expectancy occurred despite an overall decline in U.S. mortality.”