Overdose deaths among U.S. teens doubled from 2010 to 2020, according to a new study. Teen overdoses rose another 20% in the first six months of 2021, HealthDay reports.

Study author Joseph Friedman of the David Geffen School of Medicine at UCLA said this is the first time the teen overdose death rate has increased so sharply. The rise is due to substance use becoming more dangerous, not more common, he said.

“The increases are almost entirely due to illicit fentanyls, which are increasingly found in counterfeit pills,” Friedman said in a news release. “These counterfeit pills are spreading across the nation, and teens may not realize they are dangerous.”

The researchers analyzed overdose death data for teens ages 14 to 18 from the Centers for Disease Control and Prevention. They found 18 deaths, or a rate of 2.4 per 100,000, among teens in 2010. In 2020, there were 954 overdose deaths, or 4.57 per 100,000. In early 2021, there were 1,146 deaths, or 5.49 per 100,000.