A new study finds almost one-third of high school seniors who reported misusing prescription opioids but did not receive a prescription for the drugs ended up using heroin by age 35.

Researchers at the University of Michigan also found 21% of high school seniors who misused prescription opioids and later received an opioid prescription used heroin by age 35.

“It is a very timely study given the number of adolescents and young adults who were overprescribed opioids and who are now aging into adulthood,” lead researcher Sean Esteban McCabehe said in a university news release. “We need to follow these generations to assess their risk for developing later problems.”

The study followed 25 cohorts of high school seniors between 1976 and 2000, from age 18 to 35. They published their results in the Journal of Addiction Medicine.