Calls to poison control centers about children’s and teens’ exposure to medications used to treat attention deficit hyperactivity disorder (ADHD) grew more than 60 percent from 2000 to 2014, according to a new study.

The study included calls for both unintentional and intentional exposure to ADHD medications, CNN reports. Of the 156,000 calls received by poison control centers, 82 percent were considered unintentional exposures, and 18 percent were considered intentional. Three teenagers died due to intentional exposure to the medications.

About three-quarters of the calls were about children 12 and younger. Among children younger than 6, most calls involved “exploratory behavior” — children who found the medication around the house. Among children ages 6 to 12, exposures were most often associated with taking too much of a prescribed medication or taking a dose too soon after the previous dose. Among teens, about half of exposures were intentional misuse or suspected suicide attempts.