A study of teens finds almost 90 percent of those who abuse medications for attention deficit hyperactivity disorder (ADHD) say they used someone else’s medication.

The study included more than 11,000 American children and teens ages 10 to 18, who were interviewed between 2008 and 2011. The researchers found 7 percent said they had used a prescription stimulant drug in the past month, and more than half said their use of the drug was non-medical, HealthDay reports. Non-medical use included taking more pills than prescribed by their doctor, using someone else’s medication, or smoking, snorting or sniffing the medication instead of taking it orally.

Using someone else’s medication was the most frequently reported form of misuse, at 88 percent, the researchers wrote in Drug and Alcohol Dependence. The study found 39 percent took more medication than prescribed.

“It is so important for physicians and parents to counsel youth who have prescription stimulants to never share their medications,” said co-author Linda B. Cottler, PhD, MPH, of the University of Florida in a news release.

Teens who only used stimulant drugs non-medically were at increased risk for conduct problems at home, and had higher rates of using other substances, including tobacco, alcohol and illicit drugs. They were also more likely to have close friends who had tried other drugs, suggesting they are in a “circle of risk taking,” Cottler said. “It is so important for physicians and parents to counsel youth who have prescription stimulants to never share their medications,” she added.

According to the National Institute on Drug Abuse (NIDA), stimulants can increase blood pressure, heart rate, and body temperature and decrease sleep and appetite. “At high doses, they can lead to serious cardiovascular complications, including stroke,” NIDA states on its website. “Repeated abuse of stimulants can lead to feelings of hostility and paranoia.”