Paula Ross-Durow at the University of Michigan said if a teen was prescribed a drug, parents might leave the drugs available.
Ross-Durow analyzed data on 230 Michigan eighth- and ninth-graders who said they had been prescribed medications.
"About three-quarters of those teens said that they had unsupervised access to their medications," Ross-Durow said in a statement.
Ross-Durow said parents need to dispense the medication and keep them locked up -- otherwise, teens could use them when not medically needed, sell them to other teens and visiting teens couldn't steal them.
The study was published in the Journal of Adolescent Health.