A new study finds some children and teens are using more than one type of tobacco product, including cigarettes, e-cigarettes, cigars, hookahs, smokeless tobacco and pipes.

The study of almost 25,000 young people ages 9 to 18 found nearly 15 percent used one or more tobacco products. Of these, about 3 percent only smoked cigarettes and 4 percent only used one non-cigarette product exclusively. In addition, 3 percent used cigarettes with another product and 4 percent used three or more products, HealthDay reports. Twice as many children and teens used e-cigarettes alone as those who used them in addition to regular cigarettes.

“We are concerned about this because of the potential for increased harms associated with the use of multiple products, such as exposing young people to nicotine during a time when their brains are still developing or at risk for nicotine addiction,” said lead researcher Youn Ok Lee of RTI International in Research Triangle Park, North Carolina.

Boys were more likely to use several nicotine delivery devices. Children and teens who used more than one device were also more likely to use flavored products, to be influenced by advertising and to be susceptible to peer pressure, the study found.

The researchers reported their findings in Pediatrics.

“Our results also suggest that policymakers should look more closely at the potential influence of flavors and company marketing on kids’ use of multiple products. Researchers have looked at these issues when it comes to cigarettes, but less is known about them when it comes to non-cigarette products,” Lee said.