Almost one in five high school students uses e-cigarettes, cigarettes and marijuana together, according to a new study. These teens have a greater chance of engaging in risky behaviors than if they only used one substance, the researchers said.

They analyzed data from the 2019 Youth Risk Behavior Survey. They found teens who used all three substances together had the greatest chance of engaging in risky behaviors, including fighting, risky sexual behavior and other unsafe behavior such as not wearing a seat belt, HealthDay reports.

The study found 44% of teens didn’t use any of the three substances; 17% used all three; 16% used cigarettes and marijuana; and 13% used e-cigarettes alone.

Lead researcher Thomas Wills of the University of Hawaii Cancer Center said e-cigarettes are disproportionately used by teens who engage in less risky behavior, because e-cigarettes are perceived to be safer than traditional cigarettes. “But once e-cigarettes have been taken up, they seem to set in motion a process that involves other substances. So instead of a large group of single users, you now have a group of triple users,” he said.