A greater share of US teens are vaping nicotine e-cigarettes.
About 25 percent of high school seniors surveyed this year said they vaped nicotine in the previous month, up from about 21 percent the year before.
The University of Michigan study was published online on Wednesday by the New England Journal of Medicine.
Researchers surveyed more than 42,000 students across the country in grades 8, 10 and 12.
The study also found cigarette smoking declined in high school seniors, from about 8 percent to 6 percent.
The researchers have not reported how many students said they vaped marijuana.
A government survey released last week showed similar trends.