Teenagers who try e-cigarettes are more likely to try cigarettes and other tobacco products, according to a new study from researchers at University of Southern California.

It’s not proof that “vaping” leads to smoking — but it’s strong evidence that it might, experts said.

“While we cannot conclude that e-cigarette use directly leads to smoking, this research raises concerns that recent increases in youth e-cigarette use could ultimately perpetuate the epidemic of smoking-related illness,” said Adam Leventhal of the Keck School of Medicine at USC, who led the study.

His team surveyed more than 2,500 Los Angeles-area 9th graders who said they had never tried any tobacco products when they started high school, six months later and again at the start of 10th grade.

More than 200 of the kids said they had tried e-cigarettes, and six months later, nearly a third of them — 31% — said they had also tried a traditional tobacco product such as a cigarette or a cigar in the past six months. Only 8% of the kids who never tried vaping had.

A year later, 25% of the teenagers who admitted to vaping said they’d also smoked tobacco in the past 6 months, compared to 9% of non-vapers.