Researchers proved that e-cigarettes contain the nicotine substance as much as regular cigarettes, according to a recent study published in Pediatrics.

Data and urine samples collected from more than 500 teenagers showed that 40% of them had tested positive for nicotine although they were vaping what are supposed to be nicotine-free products.

“This is one of the first studies showing the amount of nicotine kids are getting from e-cigarettes. They’re getting a lot, as much or more than they would with traditional cigarettes,” the study’s lead author and a clinical associate professor of pediatrics in the Renaissance School of Medicine at Stony Brook University, Dr. Rachel Boykan, said.

Scientists are worried that the use of e-cigarettes with nicotine will cause addiction among teens who may stick with vaping, or later turn to more dangerous traditional cigarettes.

“Nicotine is a highly addictive substance and it’s being delivered at much higher concentrations than before. So they’re getting a bigger buzz,” director of the Adolescent Substance Use and Addiction Program at Boston Children’s Hospital and an associate professor of pediatrics at the Harvard Medical School, Dr. Sharon Levy told NBC News.