The nicotine in e-cigarettes seems to damage DNA in ways that may increase cancer risk, a new study in mice suggests.

The damage was seen both to DNA and its ability to repair itself, making cells more likely to mutate and develop into cancer, said lead researcher Moon-shong Tang, a professor of environmental medicine at New York University School of Medicine.

If confirmed in future studies, the finding could mean that e-cigarettes carry their own cancer risk through the nicotine they deliver, said Dr. Roy Herbst, chief of medical oncology at Yale Cancer Center.

"It's the first evidence we have that nicotine can be carcinogenic in and of itself," said Herbst, chair of the American Association for Cancer Research's Tobacco and Cancer Subcommittee. "It's certainly concerning, and certainly gives pause if one were to say e-cigarettes were safe and could be used by all people without consequences."

However, not all animal research produces similar results in humans.

For their experiment, Tang and his colleagues exposed laboratory mice to e-cigarette vapor, which contains both nicotine and liquid solvents. They also exposed mice to the nicotine and the solvents separately.

The vapors were produced using 4.2 volts of electricity, at or below the level at which most commercial e-cigarettes function, Tang noted.

Prior studies have shown that e-liquid heated using higher levels of electricity could produce harmful chemicals. This research team wanted to investigate the risk posed to people using a typical e-cigarette.

"We found the solvent alone does not cause DNA damage," Tang said. "Nicotine with e-cigarette solvent caused the same damage as nicotine alone."

The researchers also exposed cultured human lung and bladder cells to nicotine, and found the same effects—DNA damage and suppressed DNA repair.

The next step of the research is underway, in which mice are exposed to nicotine and e-cigarettevapor long-term to see if they actually develop cancer or heart disease, Tang added.

Tang could not say whether e-cigarettes are still safer than traditional cigarettes, which contain thousands of harmful chemicals produced by burning tobacco.

"We just cannot guess with the data we have," Tang said.

One scientist not involved with the research pointed to what he considered a shortcoming in the study.

It would have been nice if this study had taken the extra step of exposing mice to tobacco smoke as well, and then compared them head-to-head against the mice that inhaled e-cigarette vapor, said Anthony Alberg. He's chair of epidemiology at the University of South Carolina's Arnold School of Public Health.

"Here we've got e-cigarettes versus nothing at all, and that's where it gets tricky to talk about the broader public health impact," said Alberg. He also chairs the American Society of Clinical Oncology's Cancer Prevention Committee.

"It would seem clear based on the number of toxic substances and the lack of combustion that e-cigarettes should be lower risk than tobacco cigarettes. The magnitude of how much a reduction in risk that would be is unknown," he said.

Nicotine's potentially cancer-causing effects likely have been overlooked because prior to e-cigarettes the only folks who ever used nicotine on its own were former smokers trying to beat their habit, Alberg said.

"The way you would conduct such research would be to compare nicotine replacement therapy users versus cigarette smokers, and those people are the ones who have a heavy history of smoking already," Alberg said.

If Tang's findings are verified, they would add more impetus for public policies to require lowernicotine levels in tobacco cigarettes and to increase regulation of e-cigarettes, said Herbst, who was not involved with the current research.

The study was published Jan. 29 in the Proceedings of the National Academy of Sciences.

Earlier this month, a report from the National Academies of Sciences, Engineering and Medicine that reviewed the findings of 800 studies found e-cigarettes may lead young people to smoke conventional tobacco, but they also appear to help adults quit smoking.

Still, the report's authors acknowledged that little is known about the long-term health effects of e-cigarettes.