The US government has unveiled new federal rules that include a ban on the sale of e-cigarettes to people aged under 18.
The Food and Drug Administration (FDA) will also require manufacturers to submit products to the agency for review.
Cigars, pipe tobacco and hookah tobacco are also subject to the new rules.
On Wednesday, California introduced new anti-smoking legislation that also regulates e-cigarettes.
Electronic cigarettes can change gene expression in a similar way to tobacco, according to one of the first studies to investigate the biological effects of the devices.
Presented at the American Association for Cancer Research annual meeting on 6 April in San Diego, California, the research looked at human bronchial cells that contained some mutations found in smokers at risk of lung cancer. The cells were immortalized, grown in culture medium that had been exposed to e-cigarette vapor and their gene expression profiled
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