WASHINGTON — The young models and the candy-colored graphics that helped propel Juul to the top of the e-cigarette market are gone. In their place are people like Carolyn, a 54-year-old former smoker featured in new TV commercials touting Juul as an alternative for middle-age smokers.

“I don’t think anyone including myself thought that I could make the switch,” Carolyn says, sitting in a suburban living room as piano music quietly plays in the background.

The tagline: “Make the switch.”

Under intense scrutiny amid a wave of underage vaping, Juul is pushing into television with a multimillion-dollar campaign rebranding itself as a stop-smoking aid for adults trying to kick cigarettes. But the strategy is raising concerns from anti-smoking experts and activists who say the company is making unproven claims for its product.

On Thursday, six anti-tobacco and health groups called on the Food and Drug Administration, which regulates e-cigarettes, to investigate Juul’s marketing efforts across TV, radio and other formats.

“Juul, a product that FDA has found to be largely responsible for the current epidemic of youth usage of highly addictive e-cigarettes, is being advertised and marketed on a massive scale as a smoking cessation product, without the required review and approval by FDA,” said the letter from the American Heart Association, the Truth Initiative, the American Academy of Pediatrics and three other groups.

In a statement, FDA spokesman Michael Felberbaum said only that the agency “continues to closely scrutinize potentially false, misleading or unsubstantiated claims” to make sure the public is “not misled into mistakenly using inherently dangerous tobacco products for medical uses.”

Neither Juul nor any other e-cigarette has been approved by the FDA to help smokers quit.

Indeed, Juul’s website carries the disclaimer: “Juul products are not intended to be used as cessation products, including for the cure or treatment of nicotine addiction” — a point underscored Thursday by a Juul representative.

Over the past half-century, the FDA has granted approval to just a few kick-the-habit products, including nicotine gums, patches, lozenges and prescription drugs.

Anti-tobacco experts are perplexed that the FDA hasn’t stopped Juul from pitching its nicotine-emitting device to millions of American smokers looking to quit cigarettes.

“I think Juul is skirting the edge of the law, and I think that the FDA is letting them get away with it,” said Stan Glantz, a tobacco control researcher at the University of California San Francisco.

FDA enforcement is especially important, Glantz and others argue, because e-cigarettes are not subject to the decades-old laws that ban advertising of traditional cigarettes on TV, radio and billboards.

Most experts agree e-cigarettes are less harmful than the paper-and-tobacco variety because they don’t produce all the cancer-causing byproducts found in smoke.

But researchers are only beginning to understand the unique risks of e-cigarettes, which emerging science suggests can damage the lungs and airways and contribute to precancerous growths. Those risks have led some experts to conclude that smokers who use both cigarettes and e-cigarettes are unlikely to get any health benefit.