Joshua Raffel, another spokesman for Juul, disagreed.

“Switching is not another word for cessation or safer,” he said. “They mean very different things. For example, switching involves continuing to consume nicotine but from a different device, while cessation is about getting users to eliminate their nicotine consumption altogether.”

For more than a year now, Juul has been the central target of public health experts and the F.D.A. over its products’ appeal to youths. With a sleek vaping device that has become ubiquitous as an accessory for many young people, Juul has had a meteoric rise, culturally and financially. Its product line was released in 2015, and the company’s sales took off. By 2018 it had a valuation of $16 billion. That is less than half of its current estimated valuation of $38 billion, making the company one of the most recent successful growth stories in Silicon Valley.

Juul has engendered further criticism by selling 35 percent of the company to Altria, one of the leading cigarette companies. The union fueled criticism that Juul ultimately cared about profits at the expense of public health, an assertion Juul denied. Altria and Philip Morris International are now in merger talks, which would give Juul extraordinary pipelines to overseas markets.

Whether the F.D.A.’s action will dent Juul’s domestic sales remains unclear, although the company has already said that sales were hurt by its decision to stop shipping flavored pods to stores. The F.D.A. warned that if Juul did not comply with laws governing vaping devices, it could impose civil penalties or an injunction, or seize the company’s products.

The growing popularity of e-cigarettes has been paralleled by an evolving and unfinished regulatory framework. The core question facing regulators and public health experts has been whether the potential benefits of the new technology, such as the possibility that it can reduce smoking-related deaths, will outweigh risks, like those posed to youths.

Several deadlines have been set for e-cigarette companies to submit evidence that their products would serve the public health and come under full regulatory authority, and Juul has yet to submit an application for approval to market its products as a healthier choice. In July, a federal judge ruled that the F.D.A. must move up its deadline, calling for the companies to comply within 10 months, or by May 2020.

Juul’s defenders and advocates for e-cigarettes as a smoking cessation device, argue that the regulatory policy in the United States has been unfair to both the company and to a larger public health message. David Sweanor, a tobacco-law expert at the University of Ottawa and advocate for e-cigarettes as an alternative to smoking, said that “communicating that their products are certainly massively less hazardous than cigarettes is accurate and nonmisleading.”