Now, Juul must answer some tough safety questions to win F.D.A. approval. The company must prove its products are what is called a net public health good — that the benefits as an alternative to combustible cigarettes for adult smokers outweigh the risks of nicotine addiction, especially among minors. That requires hard scientific evidence.

Juul now has more than a 70 percent share of the e-cigarette market and employs about 2,200 people around the world, according to Ms. Andrews. She says 65 to 70 are researchers, including doctors, and others with Ph.D.s, M.B.A.s and other degrees.

The company recently started a science website promoting its new research grants. But so far, Juul has reported funding studies only by the Centre for Substance Use Research, based in Scotland, and collaborating with a few independent laboratories. The Glasgow organization, which has done most of Juul’s outside research, is well known to tobacco control advocates, who have long criticized its studies for playing down the danger of youth addiction to e-cigarettes.

On its website, the organization notes that for the past three years it has accepted money from Juul, Philip Morris International, British American Tobacco and e-cigarette makers like NJoy. Some critics contend that Juul’s funding has contributed to the unusual findings from a study it published in the May 1 issue of the American Journal of Health Behavior.

“They’ve never found anything that was contrary to the interest of the sponsoring tobacco company,” said Dr. Jackler, of Stanford, who studies the tobacco industry. “Many of their findings were at variance with a sizable volume of well-done scientific investigations at independent, reputable centers.”

The Glasgow center’s recent behavioral study of youth vaping prevalence and perceptions found that only about 4 percent of youths ages 15 to 17 reported vaping with Juul within the past 30 days; and less than 1 percent of younger children, ages 13 to 14, reported having done so. Only 45 percent of the older teenagers had heard of Juul before the survey.