As the dominant e-cigarette company in the United States, Juul may be reluctant to take the lead on a court fight while it is boxed in by another regulatory setback: On Monday, the F.D.A. accused Juul of illegally marketing its products as safer than traditional cigarettes. A ban on most flavors would be a direct hit on Juul’s business; flavored products account for about 85 percent of domestic sales, company officials have said.

Even as it faces increased scrutiny and regulatory hurdles in the United States, Juul is pushing into new markets abroad — this week in China and Portugal — where it often faces fewer restrictions regarding the age of the buyer or its ability to offer an array of flavors.

In the United States, top health officials in the Trump administration and some lawmakers hoped the flavor ban would significantly reduce the startling rise in teenage vaping. And the ban is being drafted as health officials rush to deduce what exactly is causing an outbreak of vaping-related respiratory illnesses, which now nears about 380 cases in about three dozen states and has possible links to six deaths. Many of the illnesses have been linked to vaping mixtures with T.H.C., the high-inducing chemical in marijuana, although health officials also say that some patients report using e-cigarettes as well.

An aide to Senator Richard J. Durbin, Democrat of Illinois, who met on Thursday with Dr. Ned Sharpless, the F.D.A.’s acting commissioner, along with eight other lawmakers, said the agency planned to release the details of the ban within a few weeks and have it take effect 30 days later.

But some small business owners who built up vape and smoke shops around the popular activity say regulators, in their haste to solve a problem, are pushing through new rules that effectively punish vendors for crimes they did not commit.