“Sometimes so many people are vaping in the bathrooms, it’s hard to do anything about it,” Ms. Dhiman said, adding that she doesn’t vape.

The F.D.A.’s efforts to limit teenagers’ access to flavored vapes had little effect on Lizzie Burgess’s ability to get them over the last four years in the Indianapolis suburbs. Within weeks of starting to vape at 16, she said, she was addicted. There was always a gas station, older friend or website selling e-cigarettes in flavors like banana ice cream or sour apple, she said.

At 19, she said, she was vaping THC and using a device — now advertising tobacco-free nicotine — that has as much nicotine as two packs of cigarettes, every two to three days. She said she fell ill with what started like a cold, which progressed to rapid breathing, almost-gray lips and feeling depleted. By the time she went to the emergency room, her oxygen saturation was 67, far below the normal range of 95 or higher. Ms. Burgess said she was soon in the I.C.U. with vaping-related lung injury.

She’s struggled to end her nicotine addiction and is down to two cigarettes a day.

“I think the F.D.A. should take it all off the market now,” Ms. Burgess said of the flavored vapes. “I think it will be very very hard for them to reel it all in. It’s so big and there are so many companies now.”

Synthetic nicotine remains far more expensive than the tobacco-derived product, leading some industry experts to question whether a device label of “synthetic” is accurate.

The unregulated vaping market at this point is a problem of the F.D.A.’s making, said Gregory Conley, president of the American Vaping Association, an industry trade group. He said the agency fueled the problem by overregulating a product used by millions of adults who find vaping a safer alternative to smoking.

“This country should learn some lessons from past prohibitions that failed miserably,” Mr. Conley said. “If you don’t fairly regulate a market where there is a great deal of demand from legal adults, you will fuel gray and black markets where the operators are not concerned with checking IDs before selling.”