WASHINGTON — Two months after announcing that he planned to ban the sale of most flavored e-cigarettes, President Trump on Friday once again raised concerns about such restrictions during a lively, televised White House meeting that brought together top executives from the health community and the tobacco and vaping industries.

“If you don’t give it to them, it’s going to come here illegally,” Mr. Trump said of flavored products, referring to how a “prohibition” would only increase the use of black-market products. “That’s the one problem I can’t seem to forget,” he said. “You just have to look at the history of it. Now, instead of having a flavor that’s at least safe, they’re going to be having a flavor that’s poison.”

But e-cigarettes have been on the market for more than a decade, at least, and have grown increasingly popular, with little scientific evidence or oversight to prove they are safe. Meanwhile, teenage vaping has spiraled out of control, with more than one-fourth of high school students who were surveyed reporting this year that they had used e-cigarettes within the previous 30 days, prompting concerns that a new generation is becoming hooked on nicotine.

The vaping round table on Friday afternoon brought together a diverse group of advocates and lobbyists on different sides of the issue, including Matthew L. Myers , president of the Campaign for Tobacco-Free Kids and one of the most vocal vaping critics; K.C. Crosthwaite , the new chief executive of Juul Labs; Greg Conley , president of the American Vaping Association, a trade group; and Senator Mitt Romney, Republican of Utah, who has been pushing for a nationwide flavor ban.