The Supreme Court on Monday was divided over whether the Food and Drug Administration had unlawfully rejected millions of flavored e-cigarettes for approved sale in U.S. over concerns about nicotine addiction among young people.

During oral arguments in a case that could have a significant impact on public health, the justices grappled with tobacco industry claims that the government had given unclear and shifting requirements for new product applications and failed to provide proper notice to the companies.

“FDA switched its position on what studies were required” to show that the products have benefits to existing smokers that offset risks to youth, argued Eric Heyer, the attorney representing vape manufacturers Triton Distribution and Vapetasia, which are seeking a green light to market e-liquids such as “Jimmy the Juice Man Peachy Strawberry” and “Iced Pineapple Express.”

Federal law requires sellers of new nicotine products to provide regulators with scientific evidence to show that the products would promote public health, but the statute does not spell out specifically what evidence is necessary and sufficient. The FDA’s guidance on how to meet that requirement is at the center of the case.

“Their argument is that the guidance were actually a moving target, that either they weren’t clear or you changed the guidance as time went on,” said Justice Clarence Thomas, who appeared sympathetic to vape manufacturers.

“That is their argument,” replied Biden administration lawyer Curtis Gannon, representing the FDA, adding, “But I think that the key point is that they knew from the statute that they needed to be making this comparison about what the benefits were with respect to existing smokers and weighing that against the potential costs with respect to nonsmokers and attracting youth.”

Justice Neil Gorusch suggested that the companies might not have been given “fair notice” of how they could comply with the law. “Wouldn’t due process require an opportunity for notice and a hearing?” he asked Gannon.

E-cigarettes and vapes, which deliver nicotine without some of the harmful effects of smoking, have been booming in popularity. Kid-friendly flavors, such as fruit, candy, mint, menthol and desserts, are not approved by the FDA and are on the market illegally.

While vaping among youth is declining, more than 1.6 million children use the products, according to the Centers for Disease Control and Prevention. Nearly 90% of them consume illicit flavored brands.

Manufacturers have acknowledged that their products may appeal to youth but insist that a “growing body of scientific evidence” shows that “flavors are crucial to getting adult smokers to make the switch and stay away from combustible cigarettes.”

A federal appeals court sided with the companies last year, saying the agency had acted arbitrarily. If the Supreme Court upholds that ruling, it could clear the way for broader marketing and sale of flavored nicotine products.

The Court’s three liberal justices all seemed to share the government’s view that FDA did not illegally move the goal posts during the process and that the companies simply lacked the evidence to win approval.

Since 2009, when Congress passed legislation aimed at curbing tobacco use among young people, the government has almost universally denied tobacco company requests to sell flavored nicotine e-liquids, citing risks of addiction among minors.

The FDA said the two companies in this case provided insufficient evidence that the benefits of their flavored e-products in helping tobacco smokers quit exceed the dangers of hooking children.

“I’m so totally confused,” Justice Sonia Sotomayor told Heyer. “What [FDA] said is what you provided wasn’t sufficient.”

Justice Ketanji Brown Jackson said she was “baffled” by Heyer’s argument because the FDA had explicitly articulated its standard.

“I guess I’m not really seeing what the surprise is here, or what the change is here,” said Justice Elena Kagan. “There’s just not a lot of mystery here about what FDA was doing. You might disagree with that, because you think that, in fact, the world of 40-year-olds really wants to do blueberry vaping, but you can’t say that FDA hasn’t told you all about what it’s thinking in this respect.”

Justice Brett Kavanaugh, who could be a critical vote in the case, signaled sympathy to the industry’s complaint about discretionary government regulation but suggested he wasn’t convinced FDA had acted unreasonably.

“If the agency says [your claims of benefits to adult smokers] that doesn’t outweigh the harm to youth, we’ve reviewed everything, we’re aware of everything, of course they’re aware of everything that’s out there, that’s kind of the end of it, isn’t it?” Kavanaugh asked.

Even if they lose the case, several justices noted, the vape manufacturers could reapply for approval with the FDA in a new application.

While the first Trump administration had taken a hard line against the marketing and sale of sweet and candy-flavored vapes, president-elect Donald Trump said during the campaign that he wants to “save” flavored vapes.

“We don’t know exactly what that’s going to look like,” said Heyer. But, he added, that his clients “can’t afford to wait that out.”

Nearly a quarter of high school students who use e-cigarettes consume illicit menthol-flavored varieties, according to the 2023 National Youth Tobacco Survey.

Josie Shapiro, the 2024 national youth ambassador for the Campaign for Tobacco-Free Kids who testified before Congress on the dangers of nicotine addiction, said illicit flavored vapes hooked her at age 14.

“I think that by marketing any sort of flavored product as bubble gum or any of the genres of candy, it’s going to catch the eyes of children,” Shapiro said. “I’m still addicted, and I’m still trying to fight my addiction. Honestly, the FDA needs to regulate all flavored tobaccos to flavor ‘tobacco’ products and get them off the market.”

Public health experts have credited the FDA’s restrictions on flavored nicotine products with helping to drive down the number of teenagers who vape gradually from an “epidemic” level just five years ago.

The case, Food and Drug Administration v. Wages and White Lion Investments, LLC, will be decided before the end of the Supreme Court’s term in June 2025.

ABC News’ Patty See contributed to this report.