The U.S. Supreme Court is set Monday to hear the U.S. Food and Drug Administration's defense of the agency's refusal to let two e-cigarette companies sell flavored vape products that regulators found to pose a significant health risk to youths.

The FDA under outgoing Democratic President Joe Biden's administration appealed after a lower court decided the agency failed to follow proper legal procedures under a federal law called the Administrative Procedure Act when it rejected the applications to sell these nicotine-containing products.

It is the latest case in which the justices will scrutinize actions by a U.S. regulatory agency, with Republican President-elect Donald Trump due to take office on Jan. 20 with a promise of broad deregulation.

The FDA rule at issue, which took effect in 2016 under Democratic President Barack Obama, deemed e-cigarettes to be tobacco products that, like traditional cigarettes, are subject to agency review under a 2009 federal law called the Tobacco Control Act. The law requires manufacturers of e-cigarette products to apply for authorization to sell nicotine vaping devices and e-liquids.

Two e-cigarette liquid makers, Triton Distribution and Vapetasia, filed FDA applications in 2020 for products with flavors such as sour grape, pink lemonade and crème brulee, and names including "Jimmy The Juice Man Peachy Strawberry" and "Suicide Bunny Mother’s Milk and Cookies" — offerings that critics have said were designed to appeal to minors. These were denied, as were other applications by various companies for more than one million other flavored vape products.

To secure regulatory approval, e-cigarette companies must show that their product would be "appropriate for the protection of the public health," meaning that any health benefits — like helping traditional cigarette smokers transition to generally less-harmful vaping - must outweigh the risks of bringing the new product to market.

The FDA found that nearly one in five high school students and almost one in 20 middle school students used e-cigarettes in 2020, making e-cigarettes "the most widely used tobacco product among youth by far." The agency also found that youth users consistently cited flavor as a top reason for why they vape.

The FDA over the years has approved only 34 flavored e-cigarette varieties, all tobacco or menthol flavored. The agency maintains that it has not categorically banned flavored e-cigarette products. But companies seeking the agency's approval face a particularly demanding health benefits-versus-risk legal test due to the FDA's finding that flavored e-cigarettes pose a "known and substantial risk" to youth.

Triton and Vapetasia in 2021 asked the New Orleans-based 5th U.S. Circuit Court of Appeals to review the FDA's denial of their applications.

In January, the full slate of 5th Circuit judges ruled 10-6 that the FDA had been arbitrary and capricious, in violation of the Administrative Procedure Act, by denying the applications without considering plans by the companies to prevent underage access and use.

The 5th Circuit ruling created a split with seven other federal appellate courts that have sided with the FDA in similar cases, and prompted the agency to appeal to the Supreme Court. The Supreme Court on Nov. 25 declined to hear an appeal by RJ Reynolds and other tobacco companies of a lower court's ruling upholding graphic health warnings required by the FDA on cigarette packs.

The Supreme Court, with a 6-3 conservative majority, has reined in the authority of federal agencies in a series of cases in recent years. In another high-profile case involving the FDA in June, the court decided on procedural grounds to reject a bid by anti-abortion groups and doctors to restrict access to the abortion pill mifepristone.

The justices overturned the 5th Circuit's decision to roll back FDA steps in 2016 and 2021 that eased how the abortion pill is prescribed and distributed.