The Food and Drug Administration on Thursday postponed a long-awaited decision on whether flavored e-cigarettes from best-selling brand Juul Labs and other major manufacturers can remain on the market.

The FDA said in a statement it has denied marketing applications for more than 946,000 flavored e-cigarette products because companies did not provide sufficient evidence that potential benefits for adult smokers exceeded “the public health threat posed by the well-documented, alarming levels of youth use of such products.” The agency said 80 percent of young people who use e-cigarettes use flavored products.

But the agency said it still must complete its review of remaining applications. They include those from Juul, which accounts for more than 41 percent of sales nationally; Vuse, made by Reynolds American Inc.; and NJOY.

The agency did not predict when it will be finished but said it expects to take “additional actions in the coming weeks.”

The delay surprised people on both sides of the contentious debate over e-cigarettes; it was widely assumed the FDA would announce decisions involving some of the major players by Thursday. Under a federal court order, vaping companies were required to submit marketing applications to the agency by Sept. 9, 2020, and were given a year to remain on the market while the FDA reviewed their applications. The agency previously said it had created a separate line for the market leaders.

Anti-vaping groups urged the agency to move quickly to take action on the leading brands.

Matthew L. Myers, president of the Campaign for Tobacco-Free Kids, an anti-tobacco group, welcomed the agency’s actions so far but said the most significant decisions still remain.

“The key is going to be how rapidly they make those decisions and whether they remove all flavored e-cigarette products from the market to protect our nation’s youth,” Myers said. His group was one of several that sued the agency to accelerate its reviews of vaping products.

If the FDA doesn’t act promptly, he added, “We will have no choice but to seek judicial relief.”

Vaping advocates denounced the FDA for not authorizing a single e-cigarette product so far, calling it a blow to adult smokers using vapes to quit. They predicted smoking rates will rise if people lose access to e-cigarettes, especially to products with appealing flavors.

“The FDA’s opaque review process was intentionally designed to eliminate all but the largest players from the market,” Gregory Conley, president of the American Vaping Association, said in a statement. “We look forward to lending our support to future court challenges.”

He predicted that some small and medium-size companies whose applications for e-cigarettes or e-liquids were rejected would refashion their products to use synthetic nicotine to avoid FDA regulation.

The FDA said it received applications for more than 6.5 million products from 500-plus companies by the court-imposed deadline. A single company submitted applications for 4.5 million products — but the firm’s paperwork was rejected, the agency said, because it did not meet filing requirements.

The FDA said it issued 132 marketing denials for more than 946,000 products, including those in flavors such as Apple Crumble, Dr. Cola and Cinnamon Toast Cereal.

The FDA said it would finish reviewing products as quickly as possible. Companies that do not have a pending application will be prioritized for possible enforcement action if they do not halt sales of their products, the agency said. That means they could face fines or product seizures.

Juul, which has been widely criticized for igniting a teen vaping surge after launching its products in 2015, has lost significant ground to competitors even as it remains the market leader. Company officials say the firm did not target youths and that they want to help adults quit smoking. Juul is seeking permission to keep selling its tobacco- and menthol-flavored e-cigarettes.

Regarding Thursday’s announcement, Juul said, “We respect the central role of the FDA and the required thorough science- and evidence-based review of our applications, which is key to advancing harm reduction and earning a license to operate. We remain committed to transitioning adult smokers away from combustible cigarettes while combating underage use.”

Parents Against Vaping e-cigarettes, an anti-vaping group, said it was “disappointed and frustrated” by the FDA delay on Juul. “Every day that goes by without FDA action on Juul increases the likelihood that more young people will begin using these nicotine-heavy, wildly-addictive tobacco products.” the group said.

The FDA’s scrutiny of e-cigarettes is part of a sweeping effort to review vaping products, pipes, cigars and hookahs to ensure they are “appropriate for the protection of the public health.”

The agency said Thursday the review of the applications posed “many challenges due to the size, complexity, and diversity of the submissions.”

Smaller companies complained it is difficult and expensive for them to provide the agency with the level of scientific evidence the FDA seems to be requesting. Anti-vaping groups said products should not be on the market if their manufacturers can’t prove their potential benefits outweigh their risks.

The number of applications numbered in the millions because the requirements cover bottled e-liquids, which are mostly sold in adult vape shops and come in a vast array of flavors and nicotine strengths.

Filter, a trade publication, recently reported that VaporSalon, which got turned down by the agency, wrote on Facebook that it is planning to use synthetic nicotine in its products to avoid FDA authority. The argument is that products not made from the tobacco plant are not subject to the agency’s tobacco regulations.

The underlying question about e-cigarettes — whether they can help smokers quit cigarettes — is hotly debated. Juul offers several studies, some of which have been paid for by the company, that suggest vaping might help.

But the U.S. Preventive Services Task Force, which assesses the effectiveness of preventive care services, has found that “the evidence on the use of e-cigarettes for tobacco smoking cessation in adults, including pregnant persons, is insufficient, and the balance of benefits and harms cannot be determined.”

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