U.S. health regulators have sent letters to nine Internet distributors of dietary supplements warning them against making false claims about their products’ ability to fight the flu.

The U.S. Food and Drug Administration posted the letters on its website late Thursday in an effort to steer consumers away from a variety of herbal products that the distributors claim reduce the duration or severity of the flu.

In a letter to a company called Supplementality LLC, for example, FDA said the distributor was improperly offering products intended to diagnose, mitigate, prevent, treat or cure the flu virus, and demanded the company “immediately cease marketing” in this way.

“There are no over-the-counter products that shorten the duration or severity of the flu,” Gary Coody, FDA’s national health fraud coordinator, said in an interview.

The warning covers products including Resveratrol, Garlic, Echinacea, Elderberry, Ashwagandha and Astragalus Immune System Support.

The warning letters come amid an unusually severe cold and flu season, which has pushed up demand for cold remedies.

Coody said that in some cases the infractions cited in the warning letters related to misleading claims about the supplements. In other cases they related to the sale of fake versions of the prescription antiviral drug Tamiflu.

Other companies that received warning letters include Discount Online Pharmacy, Kosher Vitamin Express, Medsnoscript, Oasis Consumer Healthcare LLC, Secure Medical Inc, Sun Drug Store, Vitalmax Vitamins, and University of Berkley, whose marketing of the “Berkley-Body-Immune Flu Prevention” product violates the law.

Coody said six companies have made corrections following their receipt of the letters, which were issued earlier this month.

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By Toni Clarke