The Food and Drug Administration has warned the maker of Purell hand sanitizers to stop claiming its products can prevent people from catching the flu, Ebola virus, the MRSA superbug and norovirus.

In a notice dated Jan. 17, the F.D.A. told Purell’s maker, Gojo Industries, that its unsubstantiated claims that Purell could reduce the potential for infection or prevent illnesses violated the Federal Food, Drug and Cosmetic Act. The agency said it was reclassifying Purell as an unapproved drug, rather than an over-the-counter product.

The F.D.A. also said it was not aware of any adequate and well-controlled studies to back claims that killing or decreasing the number of bacteria or viruses on the skin by a certain degree produced a corresponding reduction in infection or disease caused by such bacteria or viruses.

The agency took particular issue with claims on the company’s websites and social media accounts that said, “Purell Products are proven to reduce absenteeism” and Purell “kills more than 99.99 percent of the most common germs that may cause illness in a healthcare setting, including MRSA,” according to the warning letter.

The F.D.A. also criticized Gojo’s “Frequently Asked Questions,” which it said suggested that because Purell is made with ethyl alcohol, it might be effective against viruses like Ebola, norovirus and influenza.

According to the agency’s warning letter, one company passage said: “Are Purell hand sanitizer products effective against the flu? The F.D.A. does not allow hand sanitizer brands to make viral claims, but from a scientific perspective, influenza is an enveloped virus. Enveloped viruses in general are easily killed or inactivated by alcohol. The World Health Organization and the Centers for Disease Control and Prevention are recommending the use of alcohol-based hand sanitizer as a preventive measure for flu prevention.”