The U.S. Food and Drug Administration sent a warning letter to the makers of Zithromax, alleging they were over-promoting the popular antibiotic without adequate safety warnings.

The FDA’s letter said that claims made in a brochure that Pfizer produced to market Zmax (a formulation of Zithromax), are “unsubstantiated claims for Zmax.” Zmax is an extended-release formulation of the antibiotic, which means a single dose slowly dissolves in the body, taking the place of multiple pills over several days or weeks. The FDA’s warning letter holds that the makers of Zithromax are seriously downplaying Zithromax side effects.

While the warning letter concedes that Zithromax side effect risks are mentioned, it states that the brochure does not tell potential patients just how serious these reactions can be. The Zithromax side effects warning letter states that “promotional materials are misleading if they contain representations or suggestions that a drug is safer or more effective than another drug, when this has not been demonstrated by substantial evidence or substantial clinical experience.”

The FDA’s letter also states that Pfizer has oversold the benefits of Zithromax. The letter specifically states that Pfizer “broadened the indications” for Zithromax. This means that the brochure allegedly says that Zithromax can be used to treat a wider variety of infections that the drug is actually approved to treat. The letter also accuses Pfizer of making  “unsubstantiated superiority claims,” meaning the brochure allegedly claims Zithromax works better than other antibiotics without clinical trials or other tests to support this claim.

The FDA demanded several types of corrective action. First, the agency demanded that Pfizer stop distributing the brochure. Additionally, the FDA asked Pfizer to submit a written response to the letter, to ensure the drug company was aware of the violations and that it intended to take corrective action and not make similar violations in the future.

Several studies have linked serious Zithromax to side effects like liver damage, heart attacks, and Stevens Johnson Syndrome, or SJS. Pfizer is required to warn patients of potentially serious Zithromax side effects, even in advertisements.