A journal advertisement for Allergan Inc.'s (AGN) acne cream, Aczone, overstates the medicine's effectiveness and omits key safety information, the U.S. Food and Drug Administration said.

The ad completely misrepresents the results of a study by claiming it shows the medicine "Works fast 24% reduction in inflammatory lesions at 2 weeks," according to the wording of a letter the FDA sent the company on Monday. The agency posted the letter on its Web site Friday.

"This claim is a complete misrepresentation of the results" of a study involving Aczone, the FDA said. Aczone is a topical gel used to treat mild to moderate acne.

The FDA said the ad, which ran in unidentified medical journals, clearly claims the product has a substantial effect on acne after two weeks of treatment, while studies show it has only a slight benefit when compared to a placebo.

The ad in question hasn't been running since May, said Caroline Van Hove. Hove said Allergan is working with the FDA to address all of the agency's concerns.

-By Jared A. Favole, Dow Jones Newswires; 202-862-9207; jared.favole@dowjones.com