A cancer promoter is a substance that can cause a precancerous condition to develop into cancer.
Acne medicines containing benzoyl peroxide have been available without a prescription since at least 1975, and were found to be safe and effective before their introduction.
The drug was pronounced safe again in 1985 as part of a 10-year review of over-the-counter medications. But studies brought to the agency's attention since then show that benzoyl peroxide might be harmful.
That has prompted the agency to revoke the "safe and effective" finding and order more tests, which may take up to two years.
An FDA panel of scientists will hold a hearing next year on whether to order products containing benzoyl peroxide off the market until the tests are completed.
"The agency concludes that the evidence is substantial to establish benzoyl peroxide as a potent skin tumor promoter in more than one strain of mice and other laboratory animals tested," the FDA said.
Benzoyl peroxide acts as a disinfectant, killing bacteria in the pores of the skin that help cause acne. In large quantities, the chemical is toxic and can cause dermatitis and asthmatic effects, according to "Dangerous Properties of Industrial Materials," a reference book on toxic substances.
Other chemicals used in over-the-counter acne preparations have been approved by the FDA, including salicylic acid, sulfur and resorcinol and resorcinol monoacetate in combination with sulfur.