Pfizer Inc. said its generics drug division Greenstone LLC is under investigation by the U.S. Department of Justice's antitrust division.

The company said it believes the probe is related to an ongoing antitrust investigation of the generic pharmaceutical industry.

A coalition of U.S. states have filed a lawsuit against several generic drugmakers alleging price fixing for antibiotic doxycycline hyclate delayed release and oral diabetes medication glyburide.

Greenstone, which makes generic versions of Pfizer's off-patent blockbuster drugs such as Lipitor and Zithromax, also sells doxycycline hyclate capsules.