Pfizer is under investigation by the US federal government into its marketing of $13 billion cholesterol drug Lipitor, but the company insists it has done no wrong.

The probe has been initiated in the wake of a class action lawsuit, brought by health insurance plans in the USA , which claims Pfizer has been illegally promoting Lipitor for off-label uses. Law firm Grant & Eisenhofer have filed a suit against the company on behalf of various drug plans in a New Jersey federal court.

The allegations are that this off-label promotion meant the health plans were forced to pay for unwarranted prescriptions for Lipitor over the last five years, adding billions to their drug bills.

The complaint claims that “if a drug's manufacturer advertises uses not on its FDA-approved label, the drug is considered misbranded and its distribution in interstate commerce is prohibited." The plaintiffs are seeking unspecified damages and compensation from Pfizer.

Pfizer said it was cooperating with the investigation but maintained that the allegations were without merit. The company’s share fell 1.8% to $25.35 on the news.