Merck & Co’s legal team, already battling to defend thousands of lawsuits alleging injury caused by the company’s Vioxx painkiller, will now have to fend off a class action lawsuit alleging Merck was negligent in promoting its osteoporosis drug Fosamax.

The new lawsuit, filed in Florida, alleges that Merck has tried to conceal Fosamax’ side effects from patients and doctors, particularly a condition known as osteonecrosis of the jaw.

In a statement, Merck said that in all of its clinical trials of Fosamax, which have included more than 17,000 patients, it has not had any reports of osteonecrosis of the jaw. While there have been reports of patients taking Fosamax developing the condition, that does not mean the bisphosphonate drug is responsible, it maintains.

Osteonecrosis is a recognised, rare side effect that has also been linked to other drugs in the bisphosphonate class, including Novartis’ Aredia (pamidronate) and Zometa (zoledronic acid). Labelling of these drugs, and indeed for Fosamax, already notes that bisphosphonate-associated osteonecrosis has been in cancer patients treated with intravenous bisphosphonates, while some cases have occurred in patients with postmenopausal osteoporosis.

Fosamax is Merck's second best-selling drug, bringing $3.2 billion into the firm’s coffers last year, and is the top-selling drug for the prevention of osteoporosis.

On Tuesday, a jury ordered Merck to pay $9 million in punitive damages to a to a 77-year-old man who had a heart attack after taking Vioxx, concluding the drug maker misled the US Food and Drug Administration about the painkiller's heart risks. It earlier warded $4.5 million in compensatory damages to the plaintiff.