U.S. health regulators recommended limiting the use of the highest dose of Merck & Co's cholesterol drug Zocor, citing an increased risk of muscle damage.
Zocor, also known as simvastatin, is widely available as a generic drug and is also sold in combination with ezetimibe as Merck's Vytorin and niacin as Abbott's Simcor.
The highest approved dose of 80 mg should be used only by patients who have been taking it for 12 months or more without any evidence of muscle injury, the U.S. Food and Drug Administration said on its website.
"We do not see much impact here given the fact that relatively few patients were being started at this high dose," Merck Chief Medical Officer Michael Rosenblatt said.
"Most doctors were already aware that there was a disproportionate number of muscle side effects at this dose. And most of it is generic any way for simvastatin," he added.
The FDA also requires changes to the drug label to add new contraindications and dose limitations for using the drug with certain medicines.
Patients taking Zocor 80 mg daily have an increased risk of muscle injury compared to patients taking lower doses of the drug or other drugs in the same class, the FDA said.
The risk appears to be higher during the first year of treatment, is often the result of interactions with certain medicines and is frequently associated with a genetic predisposition toward simvastatin-related myopathy, the regulator said.
In March 2010, FDA said it was reviewing the safety of simvastatin based on review of data from a large clinical trial and other sources.
While muscle injury, also called myopathy, is a known side effect with all statin medications, higher doses usually pose a greater risk.
Patients with myopathy experience muscle pain, tenderness or weakness, and an elevation of a muscle enzyme in the blood.
The FDA said an estimated 2.1 million patients in the United States were prescribed a product containing 80-mg simvastatin in year 2010.
According to IMS Health data, simvastatin, including all dose strengths, was the second-most prescribed drug in the United States last year.