The Food and Drug Administration is beefing up the safety instructions for simvastatin, a cholesterol-lowering medication that was the second-most prescribed medicine in the U.S. last year.

The agency says the highest approved dose of simvastatin (80 milligrams) has been linked to a higher risk of muscle injury, a risk that is greatest during the first year of use. The symptoms of the damage, a condition called myopathy, include pain, tenderness and weakness.

FDA says doctors should no longer prescribe the 80 milligram dose to new patients. The high dose of simvastatin lowers bad cholesterol about 6 percent more than a 40 milligram dose, the agency said.

About 2.1 million people in the U.S. got a prescription for simvastatin last year. The drug, which is mainly dispensed in generic form, is also sold under the brand name Zocor and is an ingredient in the combination drugs Vytorin and Simcor.

Last year, 94 million simvastatin prescriptions were filled in the U.S., making it the second-most prescribed drug in the country, according to data from IMS Health.

The FDA says the risk of muscle injury may also be increased when simvastatin is taken in combination with other drugs, including the antibiotic erythromycin and protease inhibitors for HIV treatment. (A list of the drugs can be found here.)

The FDA said data from a seven-year-long study called Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine contributed to the decision.

Copyright 2011 National Public Radio.