Myopathy is a neuromuscular disorder in which dysfunctional muscle fiber leads to muscle weakness. Other symptoms include stiffness, muscle cramps, spasms, and the release of the muscle enzyme creatine kinase into the blood stream. The most severe type of Myopathy is known as Rhabdomyolysis, a condition characterized by the breakdown of muscle fibers resulting in the release of myoglobin into the bloodstream. Myoglobins are the contents of muscle fibers and can damage the kidneys, resulting in permanent damage or death.

In March 2010, the FDA sent out a warning regarding increased risk of Myopathy for patients taking Zocor (Simvastatin). They indicated there is greater risk for both Myopathy and Rhabdomyolysis in patients taking the maximum dose of the medication. While there is risk of developing Myopathy and Rhabdomyolysis with any statin, studies indicated the risk is highest for those taking the 80mg dose of Zocor.

The FDA reviewed data from a study known as the SEARCH trial. They evaluated several thousand patients taking 80mg Zocor and the same number of patients taking 20mg Zocor over a 6.7-year period. During that period, 52 people from the 80 mg group developed Myopathy, or 0.9 percent of the patients taking that dosage. From the 20 mg group there was one case of Myopathy, or 0.02 percent. Further, the FDA noted that 11 patients from the 80mg group developed Rhabdomyolysis, or 0.2 percent. None of the patients in the 20 mg group developed it.

The agency has also issued warnings about an increased risk of Rhabdomyolysis when doses of Simvastatin greater than 20mg are given with the medication Amiodarone. They further caution patients of Chinese heritage not to take 80mg Zocor with niacin-containing products. They advise them to use caution when taking 40mg doses of simvastatin with niacin-containing products. The incidence of Myopathy for patients of Chinese heritage taking 40mg Zocor plus a niacin-containing product was 0.43 percent. The rate among non-Chinese descended patients was 0.03 percent. They also recommend that the dose of simvastatin should be less than 40mg per day when patients are taking diltiazem due to an increased risk for Myopathy.

Despite these dose limitation and drug-drug interaction warnings, Drs are still prescribing higher doses of simvastatin with medications that increase the risk for Rhabdomyolyis. There is a long list of medications that should not be taken concurrently with Zocor, so it extremely important that your doctor knows all medications you take. It is also important to use only one pharmacy so that your pharmacist knows all the medications you take and can spot potential drug interactions.