WASHINGTON -- The Food and Drug Administration is warning that drugs used to treat high cholesterol and irregular heartbeats may cause severe muscle damage.

The FDA said doctors should use extra care when prescribing Zocor, generic Zocor or Vytorin to patients who are also taking a heart rhythm drug marketed as Cordarone or Pacerone.

The agency said the danger from the way the drugs interact is higher for patients taking more than 20 milligrams a day of the cholesterol drugs.

The generic name for the cholesterol medications is simvastatin.

Muscle injury is a risk with any of the cholesterol drugs known as statins, including Lipitor, particularly for the elderly. Although the risk of such injuries is low overall, they can be serious because they can lead to kidney failure and even death.

The FDA urged doctors to consider switching patients who are taking the heart rhythm drug to other statins for controlling cholesterol.