NEW YORK (CNN/Money) - Shares of drugmaker Merck & Co. fell more than 3 percent Thursday on concerns about the effect of a change in labeling for its blockbuster cholesterol drug.

Merck (MRK: down $1.75 to $53.20, Research, Estimates) said it will be changing the label for Zocor, incorporating a warning of potentially dangerous interaction with irregular heartbeat drug Cordadone. Those taking Cordadone should not take Zocor in dosages above 20 mg, the suggested starting dose.

Zocor, part of a family of cholesterol treatments know as statins, had about $7 billion in sales in 2001. In February the company received a six-month patent extension for the drug.

Earlier Thursday a note from Prudential Securities warned of the label changing, sending shares of the Dow component down.

But Tony Butler, pharmaceutical analyst with Lehman Bros., downplayed the action in a research note, saying the adverse events related to the label change were already known.

Butler also said it would be "virtually impossible" for competitors to take market share from Zocor, as other statins are known to induce the same adverse events and the probability of such events occurring is so low.