A Food and Drug Administration advisory panel is recommending that Amgen Inc. and Johnson & Johnson be required to include new prescribing restrictions on the labels of their top-selling anemia drugs.

The outside advisory panel looked at studies indicating that Amgen’s Aranesp and Epogen, and J & J;’s Procrit (an Epogen equivalent) raised the risk of heart attacks, strokes and death when used at high doses. The drugs are approved for anemia related to chemotherapy and kidney failure, but recently Amgen studies have attempted to justify broader uses.

The panel did not specify what the new restrictions should be, leaving that to discussions likely to take place between FDA staff and the drug makers. The FDA is not required to follow the panel’s recommendations but usually does.

The two Amgen drugs accounted for 47 percent, or $6.6 billion, of the company’s revenue last year. New restrictions could reduce the company’s sales by as much as 17 percent, analysts have speculated.

Shares of Thousand Oaks-based Amgen fell $5.77, or 9 percent, in the last hour of Nasdaq trading Thursday to close at $57.33. That is the biggest drop in more than five years. Shares of the larger and more diversified J & J; fell only $1.61, or 2.5 percent, to $62.50 on the New York Stock Exchange.