Anemia drugs made by Thousand Oaks-based Amgen Inc. may have a risk of triggering a rare condition called aplasia, which damages red blood cells, the U.S. Food and Drug Administration said Thursday.

The FDA sent an email alert to physicians and others for a group of drugs called erythropoietins, which includes Amgen’s flagship product, Epogen and its second-generation cousin Aranesp. The drugs were the company’s best-selling product group last year with combined sales of $5.1 billion.

The FDA posted its alert after the close of U.S. trading. Amgen shares fell $2.03, or 2.5 percent, to $79.61 in after-hours trading. As a result of the FDA alert, Amgen is putting a warning on its Aranesp and Epogen labels.

The alert also targeted a version of Epogen that New Brunswick, N.J.-based Johnson & Johnson markets under the name Procrit. Doctors were advised to discontinue the injected drugs for patients who develop signs of aplasia, which can itself involve severe anemia.

Potential problems with the drugs first arose a few years ago with an unusual number of cases of pure red cell aplasia developing among patients taking Johnson & Johnson’s Eprex, which the company licenses from Amgen and manufactures itself for the European market. At the time, Amgen reported few known U.S. cases that might involve its own products.

Amgen spokeswoman Mary Klem said Thursday that about half a dozen aplasia cases have been reported among the 1.2 million Epogen patients who have taken the drug since it went on the market in 1989, and only two cases have been reported among 1.3 million patients on Aranesp , which was introduced in 2001.