Shares in Amgen Inc. sunk nearly 5 percent Friday after news that its anemia drug Aranesp increased the risk of death when used to treat certain cancer patients.

Nearly 1 million Americans annually receive prescriptions for Aranesp or one of two similar products Amgen’s own Epogen and Johnson & Johnson’s Procrit. Combined sales of the three drugs amount to more than $10 billion for Amgen and Johnson & Johnson.

Concern over the over use of the drugs, which increase the number of oxygen-carrying red cells in the blood, has been raised due to a study published in The New England Journal of Medicine last November. It found that kidney disease patients treated aggressively with Procrit had a 34 percent higher risk of heart problems or death.

Amgen also reported fourth quarter net income late Thursday of $833 million (71 cents per share), a 1 percent increase from $824 million (66 cents) for the same period a year earlier, missing Wall Street expectations of 95 cents per share, according to Thompson Financial.

Revenue for Amgen was up 17 percent to $3.84 billion due to stronger-than-expected growth in its treatments for anemia and arthritis, the company said.

Shares in Thousand Oaks-based Amgen were down $3.51 to $71.34 in midday trading Friday.