The Food and Drug Administration said a "black box warning," the strongest type possible for prescription drugs, will encourage doctors to use the lowest doses possible to avoid the need for blood transfusions.
The medicines, known as erythropoiesis-stimulating agents (ESAs), are approved to treat anemia in patients with chronic kidney failure, and in cancer patients whose anemia is caused by chemotherapy. The drugs are designed to boost levels of oxygen-carrying hemoglobin in red blood cells.
"Recent reports of studies with erythropoiesis-stimulating agents (ESAs) have shown a higher chance of serious and life-threatening side effects and greater number of deaths in patients treated with these agents," an FDA public health advisory said.
"Because all ESAs work the same way, the findings from these studies apply to all ESAs; the FDA is re-evaluating the safe use of this drug class," the advisory said.
The FDA will gather further input from a May meeting of a panel of outside experts. More changes to the drugs' labels could follow that meeting, FDA officials said.
Aranesp is Amgen's best-selling drug, with sales of about $4.1 billion in 2006.
Amgen, in a statement, said it was informing doctors about the new warnings. Roger Perlmutter, Amgen's executive vice president of research and development, said most oncologists and kidney specialists did not appear to be using high doses to boost hemoglobin to high levels.
Johnson & Johnson also said it was contacting doctors about the warnings. "We remain confident in the safety and efficacy of Procrit when used according to its label," said Craig Tendler, a vice president at J&J unit Ortho Biotech.
Industry analysts said they had expected warnings about recent studies to be added to drug labels. Still, "black box warnings never help sales," Sanford Bernstein analyst Geoffrey Porges said.
Leerink Swann analyst Bruce Cranna said Procrit had become less important for Johnson & Johnson over the years. The FDA announcement might alleviate some investor concerns, he said.
"It's the devil you know. Sometimes it's better when you know the regulatory response. It removes an unknown," he said.
Amgen shares hit their lowest level since July 2005 late Friday.