Amgen Inc. said it will strengthen the warnings about potential risks on the labels of its top-selling anemia drugs.

The Thousand Oaks-based company joined Johnson & Johnson, which makes a competing anemia drug, in saying Thursday that after months of discussion with the Food and Drug Administration, they had decided to comply.

The new labeling applies to Amgen’s Epogen and Aranesp, which accounted for 47 percent of the biotech giant’s annual sales in 2006, as well as Johnson & Johnson’s Procrit. The drugs re used to treat the blood-disorder anemia in kidney-failure and chemotherapy patients.

A new boxed warning label, which is the most serious a drug can carry, will emphasize that cancer patients have increased risk of death and accelerated tumor growth when treated with elevated doses of the anemia drugs.

The additional warning labels also caution physicians that using higher doses of the drugs does not benefit patients. Doctors have historically prescribed elevated doses under the assumption it improves quality of life for the patient.

Amgen and Johnson & Johnson are the only companies in the U.S. that make the anemia drugs.

Anemia drugs are Medicare’s single biggest expense and have come under fire for being overused or improperly used. Lawmakers are also currently investigating whether Amgen’s incentive plan for doctors is paying them to prescribe the drug in larger doses.

Shares of Amgen Inc. rose slightly to $56.45 in morning trading Thursday in trading on the Nasdaq.