The U.S. Food and Drug Administration on Wednesday ordered changes in the prescribing information for two widely used anemia drugs, saying the risks of the medications outweigh the benefits for certain cancer patients.

Anemia is a frequent side-effect of cancer treatment and can leave already frail patients in an even more debilitated condition.

The drugs at issue, Aranesp and Procrit, are commonly used to treat such patients and are made by California-based Amgen. But recent studies have indicated that the treatments could worsen some cancers and might even shorten patients' lives.

Both medications are available in Canada.

Dr. Richard Pazdur, head of the FDA's oncology section, said Wednesday's directive was the first time the agency has invoked new powers Congress granted it last year to order drug makers to make changes in their prescribing literature, also known as the drug label. Previously, such changes had to be negotiated.

The FDA ordered two specific changes. The agency said the drugs should not be used with patients who are expected to be cured of cancer. Amgen had pressed for language that would have allowed the medications to be used for some patients who are expected to be cured, but who are not able to receive transfusions of red blood cells.

The second change said patients should not be treated with the drugs unless their hemoglobin levels drop below 10 grams per one-10th of a litre of blood. The company had asserted the drugs should also be an option for treating some patients with a slightly higher hemoglobin level.

The FDA's action brings the agency into line with a new Medicare policy that restricts payment for the drugs.

Amgen said in a statement the changes ordered by the FDA were "consistent" with the company's expectations.