Responding to a spate of deaths in clinical trials, the Food and Drug Administration yesterday issued its most severe warning possible for drugs widely used to treat anemia in kidney disease patients and cancer patients undergoing chemotherapy.

The "black box" warnings placed on the prescribing label for Amgen Inc.'s Epogen and Aranesp and Johnson & Johnson's Procrit are expected to result in more cautious dosing by doctors.

Use of the drugs has escalated as physicians have sought to improve the quality of life of anemic patients by using them to stimulate creation of energy-boosting red blood cells. Marketing by manufacturers has reinforced the trend. But the FDA said yesterday that recent clinical trials have shown treatment beyond recommended limits increases the risk of death from heart attack and stroke in kidney patients, and of tumor growth and death in some cancer patients.

The agency advised doctors to give patients the minimum dose required to reduce the need for blood transfusions. It said antianemia drugs should not be used in an attempt to improve the quality of life of cancer patients because those claims are unproven. The FDA allowed claims of lifestyle benefits to remain for kidney patients, but said it is re-examining the validity of patient questionnaires about factors used to support the claims.

Recent concerns about the potential dangers of antianemia drugs and their overuse have been heightened by Medicare reimbursement policies for kidney dialysis treatment, which provide a profit incentive for clinics to administer more Epogen.

Medicare loosened its policy last year to let clinics get paid even if patients exceed the FDA's recommended red blood cell limits. The National Kidney Foundation -- in a set of guidelines paid for by Amgen -- suggested last year that higher targets for red blood cell counts are appropriate, citing statistical studies that showed lower mortality. The foundation is revisiting those guidelines.

The new warnings would effectively reduce the red blood cell target to about 10 grams per deciliter, compared with the upper limit of 12 grams that remains on the label, and the 13 grams permitted under last year's updated Medicare policy. The warnings are advisory, and the targets are still left up to the discretion of physicians, the FDA said.

The FDA said it has alerted Medicare to its latest findings. A spokesman at Medicare, which spends about $2 billion annually on Epogen for dialysis patients, did not respond to a phone message yesterday.

Black box warnings, said Dr. Eric P. Winer, chief of the breast cancer center at Dana-Farber Cancer Institute in Boston, will likely make doctors more cautious about prescribing antianemia drugs.

"These are drugs that have been somewhat overused. I don't think it's been without some effort on the marketing end," he said. "There has been a tendency, I think, for patients, and to some extent health providers, to attribute more fatigue to anemia than deserves to be attributed."

The warnings tarnished the image of a class of drugs that was among the biopharmaceutical industry's first big triumphs and has generated billions of dollars for Amgen and Johnson & Johnson.

Aranesp, which accounted for $4.1 billion in sales last year, is Amgen's second-generation version of the drug. Epogen, its original form, generated $2.5 billion in revenue last year, most of it in federal Medicare reimbursements for its use in dialysis treatment. Amgen makes Procrit, which is identical to Epogen, and licenses Johnson & Johnson to sell it. Procrit sales last year totaled $3.1 billion.

Amgen's stock dropped 2.1 percent to $60.86 yesterday. Johnson & Johnson stock slipped 0.7 percent to $62.14.

In November, an article in New England Journal of Medicine described a clinical trial of Procrit called CHOIR that was cut short because of higher rates of death from heart attack and stroke in kidney patients receiving larger doses. The dosing regimen in the trial pushed red blood cell counts higher than is recommended by the FDA. The results echoed a study of Epogen in kidney dialysis patients that was suspended in 1996, also due to an unexpectedly high death rate during testing.

In October, a trial of Aranesp in Danish patients with head and neck cancers was halted early because of apparent increases in tumor growth. Amgen said it told the FDA about the trial immediately, but it did not alert investors, leading to an informal review disclosed last week by the Securities and Exchange Commission . News of the suspended study was not known until it was reported in February by The Cancer Letter , a trade publication in Washington.

In February, Amgen reported on another study of Aranesp in cancer patients not undergoing chemotherapy treatment, which resulted in a higher percentage of deaths. The causes of death have not been disclosed. The FDA has scheduled a May meeting of its Oncologic Drugs Advisory Committee to discuss the new data.