SILVER SPRING, Md. - Two months after federal regulators ordered that additional warnings be added to the labels of blockbuster anemia drugs, government advisers said yesterday that Amgen Inc. and Johnson & Johnson should be required to add more cautions and conduct more safety studies.

The Food and Drug Administration's outside panel of experts voted overwhelmingly in favor of expanding warnings about the risks of death, blood clots, and other side effects for Amgen's Aranesp and Johnson & Johnson's Procrit.

Analysts said prior to the meeting that the biggest threat to the revenue the companies received from these drugs would be if panelists wanted to add warning labels that would limit prescribing the drugs to certain patients.

Shares of Amgen rose 47 cents, to $57.80, in after-hours trading after ending the trading session down $5.77, or 9.1 percent, to $57.33. Johnson & Johnson shares rose 40 cents in after-hours trading to $62.90 and ended the trading session down $1.61, or 2.5 percent, to close at $62.50.

A drop in anemia drug sales would pose a much larger challenge to biotech maker Amgen than to diversified competitor Johnson & Johnson. Last year Aranesp was Amgen's best-selling product, with sales of $4.1 billion, or nearly 30 percent of full-year company revenue. When combined with Epogen, a version used only in kidney failure patients on dialysis, the medication accounts for more than half of Amgen's annual sales.

The panel of experts, at a meeting in a hotel here, also voted unanimously that the companies should be required to conduct new studies to definitively prove the safety of the two widely prescribed drugs. Anemia drugs accounted for $10 billion in revenue for these two companies last year alone.

The FDA is not required to follow the panel's recommendations, although it typically does.

Johnson & Johnson and Amgen have conducted more than a dozen studies of the drugs to date, although panelists yesterday said what was needed was a large, comprehensive study of whether people who take the drugs die sooner.

In March, the FDA ordered that warnings be added to the drugs' labels after recent studies showed using them outside of approved guidelines could increase the growth of tumors and the risk of death in some patients.

It was not made clear during yesterday's meeting whether the additional warnings favored by panelists would apply to all uses or just non-approved use.

Many physicians prescribe anemia drugs aggressively to raise patients' red-blood-cell levels to that of a healthy person, believing that a higher red-blood-cell level improves quality of life.

In presentations, FDA staff focused on four studies suggesting that overprescribing or prescribing for unapproved uses could in fact increase a patient's risk of death.