The US Food and Drug Administration has slapped warnings on three drugs made by US biotech heavyweight Amgen and healthcare giant Johnson & Johnson, which claim that in very rare cases the products could actually worsen the disease they are indicated for.

US regulators named Amgen’s anemia drugs Aransep (darbepoetin alfa) and Epogen (epoetin alfa) and Johnson & Johnson's Procrit (epoetin alfa) in its warning, advising doctors to discontinue treatment in patients showing signs of severe anemia.

But the companies noted that only a very small number of patients had developed antibody-related anemia, and that, if the medicines are used correctly, their benefit-risk profile is still favourable.

Aransep and Epogen are Amgen’s top-selling products, which pulled in combined sales of $5.1 billion for 2004. But investors seemed little concerned by the news, as the group’s stock dipped just 1.1% $80.78 to on December 2. Procrit raked in $3.59 billion for Johnson & Johnson last year, and the firm saw its shares slip $0.48 to $61.21.