The Food and Drug Administration Tuesday ordered stronger warnings for best-selling prescription drugs used in treating rheumatoid arthritis and other inflammatory diseases, saying that if used for over two-and-a-half years, these drugs can increase the risk of potentially fatal cancer in children and adolescents.

The drugs, called tumor necrosis factor (TNF) blockers, essentially help neutralize a protein, and are prescribed to children suffering from rheumatoid arthritis, inflammatory bowel disorder and Crohn's disease. However, the overproduction of the protein results in inflammation and damages bones, cartilage and other tissue.

The FDA decision to reinforce the "black box" warning on the TNF blockers comes after an investigation of 48 cancer cases in children - 11 of them fatal - treated with the drugs revealed "an increased risk of cancer, occurring after 30 months of treatment on average."

The stronger warning order by the FDA would affect five drugs sold in the US - which include Humira from Abbott Laboratories; Remicade and Simponi from Johnson & Johnson; Enbrel co-marketed by Amgen and Wyeth; Cimzia from Belgian drugmaker UCB.

Referring to the FDA move for stronger warnings, drugmakers Amgen and Wyeth said they would assist regulators in assessing "the potential risks and benefits" of their drug; while Johnson & Johnson said that it "will coordinate closely with the FDA to ensure that health care providers, patients and caregivers are properly informed."