US health regulators have strengthened the warnings on two blood cancer drugs to reflect the risk that they may reactivate the hepatitis B virus in patients previously infected with the disease.

The warnings affect GlaxoSmithKline's Arzerra, which was approved in the United States in 2009 to treat chronic lymphocytic leukemia (CLL); and Rituxan, a drug made by Roche Holding and Biogen Idec that is approved to treat a variety of conditions including CLL, non-Hodgkin's Lymphoma and rheumatoid arthritis.

The US Food and Drug Administration said the risk is already described in the warnings and precautions section of the label for both drugs but that cases of reactivation continue to occur and some patients have died. Now the information will be placed in a black box, indicating the most serious type of risk.

The FDA said it recommends that physicians screen all patients for hepatitis B infection before starting treatment with the drugs, and monitor patients with evidence of prior hepatitis B infection for signs that the virus has been reactivated, including for several months after therapy has stopped.

Both drugs work be suppressing the body's immune system, which is critical in fighting infections. Hepatitis B is a serious liver infection that can become chronic and lead to liver failure, liver cancer or cirrhosis, a condition that causes permanent scarring of the liver. It is spread through contact with blood and body fluids. The best way to prevent the disease is through vaccination. - Reuters