The U.S. Food and Drug Administration (FDA) has upped its warning against a pair of cancer drugs because of their potential to reactivate hepatitis B in people who have previously been infected with the virus, Reuters reports.

The drugs in question are GlaxoSmithKline’s Arzerra, approved in 2009 to treat chronic lymphocytic leukemia (CLL), and Roche’s Rituxan, which treats such diseases as CLL, non-Hodgkin’s lymphoma and rheumatoid arthritis. Both already have warnings in their labels that they can reactivate hep B, but such alerts have not been enough to stop their improper administration: There still have been cases of virus reactivation as well as some deaths. Consequently, the FDA has given the cautionary note black box status, its strongest possible warning.

The FDA advises clinicians to screen all of their patients for hep B before beginning treatment with either drug, and to look for signs of a prior infection with the virus during treatment, or for indications of viral reactivation. The monitoring should continue for several months after the end of treatment. If patients do develop hep B reactivation, the cancer drug should be discontinued and hep B treatment begun.

To read the Reuters story, click here.

To read the FDA release, click here.