The medicines, which are highly effective against hepatitis C, may trigger latent hepatitis B in patients infected with both types, the European Medicines Agency said in a statement.
It named the antivirals Daklinza, Exviera, Harvoni, Olysio, Sovaldi and Viekirax used to treat chronic hepatitis C, an infectious liver disease.
"Cases of the return of previously inactive hepatitis B infection, which can be fatal, have been reported in patients treated," with this class of drug, the agency said in a statement announcing the findings of a probe.
But a special risk assessment committee of the agency recommended that all patients be screened for hepatitis B virus before starting any of these treatments.
It also proposed that a warning be included in the drug information leaflet.
The drugs, a new class known as "direct-acting antivirals" are more effective than their predecessors.
At a cost of some 40,000 to 80,000 euros (USD 43,000- USD 86,000) for a 12-week course, they are the preserve of a lucky few.
The B and C types usually occur from contact with body fluids of an infected person. Other than for hepatitis B, there is no vaccine against hepatitis C.
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