The Food and Drug Administration is warning about the risk of reactivation of hepatitis B among patients who have had that disease and who are taking some prominent and expensive newer medicines for hepatitis C.

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The federal agency said it is requiring a so-called black-box warning in the labels for at least nine brand-name direct-acting antiviral drugs, including Sovaldi and Harvoni from Gilead Sciences Inc., Viekira Pak from AbbVie Inc. and Zepatier from Merck & Co. That is the most serious kind of warning that can be levied by the FDA.

The agency said serious liver problems or death occurred in “a few cases.” These medicines are already threatening the budgets of private and government insurers and of prison systems because of list prices ranging from $54,000 to $94,000 a person for a typical 12-week course. Hepatitis C infects millions of Americans and can lead to death after decades in the body.

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