The Food and Drug Administration is requiring that its most serious warning be placed on high-profile drugs for treating hepatitis C, saying they can cause a reactivation of hepatitis B and subsequently have led to death in some patients.

The direct-acting antiviral drugs – which include medicines under the brand names Sovaldi and Harvoni, as well as seven others – have high list prices that state governments, the Department of Veterans Affairs and prison systems have struggled to afford, with Sovaldi and Harvoni specifically costing tens of thousands of dollars for a full treatment.

More than 3 million people in the U.S. have hepatitis C, which is spread through blood and can necessitate a liver transplant.

The FDA's mandate of its so-called black box warning comes after the deaths of two patients who took a hepatitis C drug, along with one patient who needed a liver transplant. These patients saw a flare-up of a previously treated hepatitis B infection, which for most people can be easily cured but can lead to serious conditions in others.

At least 24 people who took a relevant hepatitis C medication had a hepatitis B resurgence from late November 2013 through roughly mid-July of 2016.

"This number includes only cases submitted to the FDA, so there are likely additional cases about which we are unaware," the FDA said on its website.

The FDA said it isn't sure why the hepatitis B reactivation occurs, and noted the side effect wasn't previously reported because drug companies testing their medicines didn't test them on people who had hepatitis B.