The U.S. Food and Drug Administration (FDA) issued a warning that hepatitis C treatments, Viekira Pak and Technivie, can cause serious liver injury mostly in patients with underlying advanced liver disease. The markets reacted very negatively to AbbVie Inc. (NYSE: ABBV) and this news, but Gilead Sciences Inc. (NASDAQ: GILD) came out on top because it produces a competing hepatitis C vaccine.

As a result, the FDA is requiring the manufacturer to include information about serious liver injury adverse events to the Contraindications, Warnings and Precautions, Postmarketing Experience, and Hepatic Impairment sections of the Viekira Pak and Technivie drug labels.

The FDA identified cases of hepatic decompensation and liver failure in patients with underlying liver cirrhosis who were taking these medicines, some of which resulted in liver transplantation or death. These serious outcomes were reported mostly in patients taking Viekira Pak who had evidence of advanced cirrhosis even before starting treatment with it.

Viekira Pak was approved in December 2014 and Technivie was approved in July 2015. Some 26 worldwide cases submitted to the FDA were considered to be possibly or probably related to Viekira Pak or Technivie. In most of the cases, liver injury occurred within 1 to 4 weeks of starting treatment. Some of the cases occurred in patients for whom these medicines were contraindicated or not recommended. The FDA Adverse Event Reporting System (FAERS) includes only reports submitted to the FDA, so there are likely additional cases about which the FDA is unaware.

Shares of Gilead closed Thursday up 5.8% at $107.60, with a consensus analyst price target of $124.25 and a 52-week trading range of $85.95 to $123.37.

AbbVie shares closed down 10.3% at $48.27. The stock has a consensus analyst price target of $75.07 and a 52-week trading range of $45.45 to $71.60.

Despite these reactions, Adam Feuerstein of TheStreet believes that investors have overreacted to what he sees as a small hit to AbbVie’s revenue.

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