Merck & Co. said Friday that its experimental drug combination for treating chronic hepatitis C patients demonstrated a 95% effective rate in a late-stage study.

The study combined grazoprevir and elbasvir in a once-daily tablet and was administered to previously untreated patients, some of whom also had HIV and liver cirrhosis, for 12 weeks.

"Patients with co-morbidities and varying treatment experiences represent important segments of the chronic hepatitis C population in need of additional innovative treatment options," said Dr. Eric Lawitz, vice president at the Texas Liver Institute.

A Merck spokesperson said the company remains on track to submit a New Drug Application with the U.S. Food and Drug Administration in the first half of 2015.

The combined tablet from Merck poses a potential threat to Gilead Sciences Inc.

Also on Friday, Gilead announced results from a midstage study that combined its blockbuster drugs Harvoni and Sovaldi to treat patients with advanced liver disease. Its study showed an 86% effective rate in pretransplant patients, and a 96% cure rate in post-transplant noncirrhotic patients with recurrent hepatitis C.

The two drugs represented more than half of Gilead's 2014 revenue.

Write to Lisa Beilfuss at lisa.beilfuss@wsj.com

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