The largest manager of U.S. prescription-drug benefits is taking sides in a fight between new hepatitis C treatments that cost close to $100,000 per patient.

Express Scripts Holding Co. US:ESRX will make an AbbVie Inc. ABBV, -1.13% drug the exclusive option for patients with the most common form of hepatitis C. The move will help the drug maker take market share from Gilead Sciences Inc. GILD, -1.29% , which makes the blockbuster Sovaldi, but is likely to be controversial for limiting choices.