Gilead’s shares fell about 5 percent in after-hours trading, perhaps because the extent of the discounting was greater than some investors had expected.

Image Gilead Sciences sold $10.3 billion of its new hepatitis C drug Sovaldi in 2014. Credit... Gilead Sciences, via Associated Press

All the new drugs can cure more than 80 percent of patients and have few side effects. The effectiveness has led to strong demand for the drugs among the three million or more Americans with hepatitis C, a viral infection that gradually destroys the liver. Previous treatments had onerous side effects and cured a smaller percentage of patients.

Harvoni is a combination of Sovaldi and a second drug in a single pill taken once a day. It has a list price of $94,500 for the typical 12-week course of therapy. Sovaldi, also a once-a-day pill, has a list price of $84,000 for a 12-week treatment course, but it must be taken with at least one other drug. Viekira Pak from AbbVie is four pills a day and has a list price of about $83,000 for 12 weeks.

AbbVie struck first in the battle for payers, winning exclusive status from Express Scripts, the nation’s largest pharmacy benefit manager. Gilead struck back, winning exclusive status from the No. 2 pharmacy benefit manager, CVS Health. Gilead’s drugs have also been favored by some other health plans, including Aetna, Humana and Anthem.

“I’ve never seen anything like this,” Mark A. Thierer, chairman and chief executive of Catamaran, said at the J. P. Morgan Healthcare Conference last month. He said Gilead and AbbVie kept revising the proposals they submitted to Catamaran. “This has been very beneficial to us, just sitting still and watching this happen,” he said.

The discounts and rebates should reduce costs per patient paid by insurers. But it is not clear if they will significantly reduce overall costs for treating hepatitis C. That is because in some cases, in exchange for the discounts, payers are agreeing to treat more patients, even some with little liver damage. Until now, many plans have been restricting treatment to patients with more advanced liver disease.

“We’ve negotiated with the aim of increasing access,” Paul Carter, Gilead’s executive vice president for commercial operations, said in the conference call.