The biotechnology company's shares, which rose 2.3 percent to close at $88.55, fell 3.6 percent after hours.

Quarterly net profit fell to $3.5 billion from $4.5 billion a year earlier. Adjusting for one-time items, Gilead earned $3.08 per share, which beat analysts' average estimate of $3.02, according to Thomson Reuters I/B/E/S.

Total product sales fell to $7.65 billion from $8.13 billion a year earlier. Sales of hepatitis C drugs Sovaldi and Harvoni dropped to $4 billion from $4.9 billion. Wall Street analysts, on average, had forecast hepatitis C sales of $4.09 billion, according to a poll by ISI Evercore.

Gilead drew fire more than two years ago when it charged $84,000 per course of treatment for Sovaldi, the first in a new class of hepatitis C drugs considered to be a cure for the liver-destroying disease.

The company on Monday lowered its outlook for 2016 product sales to between $29.5 billion and $30.5 billion, from a previous range of $30 billion to $31 billion.

Chief Financial Officer Robin Washington, on a conference after Gilead reported the results, attributed current trends in the hepatitis C market for the lower forecast. These include delays in starting U.S. patients on its hepatitis drugs, weak pricing in some European markets, a shorter treatment duration and loss of some market share to competition, he said.

An estimated 3.2 million Americans are infected with hepatitis C and insurers have responded by closely monitoring which patients should be treated with the new drugs.

The virus is spread by contact with contaminated blood of an infected person, by sharing needles or other equipment to inject drugs. Most patients have no symptoms, but some have fatigue, nausea, loss of appetite, and yellowing of the eyes and skin.