The logo of Gilead Sciences Inc is pictured during a news conference in New Delhi September 15, 2014. REUTERS/Anindito Mukherjee/File Photo

(Reuters) - Gilead Sciences Inc on Tuesday said fourth-quarter sales of its flagship hepatitis C drugs fell by more than half amid increased competition, and the company forecast a further slowdown for the current year.

Gilead’s shares fell slightly after the closing bell.

For full-year 2018, the biotechnology company forecast product sales of $20 billion to $21 billion - including hepatitis C drug sales of $3.5 billion to $4 billion - down from $25.7 billion last year. Gilead projected a 2018 tax rate of 21 percent to 23 percent.

The company’s forecast implies 2018 earnings per share of $6.15, according to Mizuho Securities. Wall Street analysts, on average, expected 2018 earnings per share of $6.73, according to Thomson Reuters I/B/E/S.

Gilead, which last year paid nearly $12 billion for cancer immunotherapy company Kite Pharma, said it earned $1.78 a share in the fourth quarter excluding one-time items, which beat the average Wall Street estimate of $1.67 a share.

The quarterly results were “generally in line,” RBC Capital Markets analyst Brian Abrahams said in a research note. “Most notable news was Gilead’s 2018 guidance, where competitive dynamics (plus some market shrinkage) are expected to reduce HCV (hepatitis C virus) sales.”

Including a $6 billion charge related to U.S. corporate tax changes, Gilead posted a fourth-quarter net loss of $3.87 billion.

Quarterly sales of hepatitis C drugs totaled $1.5 billion, down from $3.2 billion a year earlier but in line with analysts’ forecasts. Sales of antiviral and HIV drug rose to $3.7 billion from $3.4 billion.

Shares of Gilead, which rose 2.5 percent to close at $80.38 in regular trading on the Nasdaq, were down 1 percent at $79.55 after-hours.