Merck the second-biggest U.S. drugmaker which operates a huge facility in Durham, narrowed its full-year forecast after sales of its top-seller, diabetes pill Januvia, declined.

The company’s shares fell in early trading Monday.

Profit for 2013 will be $3.48 to $3.52 a share, the Whitehouse Station, New Jersey-based company said today in a statement. The previous outlook was for $3.45 to $3.55. Merck declined 1.6 percent to $45.80 at 8:07 a.m. New York time.

Third-quarter revenue fell 4 percent to $11 billion, missing the $11.1 billion estimate of analysts, as Januvia sales dropped 5 percent and foreign currency exchange weighed on results. Merck has struggled to keep investor support, with its stock increasing less than 1 percent in the 12 months through Oct. 25, compared with a 22 percent gain in the Standard & Poor’s 500 Pharmaceuticals Index.

“Januvia/Janumet widely missed consensus,” said Mark Schoenebaum, an analyst with International Strategy & Investment Group LLC, in a note to clients today. “We would expect the company to back off its prior guidance to expect ‘mid-single digit’ year-on-year Januvia franchise growth.” He said the stock could lose $1 to $2 today on the news.

Third-quarter net income fell to $1.12 billion, or 38 cents a share, from $1.73 billion, or 56 cents, a year earlier, the company said. Earnings, excluding one-time items, were 92 cents a share, beating by 5 cents the average of 17 analysts’ estimates compiled by Bloomberg.

Merck has several North Carolina connections. It operates a huge vaccine production facility in Durham and acquired then closed down Inspire Pharmaceuticals in Raleigh.

The company also recently decided to expand its facility in Wilson, and signed a licensing deal with RTP-based Chimerix.

“We are improving productivity and focusing our R&D and commercial resources more precisely to enable our investments in the best opportunities for innovation and growth,” CEO Kenneth Frazier said in a statement.

Job Cuts

Merck this month announced it would fire an additional 8,500 workers and overhaul research and development. The firings add to 7,500 job cuts already announced, amounting to a 20 percent reduction in Merck’s workforce. The company had about 80,000 employees as of Sept. 30. Drugmakers have been cutting expenses, research programs and positions to focus on creating new medicines, as well as selling or splitting off non- pharmaceutical businesses.

“Merck sentiment continues to get really bad,” Schoenebaum said. “I’ve become increasingly frustrated with fighting this management discount.”

Merck shareholders have been most concerned that sales of Januvia will face more competition, said Tony Butler, an analyst with Barclays Plc. Third-quarter sales of Januvia fell to $927 million from a year earlier.

Januvia Competition

Half-a-dozen new treatments that could compete with Januvia may begin selling in the next two years. The medicine has already experienced most of its potential growth, while competitors are cutting prices and marketing harder, Butler said. “Both volume and pricing opportunities for Januvia/Janumet are limited going forward,” he said in a note to clients this month.

Sales of the Gardasil vaccine rose 15 percent to $665 million from a year earlier, while revenue from the rheumatoid arthritis drug Remicade jumped 17 percent to $574 million.

Under new R&D chief Roger Perlmutter, Merck is overhauling its research labs to put more emphasis on vaccines, cancer, diabetes and hospital care. The moves will save $2.5 billion a year by 2015, according to the company.

“Merck’s belated attempts to restructure their R&D organization are encouraging, but we continue to expect material earnings disappointment compared with consensus forecasts,” Andrew Baum, an analyst with Citigroup Inc., said in a note to clients before the earnings report was released.