GlaxoSmithKline's profits dropped 13 percent in the first quarter as the British drug maker was battered by competition from generics and declining U.S. sales of the drug Avandia.

GSK, which has a U.S. headquarters and a manufacturing plant in the Triangle, reported $2.65 billion in profits for the three months ended March 31, compared with about $3 billion the same period a year ago.

Revenue for the quarter was up 1.6 percent compared with a year ago, mainly because of higher sales of GSK's asthma drug Advair, over-the-counter treatments and vaccines. The company reported $11.3 billion in sales, up from $10.9 billion a year ago.

But sales of pharmaceuticals in the U.S., where GSK generates about half of its worldwide sales, decreased 10 percent to $4.25 billion.

"Our performance this quarter was in line with our expectations," said Jean-Pierre Garnier, GSK's chief executive, in a prepared statement.

GSK reported quarterly earnings before the market opened today. The drug maker's American depositary receipts, which are like stock, closed at $43.73 Tuesday.