French drug maker Sanofi says its net profit was slashed in the first quarter from a year earlier as falling sales and patent losses on key drugs combined to hammer earnings.

Sanofi says its net profit was €1 billion ($1.3 billion) in the January-March quarter, down 45 percent from €1.8 billion a year earlier.

Revenue from Plavix, which had been the world's second best-selling drug until its U.S. patent expired last May, slid 5 percent. That, along with generic competition that cut sales of blood pressure drugs Avapro and Aprovel by 20.8 percent, dragged down total company sales by 5.3 percent.

In a statement Thursday Sanofi warned that core earnings per share could fall up to 5 percent this year, after a 12.8 percent drop in 2012.