In its filing in the US district court of Delaware, Sanofi claims that Merck Sharp & Dohme Corp. violated 10 patents held by it, including ones for its diabetes treatment Lantus and its insulin delivery device SoloStar.
The French firm said it commenced the legal proceedings against Merck after the US company's filing for new drugs applications with the US Food & Drug Administration.
A Merck spokeswoman was quoted by the Wall Street Journal as saying, the company’s product doesn’t infringe Sanofi’s patents.
Lantus accounted for nearly 20% of Sanofi's total revenues in 2015. Lantus sales in the US fell 20.5% to €4.023bn last year.
In the first half of this year, Lantus sales declined another 15% to €2.38bn.
In January 2014, Sanofi filed a suit against Eli Lilly and Company to defend its patents on Lantus.
The companies have however entered into a settlement agreement in September 2015, under which Lilly agreed to delay the launch of its insulin product to December 2016 and pay royalties to Sanofi.