The closure of the plant in Fawdon, near Newcastle upon Tyne, will be associated with the loss of 450 jobs.
The closure is being considered in the context of an adverse economic climate and the challenging pharmaceutical market in Europe,Sanofi said.
Plavix has already been affected by the arrival of generic competition in Europe. A consultation period is now underway.
Unite regional officer Bob Bolam said the closure is a total shock for the workforce.
"Unite is looking to work with the company to mitigate the losses and is calling on the company to "explore all avenues to try to save these valuable jobs," Bolam added.