Randomized , double-blind trial ( treatment period , 18 months ; median follow-up , 24 months ) ; 371 adult patients who had experienced a first episode of symptomatic unprovoked pulmonary embolism ( ie , with no major risk factor for thrombosis ) and had been treated initially for 6 uninterrupted months with a vitamin K antagonist were randomized and followed up between July 2007 and September 2014 in 14 French centers .