Blood clots sometimes form in leg veins and then break loose. When they travel to the lungs, they can cause serious or fatal consequences. The process of clot formation and breakaway is called thromboembolism, and blood clots that reach the lungs cause pulmonary embolism. Doctors can treat these problems with blood thinners. People who have had one pulmonary embolism are at increased risk for recurrent thromboembolism. The likelihood of recurrence depends on whether there is a transient problem that promotes clot formation (such as leg trauma) or no recognizable inciting event. Despite their effectiveness, blood thinners can also cause dangerous bleeding, so doctors prefer to treat patients with pulmonary embolism as briefly as possible. No one knows the safest treatment duration for a first pulmonary embolism, but doctors often give treatment for 3 months in patients with short-term risk factors and 6 months in those without known risk factors.