 Aneurysmal subarachnoid hemorrhage aSAH is characterized by bleeding onto the brain surface due to a rupture of a pouch or bulge in a brain vessel called an aneurysm. It is a rare but serious condition with a high risk of death. Even patients who have had successful repair of the aneurysm remain at risk for developing cerebral vasospasm, which can result in harmful conditions related to the lack of oxygen to parts of the brain and death. Cerebral vasospasm usually occurs within the first couple of weeks after aSAH, and it is difficult to treat. Currently there is no safe, efficacious and widely available treatment. Previous animal studies have shown that a new drug under investigation, clazosentan, was able to reverse cerebral vasospasm in animal models of SAH. A first trial conducted in a small number of patients showed some benefit of clazosentan in reversing established cerebral vasospasm after aSAH. Clazosentan has already been evaluated in the prevention of cerebral vasospasm in several large clinical trials in aSAH. This current study aims to evaluate if clazosentan is effective and safe in the treatment of cerebral vasospasm after aSAH. 