A Randomized Clinical Trial with security and dose testing of Sildenafil Citrate in patients with subarachnoid hemorrhage due to a rupture of a cerebral aneurism for prevention of cerebral vasospasm. The cerebral vasospasm is a decrease in blood flow that occurs when the intracranial vessels lose their capability of self-control of dilations and contractions. Patients with subarachnoid hemorrhage without neurological deficits who underwent endovascular or surgical correction of the aneurysm can participate in this trial. They will be randomized to a daily doses of 75 mg of Sildenafil, 150 mg of Sildenafil or Placebo from the third to the 14th day post bleeding. Today there is no proven clinical treatment for prevention of cerebral vasospasm.