Aneurysmal subarachnoid hemorrhage (SAH) is a life-threatening neurological illness: it is bleeding in the brain after a bulging blood vessel (a brain aneurysm) ruptures. Although SAH accounts for only 5% of all strokes, it often happens in middle age and it puts a significant burden on many patients during their most productive years. Complications following SAH are common, and they can cause major long-term disability. Only one medication - nimodipine - has been proven to benefit the health and wellbeing of these patients. All SAH patients should receive nimodipine for 21 days at a fixed dose. However, our early work suggested that all patients are not getting equal amounts of nimodipine into their blood. In addition, the two different forms (structural mirror images) of nimodipine might have different effects. Reduced amounts of nimodipine in the blood may lessen its benefit and contribute to worsening health and wellbeing of SAH patients. The overall goal of this research is to see what happens with different nimodipine doses and to confirm whether the two forms of nimodipine have different effects. The investigators will conduct a multi-centre study in adult patients admitted for SAH in Canada and the USA. The investigators will collect blood samples to determine the amount of each type of nimodipine in each participant's body, and then will check to see how each participant is doing at 90 days following SAH. They will also check other factors affecting nimodipine levels, so that they can in the future suggest dose recommendations that are actually tailored to each patient.