Subarachnoid haemorrhage is a devastating type of stroke, with high mortality and morbidity rate. In approximately 85% of cases, it is caused by an intracranial aneurysm rupture. Majority of patients with diagnosed intracranial aneurysm are eligible for interventional treatment, however, some patients are managed conservatively. Currently, the only recommendations for patients with conservatively managed intracranial aneurysms, are routine imaging follow-ups and minimization of rupture risk factors. There are no medications proven to decrease risk of aneurysm rupture, that might be prescribed to such patients. In preliminary study the investigators found that patients with intracranial aneurysms who took β-blockers had significantly smaller aneurysm rupture rate and dome size, as well as more favorable hemodynamic parameters. No other antihypertensive drugs showed similar associations. Therefore, in this project the investigators aim to further analyze the impact of β-blocker intake on fate of intracranial aneurysm and find possible explanations for its protective role. The investigators aim to perform a randomised, double-blind, placebo-controlled clinical trial. One hundred patients with unruptured intracranial aneurysm, , qualified to conservative management will be enrolled. Two arms (50 patients each) will be receiving nebivolol or matching placebo. Treatment in each arm will last 12 months. The following examinations will be performed at baseline and at 6 and 12 months: clinical assessment, angio-MRI with vessel wall imaging, Doppler ultrasound to extract blood flow waveforms from Internal Carotid Artery, Vertebral Artery Middle Cerebral Artery, Anterior Cerebral Artery and Posterior Cerebral Artery, as well as blood samples. Based on the results the investigators will assess changes in aneurysm size and wall contrast enhancement. The investigators will also analyze levels of possible aneurysm growth biomarkers in peripheral blood. Additionally, the investigators will prepare three-dimensional models of the artery harbouring aneurysm and perform patient-specific computer modelling of blood flow through such artery to assess hemodynamic parameters of aneurysm dome. All obtained measurements will be compared at baseline and at 6 and 12 months. The investigators hypothesize that, in comparison to the placebo group, β-blocker therapy in patients with unruptured intracranial aneurysm will contribute to favorable changes in hemodynamic parameters of aneurysm dome, decrease wall degradation process and prevent from aneurysm growth.