Methysergide is a non-selective serotonin receptor antagonist that acts primarily as a prophylactic treatment for cluster headaches by blocking 5-HT2 receptors and reducing cerebrovascular inflammation. It modulates serotonergic pathways, leading to decreased vascular reactivity and reduced frequency and severity of cluster headache episodes. However, its use is limited by potential side effects and the risk of retroperitoneal fibrosis with long-term administration.