Sumatriptan is a selective serotonin receptor (5-HT1B/1D) agonist that acts by causing vasoconstriction of cranial blood vessels and inhibiting pro-inflammatory neuropeptide release in trigeminal nerve pathways. When used for cluster headache, it rapidly alleviates pain by targeting the underlying neurogenic inflammation and vascular changes associated with these severe headaches, providing quick symptomatic relief through its specific mechanism of action.