Although the cause of migraine attacks is not fully understood, it is thought that widening of blood vessels in the brain causes the throbbing pain of migraine headaches. Rizatriptan relieves this pain by causing the blood vessels in the brain to narrow. Rizatriptan works by stimulating receptors called serotonin receptors that are found in the brain. A natural substance called serotonin normally acts on these receptors, causing blood vessels in the brain to narrow. Rizatriptan mimics this action of serotonin by directly stimulating the serotonin receptors in the brain. This narrows the blood vessels and so relieves the pain of migraine headaches.