n. an ergot derivative used in the treatment of migraine headaches. It reduces the frequency and intensity of migraine attacks in most individuals and is thought to act by opposing the action of serotonin (see serotonin antagonist). Methysergide is closely related to LSD and has similar effects at some tissue sites. Side effects of methysergide include light-headedness or dizziness, nausea or vomiting, euphoria, insomnia, and unsteadiness. U.S. trade name: Sansert.