Sumatriptan succinate, a 5-HT1D receptor agonist, constricts human cranial arteries. Two parallel-group trials for treatment of acute migraines were conducted in the United States. Adult patients were randomized and given either 6 mg of sumatriptan succinate subcutaneously (n = 734) or placebo (n = 370). At 1 hour, sumatriptan was significantly more effective than placebo in reducing moderate or severe headache pain to mild or no pain (70% vs 22%), in completely relieving headaches (49% vs 9%), and in improving clinical disability (76% vs 34%). Sumatriptan also reduced nausea and photophobia significantly better than placebo. Patients with residual migraines received another injection; those who had originally received sumatriptan received either a second active injection (n = 187) or placebo (n = 178), while those who had received placebo received a second placebo injection (n = 335). Statistical evidence for benefit of second sumatriptan injection is absent. Adverse events associated with sumatriptan were tingling, dizziness, warm-hot sensations, and injection-site reactions. Sumatriptan is effective and well tolerated in patients with acute migraine.