What is Imitrex? Imitrex (sumatriptan) is a headache medicine that narrows blood vessels around the brain. Sumatriptan also reduces substances in the body that can trigger headache pain, nausea, sensitivity to light and sound, and other migraine symptoms. Imitrex is used to treat migraine headaches. Imitrex will only treat a headache that has already begun. It will not prevent headaches or reduce the number of attacks.