A migraine is caused, at least in part, by the dilation (widening) of certain blood vessels in the brain. Serotonin, which is also called 5-hydroxytryptamine, is a neurotransmitter that constricts blood vessels. Neurotransmitters are chemicals that send messages from one neuron (brain cell) to another, or from a neuron to a tissue (such as blood vessels). Imitrex is called a serotonin-agonist because it acts like serotonin, activating serotonin receptors on the blood vessels. Activation of these receptors constricts the blood vessels which relieves the pain of migraines. Imitrex also reduces substances in the body that can trigger headaches and relieve other migraine symptoms, including nausea and sensitivity to light and sound.

The injectable and nasal spray forms of Imitrex take effect quicker than the oral form, but their effects do not last as long.

Imitrex is metabolized, or modified, by enzymes in the liver called monoamine oxidases. The injected and nasal spray forms of Imitrex are metabolized to a lesser extent than is the oral form. The time required for the concentration in the blood to be reduced by half, the half-life, is approximately 2.5 hours. Imitrex is excreted from the body through both the urine and stool. Liver damage can slow the elimination of Imitrex.