Hello dear Imitrex (sumatriptan) is a selective serotonin receptor agonist used in oral, intranasal or injectable dosage form to treat migraine headaches. It can cause neck stiffness. It also causes following side effect: dizziness, vertigo, nausea and vomiting, flushing sensation, tingling sensation, and unpleasant taste. neck stiffness, chest discomfort, neck pain, numbness, burning sensation, throat irritation, flushing, sensation of tightness, sensation of pressure, and feeling of heaviness. So please visit your doctor and tell about reaction. It can cause problem if it will become severe. Hope this may help you. Take care