Usually mild. All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most common side effects persist or become bothersome when using sumatriptan: anxiety; burning, numbness, or tingling of the skin; dizziness; drowsiness; feeling of heaviness, pressure, or tightness; feeling strange; mild, temporary flushing; muscle aches or cramps.
Do you mean Imitrex (sumatriptan)? Common side effects include numbness, hot or cold sensation, fatigue, chest pain, dizziness, and flushing. Serious side effects include heart attack, rapid heart beat, arrhythmias, high blood pressure, and others.