Pain, swelling, bleeding, or bruising at the injection site may occur. Flushing, sensations of tingling/numbness/prickling/heat, weakness, drowsiness, or dizziness may occur. If any of these effects persist or worsen, notify your doctor or pharmacist promptly.Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.Chest/jaw/neck tightness can commonly occur shortly after using sumatriptan. Only rarely are these signs of a serious condition. However, you may not be able to tell it apart from a serious reaction related to a lack of blood flow to the heart, brain or other parts of the body. Seek immediate medical attention if any of these unlikely but very serious (rarely fatal) side effects occur: chest pain, jaw/left arm pain, fainting, fast/irregular/pounding heartbeat, vision changes, weakness on one side of the body, confusion, slurred speech, sudden or severe stomach/abdominal pain, bloody diarrhea, change in the amount of urine.Tell your doctor immediately if any of these unlikely but serious side effects occur: blue fingers/toes/nails, cold sensation of hands/feet, hearing changes, mental/mood changes, seizures.This medication may increase serotonin and rarely cause a very serious condition called serotonin syndrome/toxicity. The risk increases if you are also taking other drugs that increase serotonin, so tell your doctor or pharmacist of all the drugs you take (see Drug Interactions section). Get medical help right away if you develop some of the following symptoms: fast heartbeat, hallucinations, loss of coordination, severe dizziness, severe nausea/vomiting/diarrhea, twitching muscles, unexplained fever, unusual agitation/restlessness.A serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction include: rash, itching/swelling (especially of the face/tongue/throat), dizziness, trouble breathing.This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.In the US -Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.