Headache, joint pain, redness/irritation at the injection site, swelling hands/feet/ankles, or changes in menstrual cycle may occur. If any of these effects persist or worsen, tell 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. Tell your doctor immediately if you have any side effects while this drug is being given or shortly after your treatment is finished. Examples of these side effects (infusion reaction) may include chills, fever, flushing, nausea, dizziness, tiredness, and chest pain. Tell your doctor immediately if any of these unlikely but serious side effects occur: severe headache, fast/pounding heartbeat, signs of infection (such as fever, persistent sore throat, breathing problems, unusual vaginal discharge, painful/frequent urination), mood changes (such as depression, suicidal thoughts), severe stomach/abdominal pain. This drug increases the risk of a possibly fatal brain infection (see Warning section for more details). In MS patients, the symptoms of PML can seem like an attack of worsening MS. Therefore, whether you are using this drug for MS or CD, tell your doctor immediately of any worsening symptoms that persist for several days such as: clumsiness, sudden change in your thinking (such as confusion, difficulty concentrating), difficulty moving muscles, seizure, problems with speech, vision changes. This drug may rarely cause serious liver problems. If you notice any of the following rare but very serious side effects, tell your doctor immediately: persistent nausea/vomiting, dark urine, yellowing eyes/skin, feeling tired/weak. A very serious allergic reaction to this drug is rare. However, seek immediate medical attention if you notice any of the following symptoms: rash, itching/swelling (especially of the face/tongue/throat), severe 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.