See also Warnings section. Constipation, nausea, headache, diarrhea, vomiting, stomach upset, loss of appetite, gas, tremor, or trouble sleeping 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. Because this medication weakens your immune system, you are more likely to develop infections which may be serious. Tell your doctor immediately if you develop any of the following signs of infection: persistent sore throat/fever, night sweats, flu-like symptoms, painful urination, vision changes, a sore or wound on the skin that feels warm/tender/painful and appears reddened. This drug increases the risk of a rare and possibly fatal brain infection (PML - progressive multifocal leukoencephalopathy). Tell your doctor right away if you have any of these symptoms: clumsiness, sudden change in your thinking (such as confusion, difficulty concentrating), difficulty moving muscles, seizure, difficulty speaking. Tell your doctor immediately if any of these unlikely but serious side effects occur: unusual tiredness, fast/irregular heartbeat, muscle weakness, easy bleeding/bruising, swelling of the feet or ankles, mental/mood changes, weakness on one side of the body, unusual change in the amount of urine. Seek immediate medical attention if any of these rare but very serious side effects occur: chest pain, stomach/abdominal pain, black/tarry stools, vomit that looks like coffee grounds, shortness of breath/rapid breathing. A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: 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.