Nausea, vomiting, diarrhea, loss of appetite, mouth sores, black hairy tongue, sore throat, dizziness, headache, or rectal discomfort 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. Tell your doctor immediately if any of these unlikely but serious side effects occur: sunburn (sun sensitivity), nail discoloration, muscle pain, difficult or painful swallowing, change in the amount of urine, brown/gray tooth discoloration, numbness/tingling of the hands/feet, unusual fatigue, new signs of infection (e.g., persistent sore throat, fever, chills). Tell your doctor immediately if any of these rare but very serious side effects occur: hearing changes (e.g., ringing in the ears, decreased hearing), persistent or severe headache, vision changes (e.g., blurred vision), easy bruising/bleeding, severe stomach/abdominal pain, yellowing eyes/skin, dark urine. This medication may rarely cause a severe intestinal condition (Clostridium difficile-associated diarrhea) due to a resistant bacteria. This condition may occur during treatment or weeks to months after treatment has stopped.. Do not use anti-diarrhea products or narcotic pain medications if you have the following symptoms because these products may make them worse. Tell your doctor immediately if you develop: persistent diarrhea, abdominal or stomach pain/cramping, blood/mucus in your stool.. Use of this medication for prolonged or repeated periods may result in oral thrush or a new yeast infection (oral or vaginal fungal infection). Contact your doctor if you notice white patches in your mouth, a change in vaginal discharge or other new symptoms. 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, skin lesions/sores, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing, new or worsening swelling/pain in the joints, chest pain, fast/irregular heartbeat. 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.