Do NOT use Bactrim if:

• you are allergic to any ingredient in Bactrim or to similar medicines
• you had a severe allergic reaction to any other sulfonamide (sulfa) medicine (eg, glipizide, hydrochlorothiazide)
• you are taking dofetilide
• you have anemia caused by low levels of folate in the blood or urinary blockage
• you are in week 38 of pregnancy or later (full-term) or you are breast-feeding
• the patient is a child younger than 2 months old
• you will be receiving a live oral typhoid vaccine.

Contact your doctor or health care provider right away if any of these apply to you.

Some medical conditions may interact with Bactrim. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:

• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription (especially seizure medications) or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have liver or kidney problems, blood problems (eg, anemia, porphyria), asthma, or HIV infection
• if you have severe allergies, glucose-6-phosphate dehydrogenase deficiency, or an enzyme deficiency
• if you are at risk for low levels of folate in the blood (eg, you have alcoholism, you are elderly, you do not absorb nutrients from food properly, you are in a poor nutritional state, you are taking medicine for seizures).

Some medicines may interact with Bactrim. Tell your health care provider if you are taking any other medicines, especially any of the following:

• Leucovorin because it may decrease Bactrim's effectiveness
• Angiotensin-converting enzyme (ACE) inhibitors (eg, enalapril) or diuretics (eg, hydrochlorothiazide) because they may increase the risk of Bactrim's side effects
• Anticoagulants (eg, warfarin) because the risk of bleeding may be increased
• Dofetilide because the risk of severe heart problems may be increased
• Digoxin, methotrexate, phenytoin, sulfones (eg, dapsone), or sulfonylureas (eg, glipizide) because the risk of their side effects may be increased by Bactrim
• BCG vaccine or oral typhoid vaccine because their effectiveness may be decreased by Bactrim
• Cyclosporine because its effectiveness may be decreased and the risk of liver side effects may be increased.

This may not be a complete list of all interactions that may occur. Ask your health care provider if Bactrim may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

Important safety information:

• Bactrim may cause you to become sunburned more easily. Avoid the sun, sunlamps, or tanning booths until you know how you react to Bactrim. Use a sunscreen or wear protective clothing if you must be outside for more than a short time.
• Mild diarrhea is common with antibiotic use. However, a more serious form of diarrhea (pseudomembranous colitis) may rarely occur. This may develop while you use the antibiotic or within several months after you stop using it. Contact your doctor right away if stomach pain or cramps, severe diarrhea, or bloody stools occur. Do not treat diarrhea without first checking with your doctor.
• Contact your doctor right away if stomach pain or cramps, severe diarrhea, or bloody stools occur during treatment or within several months after treatment with Bactrim. Do not treat diarrhea without first checking with your doctor.
• Be sure to use Bactrim for the full course of treatment. If you do not, the medicine may not clear up your infection completely. The bacteria could also become less sensitive to this or other medicines. This could make the infection harder to treat in the future.
• Bactrim only works against bacteria; it does not treat viral infections (eg, the common cold).
• Long-term or repeated use of Bactrim may cause a second infection. Tell your doctor if signs of a second infection occur. Your medicine may need to be changed to treat this.
• Diabetes patients - Bactrim may affect your blood sugar. Check blood sugar levels closely. Ask your doctor before you change the dose of your diabetes medicine.
• Bactrim may interfere with certain lab tests. Be sure your doctor and lab personnel know you are taking Bactrim.
• Lab tests, including complete blood cell counts and kidney function, may be performed while you use Bactrim. These tests may be used to monitor your condition or check for side effects. Be sure to keep all doctor and lab appointments.
• Use Bactrim with caution in the elderly; they may be more sensitive to its effects, especially severe skin reactions, bone marrow suppression, or high blood potassium levels.
• Bactrim should not be used in children younger 2 months; safety and effectiveness in these children have not been confirmed.
• Pregnancy and breast-feeding: If you become pregnant, contact your doctor. You will need to discuss the benefits and risks of using Bactrim while you are pregnant. Do not use Bactrim if you are in week 38 of pregnancy or later (full-term). Bactrim is found in breast milk. Do not breastfeed while taking Bactrim.

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:

Appetite loss; diarrhea; nausea; vomiting.

Seek medical attention right away if any of these severe side effects occur:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue; unusual hoarseness); blistered, peeling, red, or swollen skin; bloody or black, tarry stools; chest pain; chills, fever, or sore throat; confusion; dark urine; decreased urination; depression; hallucinations; irregular heartbeat; joint or muscle pain; mental or mood changes; painful or stiff neck; purple patches under the skin; seizures; severe diarrhea; severe or persistent cough; severe or persistent headache; severe or persistent nausea or vomiting; shortness of breath; stomach cramps/pain; unusual bruising or bleeding; unusual tiredness or weakness; unusually pale skin; vaginal irritation or discharge; yellowing of the skin or eyes.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider.