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Trimethoprim

Precautions

Before taking trimethoprim, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of: a certain type of anemia (due to folate deficiency), kidney disease, liver disease, vitamin deficiency (folate or folic acid), blood disorders (such as bone marrow suppression, G6PD deficiency), mineral imbalances (such as high level of potassium or low level of sodium in the blood).

This medication may make you more sensitive to the sun. Limit your time in the sun. Avoid tanning booths and sunlamps. Use sunscreen and wear protective clothing when outdoors. Tell your doctor right away if you get sunburned or have skin blisters/redness.

If you have diabetes, this product may affect your blood sugar. Check your blood sugar regularly as directed and share the results with your doctor. Tell your doctor right away if you have symptoms of low blood sugar (see Side Effects section). Your doctor may need to adjust your diabetes medication, exercise program, or diet.

Trimethoprim may cause live bacterial vaccines (such as typhoid vaccine) to not work well. Tell your health care professional that you are using trimethoprim before having any immunizations/vaccinations.

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Older adults may be more sensitive to the side effects of this drug, especially mineral imbalance (high potassium blood level) and allergic reactions.

Tell your doctor if you are pregnant or plan to become pregnant. You should not become pregnant while using trimethoprim. Trimethoprim may harm an unborn baby. It may lower your folic acid levels, increasing the risk of spinal cord defects. Check with your doctor to make sure you are taking enough folic acid. If you become pregnant, talk to your doctor right away about the risks and benefits of this medication.

Trimethoprim passes into breast milk. While there have been no reports of harm to nursing infants, consult your doctor before breastfeeding.

Side Effects

Diarrhea, nausea, vomiting, stomach upset, loss of appetite, changes in taste, and headache may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Remember that this medication has been prescribed because your doctor 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 right away if you have any serious side effects, including: new signs of infection (such as sore throat that doesn't go away, fever), easy bruising/bleeding, pale skin, unusual tiredness, fast/irregular heartbeat, mental/mood changes, signs of liver disease (such as nausea/vomiting that doesn't stop, dark urine, stomach/abdominal pain, yellowing eyes/skin), stiff neck, headache that doesn't go away, muscle weakness, extreme drowsiness, signs of low blood sugar (such as sudden sweating, shaking, fast heartbeat, hunger, blurred vision, dizziness, or tingling hands/feet).

This medication may rarely cause a severe intestinal condition due to a bacteria called C. difficile. This condition may occur during treatment or weeks to months after treatment has stopped. Tell your doctor right away if you develop: diarrhea that doesn't stop, abdominal or stomach pain/cramping, blood/mucus in your stool.

If you have these symptoms, do not use anti-diarrhea or opioid products because they may make symptoms worse.

Use of this medication for prolonged or repeated periods may result in oral thrush or a new yeast infection. Contact your doctor if you notice white patches in your mouth, a change in vaginal discharge, or other new symptoms.

Get medical help right away if you have any very serious side effects, including: seizures.

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: 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 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

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Drug information is selected from data included with permission and copyrighted by First DataBank, Inc. This is a summary and does not contain all possible information about this product. For complete information about this product or your specific health needs, ask your healthcare professional. Always seek the advice of your healthcare professional if you have any questions about this product or your medical condition. This information is not intended as individual medical advice and does not substitute for the knowledge and judgment of your healthcare professional. This information does not contain any assurances that this product is safe, effective or appropriate for you.

This information is intended only for residents of the United States. Products sold under the same brand names in other countries may contain different ingredients.

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Information expires December 2025.