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Trimethoprim and Sulfamethoxazole

By Hans P. Schlecht, MD, MSc, Assistant Professor of Medicine, Department of Medicine, Division of Infectious Diseases & HIV Medicine, Drexel University College of Medicine ; Christopher Bruno, MD, Assistant Professor of Medicine, Division of infectious Diseases & HIV Medicine, Drexel University College of Medicine

Trimethoprim is available as a single drug or in combination with sulfamethoxazole (a sulfonamide antibiotic).

Trimethoprim and the combination of trimethoprim and sulfamethoxazole (TMP/SMX) are effective against many gram-positive and gram-negative bacteria, including susceptible bacteria that are resistant to other antibiotics, such as methicillin-resistant Staphylococcus aureus (MRSA). Using trimethoprim and sulfamethoxazole together enhances the effectiveness of both antibiotics.

These drugs work by preventing the bacteria from producing substances they need to function.

Trimethoprim and Sulfamethoxazole


Common Uses

Some Side Effects



Infections caused by susceptible bacteria that are resistant to other antibiotics, including methicillin-resistant Staphylococcus aureus (MRSA)

Chronic infection of the prostate (prostatitis)

Bladder infections in women

Prevention of recurring urinary tract infections in women and children

Intestinal infections due to various bacteria, such as Escherichia coli (E. coli)

Treatment and prevention of pneumonia due to Pneumocystis jirovecii (a fungus)

Nausea, vomiting, and diarrhea

Allergy (including rashes)

Crystals in urine (rare)

A decrease in white blood cell and platelet counts

Sensitivity to sunlight

Possibly increased tendency to bleed if used with warfarin

Kidney failure in people who have kidneys that are not functioning well

*Trimethoprim has side effects identical to those of sulfamethoxazole, but they are less common.

Use During Pregnancy and Breastfeeding

Trimethoprim and trimethoprim/sulfamethoxazole should be used during pregnancy only when the benefits of treatment outweigh the risks. With trimethoprim, defects of the brain and spinal cord (neural tube defects), such as spina bifida, are a risk. Trimethoprim/sulfamethoxazole should not be used when the due date is near because taken at that time, this drug combination may cause jaundice and increases the risk of brain damage (kernicterus) in the newborn.

Use of trimethoprim/sulfamethoxazole during breastfeeding is usually discouraged because sulfamethoxazole passes into breast milk. Use of trimethoprim during breastfeeding is generally considered acceptable.

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