Trimethoprim is an antibiotic Overview of Antibiotics Antibiotics are drugs used to treat bacterial infections. They are ineffective against viral infections and most other infections. Antibiotics either kill microorganisms or stop them from reproducing... read more and is available as a single drug or in combination with sulfamethoxazole (a sulfonamide Sulfonamides Sulfonamides are a class of antibiotics that are effective against many gram-positive bacteria and gram-negative bacteria. Some sulfonamides are applied directly to the skin (topically) to treat... read more antibiotic).
Trimethoprim and the combination of trimethoprim and sulfamethoxazole (TMP/SMX) are effective against many gram-positive bacteria Overview of Gram-Positive Bacteria Gram-positive bacteria are classified by the color they turn after a chemical called Gram stain is applied to them. Gram-positive bacteria stain blue when this stain is applied to them. Other... read more and gram-negative bacteria Overview of Gram-Negative Bacteria Gram-negative bacteria are classified by the color they turn after a chemical process called Gram staining is used on them. Gram-negative bacteria stain red when this process is used. Other... read more , including susceptible bacteria that are resistant to other antibiotics, such as methicillin-resistant Staphylococcus aureus (MRSA Methicillin-resistant Staphylococcus aureus (MRSA) Staphylococcus aureus is the most dangerous of all of the many common staphylococcal bacteria. These gram-positive, sphere-shaped (coccal) bacteria (see figure How Bacteria Shape Up) often cause... read more 
) and some protozoa (Cyclospora Cyclosporiasis Cyclosporiasis is an infection of the small intestine caused by the parasite Cyclospora cayetanensis. The main symptoms are watery diarrhea with abdominal cramping and nausea. People can become... read more 
and Cystoisospora Cystoisosporiasis Cystoisosporiasis is infection with the parasite Cystoisospora (Isospora) belli. The main symptoms are watery diarrhea with abdominal cramping and nausea. People can become infected with Cystoisospora... read more 
) and fungi (Pneumocystis Pneumonia is infection of the lungs. Pneumonia in people whose immune system is weakened or impaired (for example, by acquired immunodeficiency syndrome [AIDS], cancer, organ transplantation... read more ). Using trimethoprim and sulfamethoxazole together enhances the effectiveness of both antibiotics.
These drugs work by preventing the bacteria from producing the type of folic acid they need to function.
(See also Overview of Antibiotics Overview of Antibiotics Antibiotics are drugs used to treat bacterial infections. They are ineffective against viral infections and most other infections. Antibiotics either kill microorganisms or stop them from reproducing... read more .)
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 Neural Tube Defects and Spina Bifida Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord. Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death. The... read more ), such as spina bifida Neural Tube Defects and Spina Bifida Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord. Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death. The... read more , 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 Jaundice in the Newborn Jaundice is a yellow color to the skin and/or eyes caused by an increase in bilirubin in the bloodstream. Bilirubin is a yellow substance formed when hemoglobin (the part of red blood cells... read more 
and increases the risk of brain damage (kernicterus Complications of jaundice Jaundice is a yellow color to the skin and/or eyes caused by an increase in bilirubin in the bloodstream. Bilirubin is a yellow substance formed when hemoglobin (the part of red blood cells... read more ) in the newborn. (See also Drug Use During Pregnancy Drug Use During Pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more .)
Use of trimethoprim/sulfamethoxazole during breastfeeding is usually discouraged because sulfamethoxazole passes into breast milk. Use of trimethoprim during breastfeeding is generally considered acceptable. (See also Drug Use During Breastfeeding Drug Use During Breastfeeding When mothers who are breastfeeding have to take a drug, they wonder whether they should stop breastfeeding. The answer depends on the following: How much of the drug passes into the milk Whether... read more .)
