How it is administered

Trimethoprim is most commonly administered by mouth in combination with sulfamethoxazole (as in the combination medication sulfamethoxazole and trimethoprim). It is available in tablet form, typically as 400 mg sulfamethoxazole with 80 mg trimethoprim (single strength) or 800 mg sulfamethoxazole with 160 mg trimethoprim (double strength). There is also an intravenous (IV) formulation for patients who cannot take oral medications.

How it works

Trimethoprim works by blocking the production of tetrahydrofolic acid from dihydrofolic acid in bacteria. It does this by binding to and reversibly inhibiting the enzyme dihydrofolate reductase. This action interferes with the bacteria's ability to make nucleic acids and proteins, which are essential for their growth and survival. When used in combination with sulfamethoxazole, which blocks an earlier step in the same pathway, the two drugs work together to block two consecutive steps in the bacterial synthesis of folic acid. This dual action makes the combination more effective and helps slow the development of bacterial resistance.

Who Should take it

Trimethoprim, in combination with sulfamethoxazole, is indicated for the treatment and prevention of certain bacterial infections. For patients with blood cancers like Hairy Cell Leukemia, it is most commonly used to prevent or treat infections caused by Pneumocystis jirovecii (a type of pneumonia), especially in those who are immunosuppressed and at higher risk for infections. It is also used to treat urinary tract infections, shigellosis, and some respiratory tract infections caused by susceptible bacteria.

Your doctor may recommend this medication if you are at increased risk for infections due to a weakened immune system, which is common in people undergoing treatment for blood cancers.

Who should not take it

Trimethoprim should not be taken by individuals who have a known allergy to trimethoprim or sulfonamides, a history of drug-induced immune thrombocytopenia with use of these medications, or documented megaloblastic anemia due to folate deficiency. It is also contraindicated in pediatric patients less than 2 months of age, those with marked liver damage, or severe renal insufficiency when kidney function cannot be monitored. It should not be used in combination with dofetilide due to the risk of serious heart rhythm problems.

If you are pregnant or breastfeeding, have severe allergies, asthma, or certain blood disorders, you should discuss with your doctor whether this medication is appropriate for you.

Commonly used with

Trimethoprim is almost always used in combination with sulfamethoxazole, as this combination is more effective than either drug alone. In patients with blood cancers, it may be used alongside other medications to prevent or treat infections, but it is not usually combined with other antibiotics unless directed by a healthcare provider.

Commonly tested with

Trimethoprim (in combination with sulfamethoxazole) is often tested with other antibiotics in studies of infection prevention in immunocompromised patients, such as those with blood cancers. It may also be compared with other agents used to prevent Pneumocystis jirovecii pneumonia, such as atovaquone or dapsone, in clinical trials.