Common brand names

• Bactrim
• Septra

How it is administered

Trimethoprim is most commonly administered as part of a combination tablet with sulfamethoxazole (often referred to as sulfamethoxazole and trimethoprim). It is available in oral tablet form (regular and double strength) and as an intravenous (IV) infusion for patients who cannot take oral medications. The oral tablets are taken by mouth, usually every 12 hours, while the IV form is given by a healthcare professional over 60 to 90 minutes.

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, which is essential for bacteria to make DNA, RNA, and proteins. By interfering with this process, trimethoprim prevents bacteria from multiplying and spreading, helping the body’s immune system to clear the infection.

When combined with sulfamethoxazole, which blocks an earlier step in the same pathway, the two drugs work together to block two consecutive steps in the bacterial production of essential folic acids. This dual action makes the combination more effective and helps slow the development of bacterial resistance.

Who should take it

Trimethoprim, usually given with sulfamethoxazole, is used to treat or prevent infections caused by susceptible bacteria. In the context of blood cancers like T-Cell Prolymphocytic Leukemia, patients often have weakened immune systems and are at higher risk for certain infections. One important use is the treatment and prevention of Pneumocystis jirovecii pneumonia (PJP), a serious lung infection that can occur in people with weakened immune systems, including those with blood cancers or undergoing chemotherapy.

It is also used to treat urinary tract infections, shigellosis, and other bacterial infections when appropriate. The decision to use trimethoprim should be based on the type of infection, the patient’s immune status, and susceptibility of the bacteria.

Who should not take it

Trimethoprim (especially when combined with sulfamethoxazole) should not be taken by people who are allergic to trimethoprim or sulfonamides, have a history of drug-induced immune thrombocytopenia with these drugs, or have documented megaloblastic anemia due to folate deficiency. It is also contraindicated in children under 2 months of age, those with marked liver damage, or severe kidney insufficiency when kidney function cannot be monitored.

Pregnant women should generally avoid trimethoprim unless the benefits outweigh the risks, as it may interfere with folic acid metabolism and increase the risk of birth defects. Patients taking dofetilide (a heart medication) should not use trimethoprim due to the risk of serious heart rhythm problems.

Commonly used with

Trimethoprim is most commonly used in combination with sulfamethoxazole, as this combination is more effective than either drug alone. In patients with blood cancers or those undergoing chemotherapy, it may also be used alongside other supportive medications such as antifungals or antivirals to prevent or treat infections.

It may be used with other antibiotics if a broader spectrum of bacterial coverage is needed, but this should be determined by your healthcare provider.

Commonly tested with

Trimethoprim is often tested in combination with sulfamethoxazole for effectiveness against a variety of bacteria, including those that cause urinary tract infections, respiratory infections, and Pneumocystis jirovecii pneumonia. In clinical settings, it may also be tested alongside other antibiotics or supportive therapies in patients with weakened immune systems, such as those with blood cancers, to determine the best approach for preventing or treating infections.