How it is administered

Trimethoprim is commonly administered in combination with sulfamethoxazole as an oral tablet or suspension, and also as an intravenous (IV) infusion for patients who cannot take oral medications. The oral tablets come in regular and double strength (DS) formulations, while the IV form is used for more severe infections or when oral intake is not possible.

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 necessary for bacteria to make nucleic acids and proteins. When combined with sulfamethoxazole, which blocks an earlier step in the same pathway, the two drugs work together to stop bacteria from growing and multiplying.

This dual action makes the combination especially effective against a wide range of bacteria, including those that can cause infections in people with weakened immune systems, such as those with blood cancers. It is particularly important for preventing and treating Pneumocystis jirovecii pneumonia (PJP), a serious lung infection that can affect people with blood cancers or those undergoing chemotherapy.

Who Should take it

Trimethoprim, usually given with sulfamethoxazole, is used to treat or prevent infections caused by susceptible bacteria. It is especially important for people with weakened immune systems, such as those with blood cancers (like leukemia or lymphoma) or those receiving chemotherapy, because they are at higher risk for certain infections.

One of the most common uses in blood cancer patients is for the treatment and prevention of Pneumocystis jirovecii pneumonia (PJP). It may also be used for urinary tract infections, shigellosis, and other bacterial infections when appropriate. Your doctor will decide if this medication is right for you based on your specific situation and risk factors.

Who should not take it

Trimethoprim (and its combination with sulfamethoxazole) should not be taken by people who have a known allergy to trimethoprim or sulfonamides, a history of drug-induced immune thrombocytopenia with these medications, or documented megaloblastic anemia due to folate deficiency. It is also not recommended for children under 2 months of age, people with marked liver damage, or those with severe kidney insufficiency when kidney function cannot be monitored.

Additionally, it should not be used with the medication dofetilide due to the risk of serious heart rhythm problems. Pregnant women should only use this medication if the potential benefit outweighs the risk, as it may affect folic acid metabolism.

Commonly used with

Trimethoprim is almost always used in combination with sulfamethoxazole, as the two drugs work together to block bacterial growth more effectively than either alone. In patients with blood cancers, it may also be used alongside other supportive medications, such as antifungals or antivirals, depending on the patient's risk for different types of infections.

Commonly tested with

Trimethoprim (with sulfamethoxazole) is often tested in clinical settings with other antimicrobial agents, especially in immunocompromised patients, to determine the best approach for preventing or treating infections. It may also be studied in combination with antifungal or antiviral medications in patients with blood cancers who are at risk for multiple types of infections.