How it is administered

Trimethoprim is most commonly administered in combination with sulfamethoxazole as an oral tablet or suspension. It is also available as an intravenous (IV) infusion for patients who cannot take oral medications or require high doses, such as those with severe infections. The oral forms are typically taken every 12 hours, while IV administration is given over 60 to 90 minutes.

There is also an ophthalmic (eye drop) formulation of trimethoprim combined with polymyxin B for treating eye infections, but this is not used for blood cancers.

How it works

Trimethoprim works by blocking an enzyme called dihydrofolate reductase, which bacteria need to produce folic acid. Folic acid is essential for bacteria to make DNA and proteins, so by blocking this enzyme, trimethoprim stops bacteria from multiplying and spreading.

When combined with sulfamethoxazole, which blocks a different step in folic acid production, the two drugs work together to make it even harder for bacteria to survive. This combination is especially effective against a wide range of bacteria, including those that cause lung infections like Pneumocystis jirovecii pneumonia (PJP), which is a risk for people with weakened immune systems, such as those with Adult T-cell Leukemia/Lymphoma (ATLL) or other blood cancers.

Who Should take it

Trimethoprim, usually in combination with sulfamethoxazole, is used to treat or prevent certain bacterial infections. In the context of blood cancers like Adult T-cell Leukemia/Lymphoma, it is especially important for preventing or treating Pneumocystis jirovecii pneumonia (PJP), a serious lung infection that can occur when the immune system is weakened by cancer or its treatment.

It may also be used for urinary tract infections, shigellosis, and other infections caused by susceptible bacteria. Your doctor will decide if you need this medication based on your risk of infection and your overall health.

Who should not take it

You should not take trimethoprim (alone or in combination with sulfamethoxazole) if you:

• Have a known allergy to trimethoprim or sulfonamides
• Have a history of drug-induced low platelets (thrombocytopenia) from these medications
• Have severe liver damage or severe kidney problems (unless your kidney function can be closely monitored)
• Have a type of anemia caused by folate deficiency
• Are a child under 2 months old
• Are taking dofetilide (a heart medication)

If you are pregnant or breastfeeding, have thyroid problems, or are prone to folate deficiency, your doctor will carefully weigh the risks and benefits before prescribing this medication.

Commonly used with

Trimethoprim is almost always used in combination with sulfamethoxazole (as in the combination known as TMP-SMX or co-trimoxazole) for treating or preventing infections in people with blood cancers. It may also be used alongside other antibiotics depending on the infection.

In patients with blood cancers, it may be given with antifungal or antiviral medications as part of a broader infection prevention strategy.

Commonly tested with

Trimethoprim (with sulfamethoxazole) is often tested in combination with other antibiotics or antifungal agents in clinical studies involving patients with weakened immune systems, such as those with Adult T-cell Leukemia/Lymphoma or other blood cancers. It is also studied alongside supportive care medications, such as those used to prevent nausea or manage side effects of cancer treatment.