In depth: The drug interaction between trimethoprim‐sulfamethoxazole and folic acid/folinic acid.

This article discusses the drug interaction between trimethoprim-sulfamethoxazole (cotrimoxazole) and folic acid/folinic acid. The interaction is frequently observed in haemato-oncological patients who are receiving trimethoprim-sulfamethoxazole for Pneumocystis Jiroveci Pneumonia (PJP) prophylaxis while also receiving folinic acid supplementation for their haematological condition. The mechanism of this interaction is the inhibition of dihydrofolate reductase (DHFR) by trimethoprim, which affects the metabolism of folic acid into active tetrahydrofolic acid (THF). While there is limited in vitro and in vivo evidence regarding the rationale of this interaction, it is recommended to prioritize folic acid over folinic acid when combining folates with trimethoprim-sulfamethoxazole prophylaxis for safety reasons. [Extracted from the article]