Trimethoprim/sulphamethoxazole and folate metabolism.

Summary Changes in folate metabolism were investigated in 24 patients receiving trimethoprim/sulphamethoxazole for a total of 104 treatment weeks. Serum microbiological assay techniques involved the use of drug resistant forms of L. casei and P. cerevisiae ( L. citrovorum ) as the test organisms together with standard haematological and biochemical methods. Serum levels of tetra-hydrofolates (a specific indicator of dihydrofolate reductase activity) remained within the normal range throughout. With recommended dosage, no significant clinical or laboratory side-effects were recorded. Animal experiments demonstrated the sensitivity of the P. cerevisiae assay which may prove useful in monitoring patients who are receiving large doses of the drug combination and whose folate metabolism is already at risk.