Cotrimoxazole and folate metabolism.

Cotrimoxazole 4 tablets daily (1 tablet = trimethoprim 80 mg and sulphamethoxzole 400 mg) was given for a period of six to fourteen days to 13 inpatients, and serum-folate levels were measured before and one day after the course of treatment. The results were compared with those from 8 patients not receiving antibacterial therapy, tested on admission and one week later. Two assay techniques were used, one employing Lactobacillus casei and the other 125I-labelled folate isotope dilution. The microbiological technique showed a significant decline in folic-acid levels in the serum after cotrimoxazole, and this decline was not seen in controls. By contrast, the radioisotope technique showed no significant alteration in serum-folate levels compared with controls. This suggests that cotrimoxazole does not depress true serum-folate and that many low microbiological results obtained during cotrimoxazole therapy reflect interference with the assay organism. There is insufficient evidence to incriminate cotrimoxazole as a significant cause of blood dyscrasias in excess of those which might occur on sulphonamide alone or even with other antibacterials.