Serum
digoxin levels can be increased by about 22% by
trimethoprim, but some individuals may show a much greater rise.
Monitor the effects of
digoxin if
trimethoprim is given, and consider measuring
digoxin levels, particularly in the elderly. Adjust the dose as necessary. Trimethoprim is a constituent of
co-trimoxazole but it is not known whether prophylactic doses of
co-trimoxazole (160 mg of
trimethoprim a day from a 960-mg dose of co-trimoxazole) will interact to a clinically significant degree. An interaction would seem likely with high-dose
co-trimoxazole, and care is needed in the elderly with any
co-trimoxazole dose.