Serious
pancytopenia and megaloblastic anaemia have been described in patients given
pyrimethamine and
co-trimoxazole (which contains trimethoprim).
Caution should be used in prescribing the combination, especially in the presence of other drugs (e.g. methotrexate) or disease states that may also affect folate
metabolism. When high-dose
pyrimethamine is used for the treatment of
toxoplasmosis, the manufacturer recommends that all patients should receive a folate supplement.