CALCIUM FOLINATE IS THE 5-FORMYL DERIVATIVE OF TETRAHYDROFOLIC ACID, THE ACTIVE FORM OF FOLIC ACID. IT IS USED AS AN ANTIDOTE TO FOLIC ACID ANTAGONISTS, SUCH AS METHOTREXATE, WHICH BLOCK THE CONVERSION OF FOLIC ACID TO TETRAHYDROFOLATE BY BINDING THE ENZYME DIHYDROFOLATE REDUCTASE. CALCIUM FOLINATE DOES NOT BLOCK THE ANTIMICROBIAL ACTION OF FOLATE ANTAGONISTS SUCH AS TRIMETHOPRIM OR PYRIMETHAMINE, BUT MAY REDUCE THEIR HAEMATOLOGICAL TOXICITIES.