Leucovorin is used as Rescue therapy for patients who are on one of the following drug therapies (LQ)
a. Methotrexate toxicity: Folinic acid (Leucovorin, citrovorum factor, 5-formyl-THFA. is an active coenzyme form, which does not need to be reduced by DHFRase before it can act.
b. Methotrexate is a DHFRase inhibitor; its toxicity is not counteracted by folic acid, but antagonized by folinic acid.
c. Folinic acid is expensive and not needed for the correction of simple folate deficiency for which folic acid is good enough.