Several studies indicate that supplemental folinic acid (5-formyltetrahydrofolate) used in large doses during low-dose methotrexate therapy for rheumatoid arthritis causes a flare in joint inflammation. However, supplemental folic acid (pteroylglutamic acid) does not lessen the efficacy of the therapy. We further hypothesize that if methotrexate efficacy is driven by aminoimidazole carboxamide ribotide transformylase inhibition, folic acid supplementation should not alter urinary levels of aminoimidazole carboxamide, adenosine, and deoxyadenosine, while folinic acid supplementation should prevent the accumulation of these compounds.