Tetrahydrofolic acid and formaldehyde are key human metabolites but their physiologically relevant chemistry is undefined. Our NMR studies confirm formaldehyde as a product of tetrahydrofolic acid degradation but also reveal their reaction regulates the stability of tetrahydrofolic acid. These observations identify a novel non-enzymatic feedback mechanism regulating formaldehyde and folate metabolism that has important implications for folate-targeting chemotherapy in cancer and other diseases.