Methotrexate works by blocking the body's use of folate (a B vitamin) to stop certain cells from growing too quickly. Folic acid is a man-made version of folate. While it helps protect healthy cells from methotrexate's side effects, too much of it can "compete" with the medicine, potentially stopping it from working properly against the disease.
This is a companion discussion topic for the original article at https://de.patient.info/treatment-medication/folic-acid-and-methotrexate-interaction