What do systematic reviews say?

Folic or folinic acid (any) vs placebo

Folic or folinic acid (any) showed no clear difference in stomatitis / mouth sores (incidence) compared with placebo (moderate certainty).

• Folic acid and folinic acid for reducing side effects in patients receiving methotrexate for rheumatoid arthritis 2013 · Cochrane Headline outcomes only
  RR 0.72 [0.49, 1.06]
  4 studies · 575 participants
  Moderate