I don't think folinic acid would likely be causing over-methylation. It's an inhibitor of SHMT, the enzyme that makes methylene-THF which is the substrate methylenetetrahydrofolate reductase (MTHFR) converts into 5-methyltetrahydrofolate (methylfolate) for use in the methylation cycle. (Less methylene-THF means less methylfolate.) That's why I was speculating early in this thread that perhaps defects in both MTHFS and SHMT together might possibly cause a folinic acid intolerance problem. (A defective MTHFS causing a buildup of folinic acid that might in turn cause a defective SHMT to be more inhibited than it might otherwise would be.) There are lots of other possibilities for an intolerance, though. The "bunch of THF's" apparently include primarily methylfolate and 5-formyltetrahydrofolate (folinic acid) in polyglutamated forms (and I think 10-formyltetrahydrofolate and some others). SHMT converts THF to methylene-THF (which is not a DHF) as mentioned above. MTHFR then converts methylene-THF to methylfolate. I think DHFR primarily comes into play with respect to the synthesis of methylfolate if one is taking the synthetic folate, folic acid.