L-5MTHF is the active form of folate, but ironically, it is far less studied than the synthetic (inactive) form folic acid. Because of that, the safe upper limit of L-5MTHF is not known precisely, but we can draw some conclusions from existing data about the risks of high-dose folic acid.

The idea that methyl folate could be a better option than folic acid in prenatal vitamins is controversial, because folic acid has been the fertility go-to treatment since it was discovered in the 1930s. The issue is, MTHFR had NOT been discovered in the 1930s, and the research hadn't really dived into the genetic needs of infertile couples yet.