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Leucovorin works (at least for some cases of CFD) because it bypasses a folate transport protein that can be nonfunctional and fail to transport ordinary folic acid into the brain. I guess that doesn't necessarily mean they couldn't do what you're suggesting though.

The dose in the pill is very high, it's not something you can just let everyone take.

The typical folate in food or even the pregnant women folic acid supplement is good at preventing these birth defects; but for someone with NDD it would actually not help and depending on dose block up the pathways for it to heal.