Evolutionary Psychiatrist Emily Deans recently tweeted that she tests resistant depression patients for MTHFR status and finds ~50% of them to be C677T homozygous, which is the most severe reduction in MTHFR activity among the common polymorphisms. This is only in about 13% of the population.
Although this is from an unpublished case series in a single clinical practice and not a peer-reviewed study of a random sample of a specific clinical population, it is still quite a staggering difference, close to 3.8 fold.
I believe anyone with mental health issues should be tested for methylation status.
I don’t find methylfolate is that helpful in heterozygotes, but for homozygotes I can take a person on 5 nonworking meds and just use a straightforward antidepressant + methylfolate. (theoretically could just use methylfolate alone but I’ve not been successful with this yet)