The active form of folate is Metafolin, by Solgar OTC. It is called methylfolate. Folic acid is not active, and there is some concern that it may compete with methylfolate at the blood brain barrier.

This is not definitive yet, but some complex research papers are now suggesting to not use folic acid, but to use the methyl form for this reason. It is not common to be low in folic acid testing.
Our foods are highly fortified today in the US with it.

People with genetic errors in methylating inactive forms of the B12 and folic acid, develop elevated homocysteine. The blood tests for both in serum do not separate out the inactive and active forms. So it is best to take the activated forms since you are paying money already. Best to pay the same $$ and get the most useful. When you have a neurological problem, that suggests a problem metabolically as well. The doctors just haven't caught up with that concept yet.

methylcobalamin == active B12
methylFolate== active folic acid (called MetaFolin).