Methylfolate can be quickly manufactured as a material that has both the 6S and 6R isomers (or L and D respectively).

These are chemically referred to as chiral molecules which are a bit like your Left and Right hand (very similar but not the same). One is usually considered the “active” ingredient in a compound and the other is often considered “inactive”. It takes extra processing steps to get rid of the inactive isomer in biochemical development (this means more time, equipment, money, labor, and therefore cost). 

L is the same as 6S.

Be sure your methylfolate is 100% 6S isomer only – you do NOT want the inactive 6R isomer contaminating your methylfolate as it may block up any folate receptors needing the active compound and render them ineffectual.

Ask your methylfolate supplement company if they can show you a COAs detailing the exact amount of the 6R isomer (as tested) in their methylfolate (this should be considered an ‘impurity’ and should show up as less than 0.15%). 

Reference: https://methyl-life.com/pages/methylfolate-types