Vitamin B9 is an essential nutrient, mainly present as folate and folic acid. It is commonly taken in supplements, and is even added to processed food in North America.
Folate is the naturally-occurring form of vitamin B9. Before entering the bloodstream, the digestive system converts it into the biologically active form of vitamin B9, 5-MTHF.
Folic acid is a synthetic form of vitamin B9. The body does not convert it into active vitamin B9 very well, so un-metabolized folic acid may build up in the bloodstream.
High levels of un-metabolized folic acid may negatively affect health by increasing cancer risk or hiding vitamin B12 deficiency. This build up happens very easily, and the full impact on health is not yet known.