Folic acid (also known as vitamin B9[1] or folacin) and folate (the naturally occurring form), as well as pteroyl-L-glutamic acid and pteroyl-L-glutamate, are forms of the water-soluble vitamin B9. Folic acid is itself not biologically active with its biological importance being due to tetrahydrofolate and other derivatives after its conversion to dihydrofolic acid in the liver.

Vitamin B9 (folic acid and folate inclusive) is essential to numerous bodily functions ranging from nucleotide biosynthesis to the remethylation of homocysteine. It is especially important during periods of rapid cell division and growth. Both children and adults require folic acid to produce healthy red blood cells and prevent anemia. Folate and folic acid derive their names from the Latin word folium (which means "leaf"). Leafy vegetables are a principle source, although in Western diets fortified cereals and bread may be a larger dietary source.

A lack of dietary folic acid leads to folate deficiency (FD). This can result in many health problems, most notably neural tube defects in developing embryos.

Leafy vegetables such as spinach, asparagus, turnip greens, lettuces, dried or fresh beans and peas, fortified cereal products, sunflower seeds and certain other fruits and vegetables are rich sources of folate. Liver and liver products also contain high amounts of folate, as does baker's yeast. Some breakfast cereals (ready-to-eat and others) are fortified with 25% to 100% of the recommended dietary allowance (RDA) for folic acid. A table of selected food sources of folate and folic acid can be found at the USDA National Nutrient Database for Standard Reference. Folic acid is added to grain products in many countries, and in these countries fortified products make up a significant source of folate. Because of the difference in bioavailability between supplemented folic acid and the different forms of folate found in food, the dietary folate equivalent (DFE) system was established. 1 DFE is defined as 1 ?g of dietary folate, or 0.6 ?g of folic acid supplement. This is reduced to 0.5 ?g of folic acid if the supplement is taken on an empty stomach.

Folic acid is an important nutrient for women who may become pregnant. Adequate folate intake during the periconceptional period, the time right before and just after a woman becomes pregnant, helps protect against a number of congenital malformations including neural tube defects (which are the most notable birth defects that occur from folate deficiency). Neural tube defects (NTDs) result in malformations of the spine (spina bifida), skull, and brain (anencephaly). The risk of neural tube defects is significantly reduced when supplemental folic acid is consumed in addition to a healthy diet prior to and during the first month following conception. The protective effect of folate during pregnancy goes beyond NTDs. Supplementation with folic acid has been shown to reduce the risk of congenital heart defects, cleft lip, limb defects, and urinary tract anomalies. Women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate-rich foods to reduce the risk of some serious birth defects. Taking 400 micrograms of synthetic folic acid daily from fortified foods and/or supplements has been suggested. The RDA for folate equivalents for pregnant women is 600-800 micrograms, twice the normal RDA of 400 micrograms for women who are not pregnant.

Folic acid may also reduce chromosomal defects in sperm to some extent, which may be relevant for men considering to father a child.[24] A benefit is indicated even for more than 700 mcg folate per day, which though below the tolerable upper intake levels of 1,000 µg/day was 1.8 times the recommended dietary allowance.