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

Vitamin B₉ (folic acid and folate inclusive) is essential to numerous bodily functions. The human body needs folate to synthesize DNA, repair DNA, and methylate DNA as well as to act as a cofactor in biological reactions involving folate. It is especially important in aiding rapid cell division and growth, such as in infancy and pregnancy. Children and adults both 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 principal source, although in Western diets fortified cereals and bread may be a larger dietary source.^{[citation needed]}

B vitamins are a group of water-soluble vitamins that play important roles in cell metabolism. The B vitamins were once thought to be a single vitamin, referred to as vitamin B (much as people refer to vitamin C or vitamin D). Later research showed that they are chemically distinct vitamins that often coexist in the same foods. In general, supplements containing all eight are referred to as a vitamin B complex. Individual B vitamin supplements are referred to by the specific name of each vitamin (e.g., B₁, B₂, B₃ etc.).

The B vitamins are necessary to:

All B vitamins are water-soluble, and are dispersed throughout the body. Most of the B vitamins must be replenished regularly, since any excess is excreted in the urine. This can result in the urine produced being a neon green-yellow colour.

B vitamins have also been hypothesized to reduce the symptoms of attention deficit hyperactivity disorder.

Several named vitamin deficiency diseases may result from the lack of sufficient B-vitamins. Deficiencies of other B vitamins result in symptoms that are not part of a named deficiency disease.