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Vitamin B9 (folate or folic acid) is a water-soluble vitamin, part of the vitamin B complex, vital for the synthesis of several amino acids, the purines adenosine and guanine and the pyrimidine thymine (three of the four nucleotide bases and hence critical for the synthesis of nucleic acids).
The antimicrobial drug class, the sulfonamides, inhibit dihydropteroate synthetase, the enzyme which forms folic acid from para-aminobenzoic acid (PABA). This reaction does not occur in mammalian cells, hence does not affect human DNA synthesis and is specific for microbes.
The chemotherapeutic methotrexate inhibits dihydrofolate reductase which converts folate into its active form tetrohydrofolic acid (THF).
Interestingly, it was formerly known as vitamin M.
Folate deficiency is the most common vitamin deficiency in the developed world. It is especially important in pregnant women as antenatally the deficiency may lead to neural tube defects. In both adults and children the deficiency may lead to a megaloblastic anemia.
Excessive doses of vitamin B9 have not been found to cause toxicity per se. However if taken in megadoses, in those with undiagnosed pernicious anemia, there is a risk of serious CNS sequelae 2.