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Menadione is a polycyclic aromatic ketone, based on 1,4-naphthoquinone, with a 2-methyl substituent.
It was formerly sometimes called vitamin K<sub>3</sub>, although derivatives of naphthoquinone without the sidechain in the 3-position cannot exert all the functions of the K vitamins. Menadione is a vitamin precursor of K<sub>2</sub> which utilizes alkylation in the liver to yield menaquinones (MK-n, n=1-13; K<sub>2</sub> vitamers), and hence, is better classified as a provitamin.
Despite the fact that it can serve as a precursor to various types of vitamin K, menadione is generally not used as a nutritional supplement. Large doses of menadione have been reported to cause adverse outcomes including hemolytic anemia due to G6PD deficiency, neonatal brain or liver damage, or neonatal death in some cases. Moreover, menadione supplements have been banned by the FDA because of their high toxicity. Menadione has been used experimentally as a chemotherapic agent for cancer, ca 1945, but has lost ground to much safer, human form, vitamin K<sub>2</sub> vitamers. Low level menadione is still used as an inexpensive micronutrient for livestock in many countries.
Lately, menadione has been mentioned again as a treatment for cancer in conjunction with vitamin C (See "The end of cancer" by April Kirkendoll) but modern researchers and trials are investigating nontoxic K<sub>2</sub> vitamers such as menaquinone-4<sup>[1]</sup> in conjunction with more comprehensive regimens.