Azelaic acid is a dicarboxylic acid found in food (whole-grain cereals and animal products). There is a normal human plasma level (20–80 ng/mL); topical application does not significantly alter this level. The mechanism of action is thought to be normalization of the keratinization process (decreased thickness of the stratum corneum, decreased number and size of keratohyaline granules, and decreased amount of filaggrin). There are reports of in-vitro activity against Propionibacterium acnes and Staphylococcus epidermidis, which may be due to protein synthesis inhibition. In aerobic microorganisms, there is inhibition of oxidoreductive enzymes (such as tyrosinase, mitochondrial enzymes of the respiratory chain, 5α-reductase, and DNA polymerases). In anaerobic bacteria, there is disruption of glycolysis. Azelaic acid is used principally in the treatment of acne vulgaris and rosacea, although there are some advocates for its use in the treatment of hyperpigmentation (such as melasma for which it was initially developed). However, the US Food and Drug Administration has not approved the drug for this indication. Azelaic acid is available as a 15% gel or 20% cream preparation.