Biotin is a water-soluble B vitamin that is an essential cofactor for four carboxylase enzymes, each of which catalyzes an essential step in intermediary metabolism. Because humans and other mammals cannot synthesize biotin, it must be derived from dietary sources and de novo synthesis by intestinal bacteria. Biotin was originally recognized when rats fed protein derived from egg whites developed severe dermatitis, hair loss, and neuromuscular dysfunction. A growth factor found in liver, then called “Protective Factor X,” cured the condition; this growth factor is now known as biotin. It was later discovered that uncooked egg whites contain a glycoprotein, avidin, that binds to biotin and prevents its absorption, whether biotin is from the diet or from intestinal bacterial synthesis.1 Besides genetic inborn errors of metabolism, biotin deficiency can occur during extended parenteral nutrition, pregnancy, or long-term anticonvulsant therapy. Conditions that may benefit from biotin supplementation include dyslipidemia, brittle nails, diabetes, dermatitis, and candidiasis.