Biotin is a water-soluble B vitamin (vitamin B7) that human cannot synthesise and therefore depend on dietary and supplements intake.
It was established as a vitamin after 40 years of additional research after its initial discovery in 1927. 
Dietary biotin exists in protein-bound and free forms. However, most of the biotin in foods is bound to protein and very little dietary biotin is in the free form. [1,2]
Toxicity is rare due to the limited intestinal absorption of biotin. [1,3]