Vitamins have traditionally played the role of coenzymes, organic molecules that facilitate the chemical reactions catalyzed by enzymes. However, several vitamins assume additional endocrine-like actions; this review will discuss four such vitamins. Vitamin K2 is involved in the gamma-carboxylation of coagulation factors and bone proteins, but it can also bind and activate the steroid and xenobiotic receptor in order to mediate transcription in bone tissue, and has been used to treat osteoporosis. Biotin is critical for some carboxylation reactions, but it also induces epidermal differentiation and has been used to treat lameness in animals and brittle nails in humans. Pyridoxal phosphate (the active form of vitamin B6) is involved in a multitude of reactions, including decarboxylation and transamination; it can also inhibit DNA polymerases and several steroid receptors and may prove useful as an adjunct in cancer chemotherapy. Finally, nicotinic acid is converted to NAD+ and NADP+, which are used as hydrogen/electron carriers in redox reactions. However, it also possesses vasodilatory and antilipolytic activities.