Increasingly, veterinary clients are using nutraceuticals as adjuvant treatment for a variety of ailments in their pets. Recent evidence suggests that more than 50% of clients who have companion animals with cancer are using a nutraceutical and/or supplement as part of the treatment plan, and 65% of these clients say their veterinarian approves. This trend does not extend to healthy pets, of which only 10% receive routine supplements. Unfortunately, few resources regarding the safety of nutraceutical use exist, and recent evidence has shown that doses of human-formulated supplements such as lipoic acid can be toxic to cats. Veterinarians need a better understanding of the metabolism and safe upper limits for commonly used nutraceuticals because many clients are giving these products to their pets, often in human formulations. This article focuses primarily on two flavonoids: epigallocathechin-3-O-gallate (EGCG), a flavanol, and genistein, an isoflavone.