If you follow the diet recommended for your small pet, you do not need to add vitamins to its water supply. Adding vitamins to water can make the water taste disagreeable and promote bacterial growth. In addition, many vitamins are inactivated shortly after they are exposed to air, water or light. Although a rare problem, it is possible to overdose a pet on vitamins, which can result in disease. Medications should be added to water only under the advice of a veterinarian.