The Veterinary Oral Health Council
exists to recognize products that meet pre-set standards of plaque and
calculus (tartar) retardation in dogs and cats. Products are awarded
the VOHC Seal of Acceptance following review of data from trials
conducted according to VOHC protocols. The VOHC does not test products
Their protocols seem reasonably strict (a large part of my job is experimental design), and their list of approved products is relatively small (whether that's because they turn down some applications or because the industry doesn't pay attention is not really clear). I would estimate that any product that they've approved would be at least "best in class" for promoting oral health.
There is only one company with water additive products approved by the VOHC, HealthyMouth and their cat:essential. This product is approved for plaque removal, but not tartar. VOHC defines the difference as:
Plaque: The soft, bacteria-rich layer that rapidly forms on the
surface of the teeth if the teeth are not frequently chewed or brushed
Tartar: Calcium salts secreted in saliva that are deposited on
the surface of the teeth as a hard substance that is resistant to
removal by chewing or brushing.
However, the criticisms of the studies used for VOHC approval of this product noted that the experiments have not been peer reviewed, randomized, and or conducted blind (a term that means the people running the test knew which set of animals was getting each treatment, so may have (even accidentally) affected the results).
Dr. Fraser Hale, DVM, FAVD, DiplAVDC Board-Certified Veterinary Dental Specialist, remarks:
My assessment (and that of the VOHC) is that, while there are no
miracle products out there, healthymouth™ water additives and topical
gels may be a valuable tool in the ongoing effort to maintain good
oral hygiene and periodontal health when used as part of a
comprehensive oral care program.