As a general rule, before supplementing your cat's diet, you should discuss with your veterinarian the available evidence or recommendations supporting the use of nutriceuticals and dietary supplements. Be certain to avoid high levels of supplementation of any single nutrient unless you're certain that it's safe and won't interfere with any other medications your pet may take.
Supplements fall into two general and very large categories: vitamin and mineral supplements and nutriceuticals. Nutriceuticals are nutrient supplements given to obtain a pharmacologic (drug-like) effect or prevent a specific disease. The overall benefit of vitamin and mineral supplements is hotly debated. Cats that eat an appropriate balanced diet do not need supplements, based on most feeding studies of healthy cats. Nevertheless, many of us take dietary supplements and many of us wish to provide our pets with the same potential benefits.
Of course, dietary supplements can also be dangerous. Excessive supplementation with calcium salts, for example, can lead to significant bone diseases in growing cats. Vitamin D supplementation can lead to harmful elevations of the blood calcium and damage to the kidneys. Nutriceuticals fall into a different category since they are used to either prevent or treat specific diseases. Examples include taurine (an amino acid essential to cats) and Cosequin (a protein complex of possible benefit in joint health). There are others, such as L-carnitine (sometimes used for heart conditions), rutin (used for a serious condition called chylothorax) and co-enzyme Q10. Be aware that the Food and Drug Administration does not regulate supplements in the same way that drugs are regulated and controlled. The proof of effectiveness and safety demanded for a drug pharmaceutical is not required for nutriceuticals or vitamins.