Cats eating commercial cat food don't need "vitamins" (and some vitamins and minerals are dangerous to supplement), but there are many other supplements, such as antioxidants, Omega-3 fatty acids, probiotics and enzymes, that are good for them. The dosages are all given in this article: http://www.littlebigcat.com/index.php?a ... atsanddogs
either with the supplement, or use this rule: "Where dosage information is not specified, give a cat or small dog about 1/10 to 1/6 the human dosage, as specified on the label, per day. (Human doses are based on a 150 lb. person)"