i don't give Zack a multivitamin, i don't know if he needs it. the ingreidents on the bag of food sound nutritionally thorough, plus raw veggies he gets and occasional other stuff. But i don't know. I don't take a multivitamin myself. i don't get sick much and get mild cases when i do, never miss a day at work from illness, not in many years. i've taken multivitimins and other supplements at different times and have to say i could never tell if it was doing me any good or not because i didn't feel any different one way or the other.
The one thing that made a dramatic difference in how i felt and actually changed my life, was changing my food. On the advice of a highly recommended very intuitive nutritionist, i made very big changes in what i did and didn't eat, and almost right away felt a difference, and as the months went by and then the years, i felt better and better and ended up very healthy. In essence, it was a low allergen selection of foods, and also no salt and no sugar. Very powerful effect in terms of my health.
My vet recommended that i sprinkle Missing Link on Zack's food. I got some. Reading the label on that can really sounded like the healthiest food in the world in terms of nutrition, every possible nutrient a body could need. i didn't end up giving it to him though, based on the 'if it ain't broken, don't fix it' principle.