Before supplementing your dog, it is very important to speak with your veterinarian about your dogs current diet and any changes you want to make. If your dog is eating a commercially made diet labeled as complete and balanced (almost all dog foods) then he is already getting vitamins and minerals from the food. Too much of a good thing ends up being a bad thing.
Even though complete and balanced dog foods do have a lot of vitamins in them, certain vitamins may also benefit from supplementation. Here are some important vitamins for healthy dogs, and whether or not a dog might benefit from supplementing them.
Vitamin C
Most dog foods actually don’t provide a source of vitamin C, because dogs are capable of manufacturing it on their own. (In fact, very few animals can’t—humans are one of them.) If your dog is a happy, healthy, normal dog he most likely doesn’t need vitamin C either, however certain situations can cause your dog’s need for vitamin C to outpace his ability to manufacture it. If you’re dog is sick, stressed, or older, talk to your veterinarian about starting a vitamin c supplement as it may be beneficial.

Vitamin A
If your dog has a vitamin A deficiency, you’ll notice a dry and lifeless coat, and your dog will no longer be able to see well in the dark. If the deficiency is allowed to progress, the dog will start to lose muscle tone and become weak. If your veterinarian has diagnosed your dog with a vitamin A deficiency, your dog should absolutely be on a supplement, as well as if it is recommended by your veterinarian to compliment a home made diet.
Care should be taken not to over supplement your pet. This is a fat soluble vitamin which means that if the dog receives it in excess, the only way for it to get rid of it is to store it in fat.
Vitamin B
The vitamin B complex, which includes a range of vitamins, is essential to your dog’s health. Although it is included in most commercial dog foods, vitamin B is very fragile and may be destroyed during the cooking process.
Symptoms of Vitamin B deficiency include excess shedding, premature graying, extra dirty decaying teeth, and so much more. Dogs on a vegetarian diet are particularly prone to lack of vitamin B.
Vitamin B is water soluble, which means if your dog receives too much he will simply flush it away in his urine.

Vitamin D
It’s not just humans in northern countries that suffer from vitamin D deficiencies. If your dog lives mostly indoors, chances are he could use a little more vitamin D in his diet. Many dogs don’t synthesize enough from the sun, and do require supplementation.
Talk with your veterinarian before starting your dog on a vitamin D supplement, as vitamin D poisoning from too much of this vitamin is more common than a deficiency.
There are many different vitamins for dogs out there. If you’re concerned about your pet’s health, a good multivitamin for dogs could help top off any deficiencies he has and make him a happier and healthier pet.