Really depends on the guaranteed analysis of the food you are feeding. If you dog is a giant or large breed and is within the ideal weight limits then a vitamin supplement would only be called for if you noticed dull fur or some other tell tale sign of a vitamin deficiency. If you are using a good high quality food with the right amount of protiens and fats you probably wont need any supplements till she is in heat and pregnant or nursing. Just keep an eye on her and make sure she is eating her entire ration within 10-15 minutes of feeding and not eating poop or grass. If she is eating well and her coats is shiny and soft and her eyes are bright you should be just fine. If any of those are not so then a multivitamin supplement wont hurt a bit. I tend to try to keep my dogs at the low end of the max weight range and keep their fat content in there food at or below 16% and high quality protein between 25 and 30%. Check out Dogfoodadvisor.com for a collection of the most common foods gaauranteed analysis numbers and any special stuff like Omega 3 fatty oils and joint supplements.