Pet Education explains that healthy dogs produce all the vitamin C they need in their livers and therefore do not need to take vitamin C supplements. The Dog Food Advisor corroborates this, further stating that claims about vitamin C curing diseases in dogs have not been confirmed.
The Dog Food Project notes that vitamin C given to dogs from external sources such as fruit will not produce toxicity because the nutrient is water soluble and readily excreted. However, external vitamin C sources may cause diarrhea because the nutrient is a natural laxative. Recent studies indicate that excessive vitamin C in dogs may lead to kidney stones or difficulty absorbing vitamin B12.