More on Vitamin C for dogs can be found at this website (and others, if you google it):
Vitamin C Overdose in Dogs | eHow.com Purpose of Vitamin C in Dogs
Vitamin C, also known as ascorbic acid, is responsible for a dog's immune system, blood and skin, collagen, and vitamin E production.
Natural Vitamin C
Dogs naturally produce vitamin C in their bodies. Every day, a dog will produce approximately 18 mg of vitamin C per pound. This means that a sixty pound dog will produce more than 1000 mg on their own each day. That's more than two supplement tablets, 500 mg each.
Vitamin C Supplements
There are a number of over-the-counter supplements available for dogs. Vitamin C supplements are oftentimes combined with other minerals and nutrients, like glucosamine. Supplements are typically in tablet form, in 325 mg and 500 mg strengths.
Uses for Vitamin C Supplements
It is not recommended to provide a daily vitamin C supplement to a dog, unless otherwise directed by a veterinarian. Because dogs naturally produce vitamin C, a supplement used long term can be detrimental to the natural production of the vitamin. Using vitamin C in a strong dose over a short period of time to help treat a specific ailment, such as an injury or infection, is appropriate and can be beneficial.
Overdose Dangers
The danger of vitamin C overdosing is that symptoms may not appear until permanent damage has been done. Damage can range from the inability to naturally produce his own vitamin C, to developing calcium oxalate crystals or stones in the urinary tract.