A healthy dog does not need vitamin C (ascorbic acid) in its food: its liver makes it from glucose, which is why the AAFCO nutrient profiles set no minimum for it in dog food. The vitamin is an antioxidant and takes part in making collagen. Studies that gave extra vitamin C to healthy or athletic dogs found little or no effect, and in one of them racing greyhounds ran more slowly.
Why dogs make their own, and why it is still on labels
Ascorbic acid is a water-soluble vitamin. It neutralises free radicals and is needed to make collagen. People have to get it from food; dogs do not. According to the MSD Veterinary Manual, AAFCO (the body that sets nutrient profiles in the United States) has no vitamin C requirement for dogs and cats, because both synthesise it from glucose in the liver. A 2001 pharmacokinetic study opens by noting that the dog can synthesise ascorbic acid but is often given it in the hope of improving health or performance.
In the EU, vitamin C is authorised as a feed additive for all species in several forms: ascorbic acid, sodium ascorbate, calcium ascorbate, ascorbyl palmitate, sodium ascorbyl phosphate and sodium calcium ascorbyl phosphate. In its 2025 re-evaluation, the European Food Safety Authority (EFSA) concluded that all of them are still safe for animals under the current conditions of use, which set no maximum content. Back in 2013 it had not considered a maximum necessary either. Some forms are authorised as a nutrient and others as a technological antioxidant. In that second role, according to the MSD Manual, vitamin C regenerates the tocopherols that keep the fat in dry food from going rancid. So it can appear on a label even when nobody added it with the dog in mind.
What research in dogs has found
Racing greyhounds: slower
Five trained female racing greyhounds went through three four-week phases in a study published in 2002: no vitamin C, 1 g a day after running, and 1 g a day one hour before running. They ran 500 m twice a week. On vitamin C they were on average 0.2 seconds slower, about 3 metres behind at the finish line, and blood markers of oxidation did not change. Five dogs is very few, but the result went the opposite way from the one the researchers were looking for.
Muscle damage in sled dogs
A trial with 41 trained sled dogs gave 21 of them a daily supplement for three weeks containing 706 mg of vitamin C, 457 IU of vitamin E and 5.1 mg of beta-carotene. The other 20 got a supplement with almost no antioxidants. After three days of endurance exercise, creatine kinase (CK), a marker of muscle damage, rose by the same amount in both groups. The supplement raised vitamin E in the blood but not vitamin C, which went up with exercise in both groups.
Antioxidants and immunity in healthy dogs
A 2009 crossover study gave 15 healthy dogs 0, 30 or 60 mg of vitamin C a day, always alongside 60 mg of vitamin E, in 36-day periods. Blood vitamin C tended to rise but the change did not reach significance. Oxidation markers and immunoglobulins stayed the same. The count of one type of lymphocyte (CD4+) went up, and the lymphocytes' response in the laboratory increased at 30 mg but fell at 60 mg. The authors concluded there was no clear evidence of an antioxidant effect in dogs whose diet already had plenty of vitamin E.
Vitamin C was also part of a mix tested in a 2024 study with 40 dogs: a food with more vitamin E, 100 mg/kg of vitamin C and beta-carotene reduced DNA damage and improved antioxidant status over 84 days. The study was designed to compare vitamin E levels, though, and cannot separate out what the vitamin C did.
Bones in growing puppies
It was once suggested that hypertrophic osteodystrophy, a bone disease of fast-growing puppies, was caused by a lack of vitamin C. In a 1979 study, Labrador puppies fed a ration high in protein, energy and calcium developed the bone lesions typical of overfeeding, including hypertrophic osteodystrophy. Adding 0.5 g of vitamin C twice a day did not improve the lesions and raised blood calcium. The authors concluded that treating dogs with this disease with vitamin C is contraindicated. They also found that a dose of 0.5 or 1 g raised blood vitamin C for less than eight hours.
Dogs with liver disease
Since dogs make vitamin C in the liver, you might expect a diseased liver to make less. A 2006 study measured blood vitamin C in 15 dogs with a portosystemic shunt (an abnormal vessel that diverts blood away from the liver), all with abnormal liver tests. Only two had levels below the reference range, and the authors concluded that their vitamin C status was not affected.
How it is absorbed
The 2001 pharmacokinetic study gave 20 dogs 15 or 50 mg/kg by mouth. Vitamin C rose quickly in the blood, which points to possible active transport in the gut, but levels did not rise in proportion to the dose. The crystalline form and another commercial form gave similar results.
The doses behind these results
If your vet sees a reason to give vitamin C to your dog, the vet should set the amount.
| Study | Dogs | Dose | Result |
|---|---|---|---|
| Greyhounds, 2002 | 5 female racing greyhounds | 1 g/day, 4-week phases | 0.2 s slower over 500 m; oxidation unchanged |
| Sled dogs, 2000 | 41 sled dogs | 706 mg/day with vitamin E and beta-carotene, 3 weeks | No change in muscle damage |
| Healthy dogs, 2009 | 15 dogs | 30 or 60 mg/day with 60 mg of vitamin E, 36-day periods | No clear antioxidant effect |
| Dry food, 2024 | 40 dogs | 100 mg/kg of food, with vitamin E and beta-carotene, 84 days | Less DNA damage; effect not separable from that of vitamin E |
| Puppies, 1979 | Growing Labradors | 0.5 g twice a day | No improvement in the bone lesions; more calcium in the blood |
| Absorption, 2001 | 20 dogs | 15 or 50 mg/kg by mouth | Fast rise in the blood, not proportional to the dose |
What this means for a healthy dog
In a healthy dog there is no shortage for a supplement to make up, so at most it is one more antioxidant, like the catechins in green tea. The studies in dogs do not show that extra vitamin C improves performance, muscle damage after exercise, immunity or bones. Nor do they show serious harm at the doses used, though some dogs need particular care with it.
Oxalate stones, growing puppies and other cautions
- If your dog has had calcium oxalate stones, do not give extra vitamin C without talking to the vet first. The MSD Manual warns that ascorbic acid breaks down into oxalate without any enzyme involved and increases the oxalate passed in the urine.
- Check anything added to a fast-growing large-breed puppy's diet with the vet first. In the 1979 study of Labrador puppies, extra vitamin C raised blood calcium.
- In greyhounds, 1 g a day was linked to slower races, which matters for any dog that competes.
- We have not found an established toxic dose for dogs, and the European authorisation sets no maximum in feed. That does not make high doses useful.
Vomiting or diarrhoea after starting any new product is a reason to see the vet. So is a dog that strains to urinate, has blood in its urine, or goes often and passes only a little each time, especially one that has had stones before.
Sources
- Nutritional Requirements of Small Animals. MSD Veterinary Manual.
- Pharmacokinetics in dogs after oral administration of two different forms of ascorbic acid. Research in Veterinary Science, 2001.
- Assessment of the feed additives consisting of ascorbic acid (3a300), sodium ascorbate (1b301), calcium ascorbate (1b302), ascorbyl palmitate (1b304), sodium calcium ascorbyl phosphate (3a312) and sodium ascorbyl phosphate (3a311) (as sources of vitamin C) for all animal species for the renewal of their authorisations. EFSA Journal, 2025.
- Supplemental vitamin C appears to slow racing greyhounds. The Journal of Nutrition, 2002.
- Effect of dietary supplements containing antioxidants on attenuation of muscle damage in exercising sled dogs. American Journal of Veterinary Research, 2000.
- The effect of vitamin C supplementation in healthy dogs on antioxidative capacity and immune parameters. Journal of Animal Physiology and Animal Nutrition, 2009.
- Effect of dietary antioxidants on free radical damage in dogs and cats. Journal of Animal Science, 2024.
- Ascorbic acid deficiency and hypertrophic osteodystrophy in the dog: a rebuttal. The Cornell Veterinarian, 1979.
- Plasma concentration of vitamin C in dogs with a portosystemic shunt. Canadian Journal of Veterinary Research, 2006.