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Side Effects of Dog Health Supplements – Common Side Effects What To Be Careful About

Written by Jay
BsC (Hons) Animal Behaviour & Welfare graduate with a passion for advocating for misunderstood animals.
Published on
Tuesday 1 March 2022
Last updated on
Tuesday 9 May 2023
Side Effects of Dog Health Supplements
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As pet parents, we are inundated with hundreds of supplements everywhere we look. They are available in many places, ranging from your local pet store to your veterinarian’s office. But is it worth giving dog health supplements? Are the side effects of dog health supplements more of a risk than a benefit? And when is it appropriate to give your pooch a serving of health supplements?

Dog supplement side effects are a growing risk for pets. Although we all have good intentions when we give our dogs extra supplements, giving too much of a good thing can be a bad thing. For example, giving too much calcium can lead to hypercalcemia in dogs. Similarly, giving too much iron can cause bloody vomit and diarrhea. To find out more about these problems, read on with us.

When To Give Supplements To Your Dog

If you have a dog with special medical needs, supplements can be a great way to help your dog to live a happy, healthy lifestyle. Most dogs don’t need any supplements until they are older or develop a medical condition.

Because of this, it is important to not give extra supplements to your pup with the thought that you are making them healthier. Vitamin D, for example, has many health benefits, but can also be deadly for dogs when given in excess. Just because someone you know swears by a certain supplement for their furry friend, it doesn’t mean that the supplement will have the exact same effect on your pet.

With your vet’s approval, dogs with these conditions may benefit from these supplements:

It is crucial that you talk to your vet before giving your pet a supplement for their medical condition. Certain supplements can interact with your dog’s medication or make them feel worse. Your vet will also want to monitor your pet’s progress on the supplement so that they can adapt their treatment plan.

vitamin supplements for dogs
Dogs fed a homemade diet, may require supplements to ensure that they are getting enough vitamins with their meals.

Health Supplements and Their Side Effects

While supplements for dogs undoubtedly help thousands of dogs every day, they can also do more harm than good for other dogs. Many pet parents give their pups extra doses of supplements in hopes of making them even healthier, only to result in ill health for their furry friend. So, what are the risks of giving each supplement? And what signs should pet parents look out for when it comes to the side effects of dog health supplements?

Vitamin E

Vitamin E is a category of essential, fat-soluble vitamins. It is good for your dog’s immune system, muscles, nerve cells, and skin. Luckily, most high-quality dog foods contain just enough vitamin E for your dog. To date, there are no published reports about vitamin E toxicosis in dogs.

However, vets warn that excess supplements could lead to blood clotting problems for dogs. In cats, high levels of dietary vitamin E can cause prolonged bleeding times. This is especially a risk for dogs with bleeding disorders. To date, though, no exact toxic amount is described for dogs, and the other dog supplement side effects of vitamin E are unknown.

Vitamin B3

Vitamin B3 is a vitamin family made up of three forms: niacinamide, niacin, and nicotinamide riboside. Niacinamide is sometimes given as a supplement to treat inflammatory skin conditions. However, limited studies have been done on animals. So, we do not yet know the full range of effects when too much is given.

In general, though, niacinamide may be harmful to dogs with seizures or diabetes and can interact with skin allergy testing. Similarly, niacin is sometimes given to reduce hypercholesterolemia in dogs. However, there are few studies on the effects of an overdose. In one study, dogs developed bloody diarrhea when given x250 the adult requirement for niacin, and exposure to it for 11 days resulted in death.

Vitamin A

Vitamin A is an essential vitamin for dogs. Your dog needs vitamin A to maintain healthy skin, a smooth and shiny coat, and to maintain nerve function. Without enough of this vitamin, dogs can suffer from serious illness and blindness. However, vitamin A toxicity is just as much of a danger for our furry friends.

Vitamin A toxicity can occur from eating too much organ meat or by taking too many supplements. Cod liver oil is a common culprit. However, there is a lot of variation between individual dogs when it comes to vitamin A toxicity. While one dog can eat large quantities and appear normal, others seem much more prone to developing signs of toxicity. The signs of vitamin A toxicosis include anorexia, nausea, peeling skin, tremors, seizures, paralysis, and death.

Calcium & Vitamin D

Calcium is a mineral that is found in the body. It plays an important role in muscle contraction, the transmission of nerve impulses, and bone growth, among many other things. Because it’s so useful, some pet parents mistakenly give their pup more calcium than they really need. While it is uncommon for dogs to suffer hypercalcemia through supplementation, there are several documented cases of it happening.

One study describes a dog whose diet contained excessive amounts of vitamin D, causing hypercalcemia which resolved when the diet was changed. A second dog, who was on the same diet as the first dog, was also hypercalcemic. Both dogs suffered lethargy, a stiff gait, polydipsia, and polyuria. Also, excessive calcium intake during pregnancy increases the risk of low blood calcium levels and eclampsia. This is because it suppresses normal parathyroid hormone production.

Vitamin B6

Vitamin B6, or pyridoxine, is a water-soluble vitamin supplement that benefits the nervous system and the body’s metabolism. One of its most important roles is maintaining your dog’s potassium-sodium balance. The effects of vitamin B6 toxicosis are not well-studied.

However, chronic high doses of more than x100 the requirement for dogs can cause ataxia and tonic seizures in dogs within one week. This is because vitamin B6 is necessary for making nerve messages in the body. Just because the effects are well-studied, it doesn’t make the supplement inherently safe in large doses.

Vitamin C

Vitamin C is an essential antioxidant for dogs. It supports healthy aging and reduces inflammation in the body, thereby supporting the immune system and giving a boost of energy to your pup. Your healthy adult dog has a metabolism that allows them to make their own vitamin C in their liver when given a high-quality diet.

When given in high doses, the vitamin is able to alter the makeup of your dog’s urine, which can cause issues for your dog. Whatever your dog doesn’t use goes into their urine as oxalate. This increases your dog’s risk of calcium oxalate bladder stones, especially in male dogs. Because of this, breeds with a high risk of urinary oxalate stone formation should not get any vitamin C supplements. These include Schnauzers, Lhasa Apsos, Shih Tzus, and Bichon Frise.


Your dog’ needs iron. Without iron, dogs develop anemia just like people do. This mineral is very important because it is necessary for the production of red blood cells. However, giving too much iron can have devastating effects. Iron has a direct irritant effect on the gastrointestinal tract.

Once too much iron goes into your dog’s blood serum, the free iron goes to the liver. This is where it damages the mitochondria of the cells inside, which causes the tissues to die. The signs of iron toxicosis develop within 6 hours and start with vomiting and diarrhea, which may or may not contain blood. Hypovolemic shock, fever, and liver failure develop 12 to 24 hours later.

multivitamins not needed
Giving vitamin or mineral supplements to a pet eating a balanced diet may put them at risk for toxicity.

Side Effects of Dog Health Supplements – FAQ

So, still, wondering about the side effects of dog health supplements? Feel free to check out our FAQ for more details. If in doubt about your dog’s health, always ask your vet for advice.

Can vitamins be dangerous? 

If your pet is already receiving a high-quality, balanced diet and has no health problems, giving extra vitamins can be dangerous. Vitamin toxicity is a genuine risk for our furry friends, especially with chronic exposure. Dog vitamins can also be dangerous if given inappropriately. For example, pregnant or nursing dogs should not have any calcium or vitamin D supplements. Be sure to talk to your vet about the side effects of dog health supplements.

Are dog supplements necessary?

Dogs fed a high-quality diet should not need any extra vitamin supplements unless a vet says so. Your dog should get all of the nutrition they need from their diet if it is balanced and complete. However, if your dog has a medical condition, your vet may recommend certain supplements to support the body’s health. Make sure to check in with your vet first before giving your dog any medicines.

Is sunlight good for dogs?

Sunlight is good for your dog. Not only does it promote vitamin D absorption, but it also keeps your dog’s biorhythms ticking along. However, it’s important to be aware of the risk of heatstroke when your dog spends a long time in sunlight. Some health conditions can worsen with prolonged exposure to sunlight. Take discoid lupus for example; this condition is made worse by the sun, and dogs with the illness should avoid intense sunlight.

Can I give my dog human vitamins? 

Never give your dog vitamins made for humans. The vitamin requirements for humans and dogs are different, and while humans may be able to cope with overdoses of some, dogs may not. As well as this, human vitamins may contain harmful ingredients like xylitol that can be toxic to dogs.

What happens if my dog takes human vitamins?

While you may think that your vitamins pose little risk to your pup, they can be toxic in large amounts – liver failure, high calcium levels in the body, and organ damage are all potential risks. If your dog ingests vitamins for people, be sure to contact your vet right away.

The side effects of dog health supplements are a growing problem for pet parents. With pet supplements on the rise, we must continue to feed our dogs a complete, balanced, and high-quality diet that meets their needs. And, as always, talk to your vet if you have any worries about your pup’s health.

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