“I almost killed my dog with fish oil” is the story of a dog who regularly took supplements of fish oil only for this to lead to a vitamin E deficiency with painful but not fatal consequences. The story is a cautionary tale because fish oil is one of the best selling supplements for dogs (and humans.) Fish oil isn’t toxic for dogs but it does need to be used carefully.
Fish oil is high in an important fat called Omega 3. And omega 3s can boost a dog’s health in lots of ways- the main one being a healthy heart. Like all supplements, fish oil does have side effects and I examine how much truth there is about vitamin E deficiency. The amount of omega 3 in a fish oil varies depending on the species of fish which are used- and so I have created a chart highlighting the top species. Other related issues that I look at are whether dogs can be fed fish oil capsules that are designed for human consumption and how healthy alternatives to fish oils are- oils such as cod liver oil and krill oil.
Table of Contents
- What is fish oil?
- What are the main nutrients in fish oil?
- How does fish oil help dogs?
- How might fish oil hinder dogs?
- Which fish oil contains the most Omega 3s?
- Is human fish oil safe for dogs?
What is fish oil?
Fish oil is the oil extracted from fish. Fish oil tends to come from fatty or oily species of fish such as salmon, herring, sardines, tuna or mackerel. Whole fish are processed in order to extract the oil from them- solid protein and water are also extracted from the whole fish. The five stage process that the whole fish are taken through are: cooking, pressing, separating, drying and milling.
What are the main nutrients in fish oil?
The main nutrient in fish oil is a type of fatty acid called Omega 3. The three most important Omega 3s are ALA (alpha-linolenic acid), DHA (docosahexaenoic acid), and EPA (eicosapentaenoic acid). Whilst ALAs are found mostly in plants, DHA and EPA are found in animal foods such as fish. DHA and EPA are the most important Omega 3’s in any type of fish oil.
Omega 3s are essential fatty acids. Omega 3s are critical to the good health of a dog. Every cell wall in a dog’s body contains Omega 3 and they are an important source of energy for dogs. Dogs cannot synthesise (or make these) fatty acids on their own and so they need to get them from the food that they eat.
Although the nutrients in oil from different types of fish varies, about 30% of fish oil consists of Omega 3 fats whilst the other 70% of the oil contains other types of fat.
How does fish oil help dogs?
Fish oil helps dogs in 7 main ways. Research has shown that adding fish oil (Omega 3) into a puppy’s or dog’s diet will:
- Fish oil helps osteoarthritis
- Fish oil helps maintain a healthy heart
- Fish oil helps to protect the kidneys
- Fish oil helps to fight inflammation
- Fish oil helps develop eyesight
- Fish oil helps the skin and coat
- Fish oil helps reduce symptoms of anxiety.
1. Fish oil helps osteoarthritis
Osteoarthritis is also known as degenerative joint disease (DJD). It is caused by inflammation in the joints of dogs, which over time gets worse. The condition mostly affects the dog’s legs and their lower spine. In a healthy joint, a layer of cartilage acts as a cushion allowing a joint to move freely. Osteoarthritis causes the cartilage to deteriorate which leads to a loss of movement and an increase in pain.
In a 6 month trial involving one hundred and twenty seven dogs with osteoarthritis, Roush et al fed some of these dogs on commercial dog food and whilst other dogs were fed with a food that contained increased amounts of Omega 3 fatty acids.
The dogs who were fed the Omega 3 enriched food were found by their owners to be better able to play and walk than dogs who were just fed the commercial dog food.
[2] Fish oil helps maintain a healthy heart
Cardiovascular disease is also called heart disease. It is thought that about 10% of dogs that are seen by vets could have this disease. It is a very broad disease which includes any abnormality of the heart- such as its physical structure or how it functions.
In 2010 Freeman discovered that Omega 3s may support a healthy heart in dogs by controlling heart rate and preventing arrhythmia (which means a heart that beats too slowly, too fast or irregularly.) Omega 3s can also help to reduce inflammation of the arteries.
[3] Fish oil helps to protect the kidneys
In a study in 1998, SA Brown et al found that when fish oil was given as a supplement to dogs who had kidney disease the fish oil helped to slow the progression of the disease.
[4] Fish oil helps to fight inflammation
The nutrients DHA and EPA that are in Omega 3 fats help fight inflammation in different areas of a dog’s body. As well as helping fight inflammation with arthritis, the heart and the kidneys, Omega 3s can reduce inflammation caused by skin allergies and diabetes.
[5] Fish oil helps develop eyesight
Bauer in 2007 found that 12 week old puppies (whose mother’s had been eating a diet supplemented with fish oil) had better eyesight than puppies whose mothers weren’t on this supplement. The puppies who had been eating fish oil demonstrated that they had better vision in low light.
[6] Fish oil helps the skin and coat
Mueller et al in 2004, used an oil containing DHA as a supplement in a study involving thirty dogs with dermatitis (skin irritation.) The study lasted for ten weeks. Those dogs that were given the DHA oil had less skin irritation at the end of the study.
The DHA oil works in three ways. Firstly it moisturises the skin and forms a protective layer and secondly the oil provides a protective layer. Thirdly, because the DHA has anti inflammatory properties, it will help to reduce any swelling from any infection
[7] Fish oil helps reduce symptoms of anxiety.
Although there has been no research into the effect that fish oil has on the anxiety levels of dogs, there have been a few studies which have looked at its impact on human behaviour. Taking a daily supplement of omega 3s keeps the part of the brain responsible for regulating our moods more active (Conklin SM et al 2007) whereas people with lower than normal levels of omega 3s were more likely to display hostile and anxious behaviour (Green P et al 2006.)
How might fish oil hinder dogs?
Fish oil hinders dogs in three ways.
- Prolonged use of fish oil might lead to a vitamin E deficiency
- Dog’s can overdose on fish oil.
- Fish oil can spoil- it needs to be stored correctly.
[1] Prolonged use of fish oil can lead to vitamin E deficiency
This is the original “I almost killed my dog with fish oil” story. It’s about a dog called Neo who had been on fish oil supplements for a sustained period and as a result Neo developed a vitamin E deficiency. The side effects of a vitamin E deficiency are painful and include problems with their coordination, suffer from fatigue and develop problems with their eyesight- particularly their night vision. Neo was struggling to get up and move around. The reason that ingesting fish oil over a long period can lead to a vitamin E deficiency is that a dog’s body uses vitamin E to help it absorb fish oil.
I can’t find any research that demonstrates that this might happen in dogs but I did find some research which shows that it happens in women. In 1991, 25 women were given fish oil supplements over three months. Their vitamin E levels were taken at the start and the end of the trial. Their vitamin E levels were lower at the end of the three months.
[2] Dogs can overdose on fish oil
Dogs can overdose on fish oil. Ingesting too much fish oil can lead to a variety of side effects including vomiting and diarrhea and delayed wound healing.
If you are using a fish oil supplement which is specifically created for dogs then there should be very clear dosing instructions on the packaging. Otherwise there’s a recommended and maximum daily dosage guideline.
The recommended daily dosage for a dog is that they should only consume 100 mg of fish oil for every kilogram of body weight.
In the case of my Golden Retrievers, who are 65 lbs (29.5 kg) they should be given 2950 (29.5 x 100) mg fish oil per day.
The maximum daily dose for a dog is that they should consume 310 mg of fish oil or every kilogram of body weight.
In the case of my Golden Retrievers, who are 65 lbs (29.5 kg), they should be given 3924 mg fish oil per day.
[3] Fish oil can spoil- and might already be spoiled
Fish oil spoils more easily than other foods because it contains Omega 3 fats. Whole fish also spoil more easily than other foods for exactly the same reason. Omega 3 fats spoil because they oxidise- which happens when they have prolonged contact with air, heat or light. Once fish oil starts to oxidise it can’t be stopped. And rancid fish oil has a very strong and unpleasant taste and smell to it. Oxidised fish oils are less effective than fresh fish oils because they have a different chemical structure and don’t work in the same way.
Although there have been no studies into how common spoiled fish oil is in products specifically marketed to dogs, a recent study published by The Guardian in 2022, found that more than one in ten best selling fish oil products knowingly used spoiled or rancid fish oil- which were covered up by the use of flavourings.
Which fish oil contains the most Omega 3s?
In the chart below I have listed five of the most common species of fish that are used in fish oil- salmon, menhaden, mackerel, sardines and herring. The fish are listed in order of the amount of Omega 3 that they contain.
| Type of Fish Oil | % of Omega 3 |
| Salmon | 34.25 |
| Menhaden | 26.64 |
| Mackerel | 24-26 |
| Sardines | 22.77 |
| Herring | 11.10 |
The chart shows that the type of fish that contains the most Omega 3s is salmon oil. 34.25% of salmon oil is omega 3. In second place is menhaden oil. 26.64% of menhaden oil is omega 3. In third place is mackerel oil – between 24% and 26% is omega 3. Next up is sardine oil of which 22.77% are omega 3. The last place on my list is herring oil. This oil has the least amount of Omega 3 in it. Only 11.10% of herring oil is Omega 3.
A quick look on Amazon UK showed me that most of the best selling fish oils that are made specifically for dogs and sold in the UK are salmon oil. However on Amazon.com, most of the best selling fish oils that are made specifically for dogs are just labelled as “fish oil” with no mention of specific species of fish.
Is human fish oil safe for dogs?
Human fish oil is fish oil that is manufactured with the intention of humans eating it. Human fish oil is safe for dogs to ingest as long as they are given the correct amounts. Most of the best selling fish oils for people come in easy to eat capsules. Looking at the top 10 best selling fish oils, the amount of omega 3 in each capsule ranges from 800 mg (micrograms) to 3600 mg. Most of the capsules contain 1200 mg.
Earlier, I explained that dogs should only have 100 micrograms of fish oil for every kilogram of their body weight. An 800 mg capsule of fish oil is the perfect daily amount for an 8 kilogram dog (17 lbs 10 oz.) A 1200 microgram capsule of fish oil is the perfect daily amount for a twelve kilogram dog (26 lbs 7 oz) and a 3600 mg fish oil capsule is the perfect daily amount for a thirty six kilogram (79 lbs 6 oz) dog.
Can I feed my dog cod liver oil?
Dogs shouldn’t be given cod liver oil but this isn’t because it contains omega 3 fats. Dogs shouldn’t be given cod liver oil because it contains very high quantities of vitamin A which can harm your dog. Cod liver oil is oil made from the liver of a species of fish called cod. 18.8% of cod liver oil is Omega 3, which in our omega 3 chart from earlier would rank cod liver oil in 5th place below sardine oil which is 22.77% omega 3.
The biggest difference between cod liver oil and other fish oils, is that cod liver oil contains vitamins A and D- which other fish oils don’t contain. Cod liver oil contains huge quantities of vitamin A and D. The concentration of vitamin A is so large that dogs that have cod liver oil added to their diet for prolonged periods might be poisoned by it.
Can dogs eat krill oil?
No, you shouldn’t feed your dog krill oil. Although it is a good source of Omega 3, krill oil is very rich in an antioxidant called astaxanthin. Krill oil is only available in capsules made for people and so the amount of astaxanthin in one capsule is too much for most dogs.
Krill oil is oil that is made from krill. Krill are small shrimp like crustaceans. They grow up to about 6 cm in length and they can live up to five years. They live in every ocean in the world. Like fish oil, krill oil is high in the omega 3s DHA and EPA. 25% of krill oil is Omega 3.
Astaxanthin makes up 36% of krill oil. I can’t find any specific research on the use of astaxanthin with dogs but there is plenty of research on the use of it with humans. Astaxanthin helps in a number of ways including protecting and repairing our skin and in reducing inflammation which is important for the health of the joints in our body.
