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Fish oil is probably the most common supplement added to the pet diet. This is not without good reason. Growing numbers of studies confirm that the anti-inflammatory effect of omega-3 fats in fish oil has a beneficial effect in treating a host of abnormalities in pets. Research confirming these same effects abound in the human literature.

Now treatment for cancer, joint, heart, kidney, skin and intestinal problems, as well as geriatric dementia, often include generous amounts of fish oil and its abundant DHA and EPA omega-3 fatty acids. The positive effect on skin and coat quality has moved large numbers of pet owners to add fish oil to the diets of their young and normal pets. Overall, the trend to supplement with fish oil is positive for pet health, but there is a flip side to that coin. Too much of a good thing may have adverse effects on health.

Side Effects of Fish Oil Supplements for Pets

  1. The anti-inflammatory effect of EPA and DHA increases the production of certain chemicals that alter platelet function. Platelets or thrombocytes are cells produced in the bone marrow that aid in the formation of blood clots. This is an important first line of defense to prevent blood loss from trauma or other events or conditions causing hemorrhage. The chemicals produced by EPA and DHA decrease platelet activity and aggregation to form clots. Animals fed excessive amounts of fish oil would have a tendency to suffer higher blood loss when injured or afflicted by conditions that cause bleeding. This would also be an important consideration for pets needing surgery, especially procedures on body organs or parts of the body with heavy blood flow.
  2. The anti-inflammatory properties of EPA and DHA also interfere with wound healing. Inflammation at the site of a wound promotes the migration of white blood cells to the site to begin early wound healing processes. EPA and DHA decreases this necessary wound healing step and slows the body’s ability to repair the skin and promote new skin production. This is especially pronounced in the first five days of the wound healing process. Such an effect could be serious for an animal undergoing an extensive surgical procedure that was also fed high levels of dietary fish oil.
  3. The inflammatory response of immune system and white blood cells is important to effectively control threats from infection, cancer, and other abnormalities. This results in the production of a host of chemicals that promote the inflammatory response. The anti-inflammatory effects of EPA and DHA interfere with this important function. That is why fish oil is so helpful for treating conditions with an excessive inflammatory response like allergies and the skin problems associated with them. However a necessary level of inflammatory response must be maintained to protect the body and excessive amounts of EPA and DHA can interfere with that process.

Safe Levels of Fish Oil for Pets

The National Research Council has established a safe upper limit of EPA and DHA for dogs. It has yet to establish one for cats. In light of that, it is probably safe to use the guidelines for dogs for both species. Translating the data suggests that a dose between 20-55mg combined EPA and DHA per pound of body weight is safe for dogs and cats. This dose is far less than those used to treat serious conditions where the risk of side effects is less important than the benefits of treatment. Consult with your veterinarian when treating conditions requiring higher dosages.

Dr. Ken Tudor

Image: Composite / Shutterstock

Comments  10

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  • 08/08/2013 11:13am

    This is very interesting, thank you for explaining the downside of fish oils. Would you be able to post the dose that is used to treat certain conditions as a comparison to what has been established as a safe dose as a nutritional supplement? I have never given my dogs a dose higher than what you posted as being within the safe range, but I am wondering how much more is sometimes recommended as a therapeutic dose.

  • Omega 3s
    08/08/2013 12:13pm

    Are they saying 20-55mg EPA /DHA per lb? Meaning a 250 lb dog would get at lower end of spectrum 5000 mg EPA? I've always went with 180 mg EPA and 120 mg DHA per 20 lbs body weight for normal maintenance and per 10 lbs body weight for kidney/heart related conditions. Seems that the 20-55 mg per lb dosing even higher than what I was using?

  • 08/08/2013 01:41pm

    You are correct. Most do not dose this level, but the data would suggest that this level is safe if necessary to treat inflammatory conditions. Your dosing, which is similar to mine, is more appropriate for the normal pet. Thanks for your input.

    Dr. T

  • Overdose!
    08/08/2013 06:03pm

    One unpleasant side-effect of too much fish oil is diarrhea. I found that out the hard way. I put out one plate of canned cat food for multiple cats and, not knowing how much each would eat, I unintentionally added too much fish oil.

    It didn't take long for the full effect of my mistake to evidence itself.

  • Fish Oil debunked
    07/06/2014 02:27pm

    Since the original comment human trials have demonstrated absolutely no role for fish oil in mitigating cardiovascular risk and risk of dementia. I suspect that randomized control trials(RCT), if they could be done in animals, would also demonstrate no benefits. This is similar to the findings with Glucosamine and chondroitin in humans in recent trials, which demonstrate no benefit over placebo.

  • omega 3 for dog with dege
    11/03/2014 06:00pm

    hello. i found this article very helpful, however, the info given of 20-55mg of omega 3 per pound of dog doesn't say if that amount is per day or per week. also, i'm trying omega 3 for my dog who has degenerative knee problems. i was wondering what amount would be safe to give him to help his joints.

    thank you...

  • 11/03/2014 06:10pm

    The dosage amount is per day. It is also refers to a combined total of DHA and EPA in the oil. You need to add the amount of DHA and EPA together for each serving size to determine the correct dosage for your pet. They are also based on using fish body oil instead of fish liver oil. These dosages may deliver an unsafe dose of vitamin D if fish liver oil is used. The dosing amount I offered was to give readers an idea of the limits for a safe amount of fish oil and not a prescription for individual pets. I would advise you check with your vet before supplementing with any fish oil in case your pet has other problems that might preclude the use of fish oil or an adjustment in amount of it.

  • 11/03/2014 06:29pm

    thank you dr. i appreciate your time :-)

  • Plant based oils?
    05/27/2015 07:41am

    Hello Dr. Ken Tudor

    As you mentioned above, the fish oil's drawbacks are caused from EPA and DHA acids.The plant based oils have the ALA omega 3. Does that mean the plant based oil would have less drawbacks? Also I read on about fish oils sometimes would have mercury in them due to pollution from the sea etc. I was wondering if Plant based omega 3 oils are better alternative to Fish based oils?

    Kind Regards,

    Tsogt

  • 05/27/2015 01:51pm

    The side effects of overdosing EPA and DHA can occur no matter what the source, fish or plant. Because the EPA and DHA in fish oil is preformed and can be immediately used by the body it has been straight forward for researchers to determine possible doses that might be unsafe. The alpha linolenic acid in plant sources does not contain preformed EPA and DHA. The body needs to convert the ALA to these fatty acids. That conversion process is one of the body's least efficient systems and is dependent on sex, age and health of the individual. So a dose of flaxseed oil that would produce these side effects is not readily known and would be dependent on the individual animal's conversion capabilities.

    Further, we know ALA is converted to EPA but research has shown that DHA is not produced. Instead DHP, a precursor of DHA is produced that needs to be converted in the retinas of the eye and neurological tissues. So far the efficiency of that conversion system is not readily known and we don't know have accurate numbers of the amount of DHA that is released from the nervous system into the blood stream.

    My preference is fish oil because we can rely on the amount of preformed EPA and DHA our dogs are getting in our homemade dog food program.

    The amount of mercury in fish oil is largely dependent on the source. The larger the fish, the more potential for mercury because of how it becomes more concentrated as it moves up the food chain from algae. That is why some people prefer krill oil to fish oil. They both contain EPA and DHA, but krill are lower in the food chain and the small amount of mercury it contains in immeasurable with today's technology. Fish oil is generally much cheaper than krill oil and many manufactures have instituted technologies to purify it and remove contaminants like mercury.

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