Garlic for Pets- What’s it Good for?

One of the most popular plant products touted for medicinal use in pets is garlic Allium sativum. A recent newsletter by Dr. Deva Khalsa, a popular alternative veterinarian, makes some typical claims:

Garlic is a miracle herb!

Garlic (Allium sativum) has been valued for thousands of years for medicinal purposes.

reported adverse affects from garlic add up to a total non-event over the past 22 years….This proves beyond the shadow of a doubt that the risk of using garlic is so infintessimally low that it’s simply statistically insignificant. What is significant is all the positive research delineating the medicinal powers of garlic.

a natural antibioitic

it’s antifungal, antiviral and antibacterial effects can contribute to the healing process.

Garlic increases general immune activity

Uncooked garlic helps to lower blood triglycerides and choleterol making it useful for certain breeds (Schnauzers and Beagles) that are predisposed to this problem.

Compounds in garlic act as antioxidants and help flush toxins out.

garlic has been fed to dogs in order to help prevent flea infestation.

A host of studies provide evidence that the allicin in garlic works to inhibit cancer formation.

Since these pretty fairly represent the claims made for garlic, I will address each in turn.

1. Garlic is a miracle herb-
Well, miracle is a pretty tough word to justify when applied to any medical therapy, all of which come with pros and cons. I’d call this a warning sign!

2. Garlic has been used medicinally for thousands of years.
True, but irrelevant to whether or not it works. The traditional use of plants as medicines has a poor record of predicting uses that are actually proven valid by controlled scientific testing. Here are some of the traditional uses record for garlic (from Wynn SG. Fougere BJ. (2007). Veterinary Herbal Medicine St. Louis: Mosby Elsevier.)

Traditionally, garlic has been used in humans [or animals] for abnormal growths, bronchitis, pneumonia, digestive problems, intestinal infection, tuberculosis, dysentery, earache and ear infection, vascular disorders…influenza…relieve toxicities, and kill parasites…treatment of a cow that was unable to rise after calving…coccidiosis in poultry…breast tumors on dogs (topically), for distemper, for jaundice, for rheumatism, and for thyroid complaints and worms…for fungal infection and swelling of the tongue, oral blisters and wounds…contagious abortion, tetanus, milky diarrhea, abdominal pain, asthma, polyuria, sores, compound fractures, epilepsy and swelling of the kidneys, gangrene of the lung…

When something is used to treat such a wide variety of apparently unrelated medical conditions, either it is a miracle or it is an example of how easily people can be fooled into thinking a useless therapy is helping. A truly miraculous therapy shouldn’t have any trouble proving itself in scientific testing, so when the evidence is lacking despite centuries of use and decades of research, the most likely explanation is human fallibility rather than miracle cure.

3. Risk of using garlic is negligible.
Mostly true. A number of in vitro and lab animal studies have shown the potential for garlic to cause anemia and other problems. Studies looking at garlic as a potential antimicrobial in swine feed found the pigs ate less and gained less weight (and also had and “objectionable” taste, for those carnivores among you). However, the amount of garlic which must be fed to cause clinical illness in dogs is probably much larger than is typically used. Cats are much more sensitive, but they are also less likely to accept garlic.

There are a couple of important caveats here, though. For one thing, the in vitro and lab animal studies showing potential dangers of garlic, which Dr. Deva and other proponents of garlic dismiss as not relevant to actual use, are exactly the same kinds of studies which show potential benefits and which they constantly cite to justify actual use of garlic. This is a classic case of confirmation bias in which people accept as valid evidence which supports their position but reject evidence which contradicts it.

The other factor is that there are case reports of individual dogs who experienced anemia, high blood pressure, and other serious adverse effects from consuming relatively small amounts of garlic (e.g. 1, 2). It is often the case with pharmaceuticals, which appear safe in relatively small test populations of a few thousand people, less common sensitivities in some individuals appear when they are given to much larger numbers of patients. This is part of the business of balancing the risks and benefits of any treatment. So while the risks of garlic appear to be low for most dogs, it is not appropriate to assume they are inconsequential, especially when evidence for benefits is so poor (as we will see).

4. Garlic has antibiotic, antifungal, and anti-parasitic properties.
Maybe. The basis of this claim, apart from low-reliability evidence such as anecdote and opinion, is in in vitro and animal model studies. Chemicals from garlic do have effects on bacteria, fungi, and parasitic organisms in the laboratory. This only suggests that these chemicals might have similar effects in the real world. It doesn’t prove anything. What an isolated chemical does in a petri dish is not necessarily what feeding garlic will do in a living animal. And as I often remind people, bleach has antibiotic, antifungal, and anti-parasitic properties in the lab, but I wouldn’t recommend it as a dietary supplement!

The research in real animals is mixed. Garlic has failed to show benefit in at least as many studies as it has shown some effects, so the balance of the evidence is unclear. And the published research is almost all in agricultural animals, not companion animals.

5. Garlic “boosts” the immune system.
Nonsense. The notion of a general immune stimulant which has clinically beneficial effects and no side effects is a fantasy that ignores the incredible complexity of the immune system and how it interacts with all the potential threats it is exposed to. This empty bit of marketing is discussed, and demolished, in detail in this article.

6. Garlic lowers triglycerides and cholesterol.
Probably not. A 2009 review of the research in humans found that “the available evidence from randomized controlled trial does not demonstrate any beneficial effects of garlic on serum cholesterol.” The point is largely moot since blood lipids are almost never a meaningful health issue in companion animal species. It is true, as Dr. Deva suggests, that it is an uncommon problem in certain breeds. However, I would note that both of the dogs in the case reports I mentioned before who suffered harm from eating garlic were Schnauzers, the same breed that most often exhibits this otherwise rare problem. So I’d be very careful about suggesting garlic as a remedy for this issue in this breed given the lack of evidence for a benefit and the presence of evidence for possible risk.

7. Garlic is an anti-oxidant and helps remove “toxins”
A hypothetical treatment for a hypothetical problem. First off, the subject of oxidation and anti-oxidants is about as complex as the subject of immune system activation, and there is growing evidence that anti-oxidant therapies do little good and some harm (e.g. 3, 4, 5). So while garlic compounds show some antioxidant properties in the lab, they also show the ability to cause oxidative damage (that’s where the anemia comes from), and there is zero evidence in real cats and dogs for any benefit from garlic as an anti-oxidant.

Secondly, the mysterious “toxins” so often mentioned as a target for health promoting therapies are a classic sign of snake oil marketing. Without information about specific toxins, evidence for the harm they do, information about how they are processed and eliminated by the body, and evidence concerning how garlic can aid in this process, this claim is totally meaningless. And, not surprisingly, none of the information or evidence is offered for garlic.

8. Garlic is an effective flea control method.
Doubtful. There are, of course, plenty of anecdotes from people saying that garlic drove the fleas away from their pets like magic. There are just as many that say garlic didn’t help at all. There is absolutely no research evidence showing garlic supplements to be an effective form of flea control. It is often given with Brewer’s yeast, which has been shown to be ineffective as a flea repellant. So for all the confidence with which this claim is made, it is simply made up.

9. Garlic is useful in prevention or treatment of cancer.
Maybe. There is low to medium level evidence in humans that eating garlic may reduce the risk of some kinds of cancer. The American Cancer Society concludes

there is not enough evidence at this time to support eating large amounts of garlic or taking garlic supplements for cancer prevention. Garlic may have the potential to interfere with anesthesia or other medicines. It is reasonable to include garlic as part of a balanced diet, unless one has a particular health problem or is taking medication that has been shown to be adversely affected by garlic.

Other research summaries show a mix of possible benefits and risks, including bleeding and interference with the action of a number of medicine. So like anything with the potential for benefit, there is the potential for harm.

And, as usual, none of this research has been done in companion animal species. So while extrapolation from research in humans is often necessary, it is also a great way to draw the wrong conclusions. Such research strongly suggests which ideas may be worth pursuing and which may not, but it certainly doesn’t provide a confident, reliable answer about what to do for our pets.

Bottom Line
In vitro and lab animal research suggests a number of potential uses for garlic. The same level of research suggests some potential danger to garlic use in dogs and cats. There is almost no clinical research to substantiate any of the claims made for garlic use in pets. Traditional use suggests almost every pant is a cure-all, and the majority of such claims have yet to pass the test of scientific study. So at this point, hysterical claims of garlic as a “miracle food” are totally unjustified. The potential benefits deserve to be investigated, but they are mostly theoretical and unproven. The risks are probably low overall, but some individuals have been harmed by eating garlic, and we are currently unable to predict accurately which patients will be adversely affected.

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20 Responses to Garlic for Pets- What’s it Good for?

  1. Bartimaeus says:

    I had one patient with a portosystemic shunt which was being fed a homemade diet by it’s owner. It had heinz body anemia which resolved after the owner stopped feeding the dog garlic. The difficult part was convincing the owner that the garlic was not the miracle food she had been told it was. I have seen at least one other normal dog with anemia and bilirubinuria that we traced back to fried onions. Feeding garlic can and does cause harm under certain circumstances. There just is no reporting system for these kind of events to keep track of how common they really are.

  2. Tomcat says:

    Thanks for sharing…

    Wow…I posted this link on our Veterinary News Network Facebook page and there was an immediate and visceral reaction from a veterinarian who took offense to it. I am always amazed with how aggressive people will be while voicing their opinion online.

  3. skeptvet says:

    Isn’t it amazing? I’ve often wondered where the anger comes from. I think the idea of accepting that our experience and intuition are unreliable and that many things are beyond our control is frightening, and people react angrily when told they don’t necessarily know what they think they know. And though there are plenty of alternative veterinarians who are rational and reasonable about issues of evidence, there is a certain degree of faith-based medicine about much of the alternative medicine community. And no one reacts well to having their faith challenged.

    Given this is a fairly mild article about garlic, you can imagine the hate mail I get for my truly controversial opinions! 🙂

  4. v.t. says:

    Thanks for posting this, skeptvet, it’s an important issue and just another one of those thorns in my side (Rachael Ray anyone?).

    Heinz body anemia is nothing to scoff at, and those pet owners who have never seen a pet suffer with it, choose to ignore the risks. Why anyone would subject their pet to such risk is beyond me, but I digress.

    I too would like to know where the anger comes from. But, as we’ve seen a thousand times, the alties use anger as a defense mechanism, not giving any real thought to their own behavior (which doesn’t lend them much credibility with their behavior).

  5. Shawna says:

    My initial post with references was considered “spammy” so I will not be able to include the reference links.

    Dr. William Li discusses foods that are antiangiogenic — garlic is one of those foods.

    Google scholar lists several articles stating garlic induces apoptosis (allicin actually).

    Colorado State University Extension Office has an article on garlics benefit as a mosquito repellant. Knowing this, its not hard to infer it might work against fleas as well.

    Medical News today has an ariticle stating allicin/garlic kills the antibiotic resistant hospital super bug MRSA.

    Penn Medical School has discovered that the good gut bacteria “prime” neutrophils thereby stimulating the immune system. The inulin/FOS in garlic feeds those good gut bacteria (prebiotic).

    Those same beneficial gut bacteria, when fed, act as a nitrogen trap and in such clean (or detox) the blood. Very useful in kidney and liver disease.

  6. Shawna says:

    I would like to mention, the dog pictured in my avatar has congenital kidney disease. She had polyuria and polydipsia even before being fully weaned. She was officially diagnosed at one year of age when taken in to be spayed. Over the next year three subsequent blood draws were done as well as other testing all concluding chronic kidney disease.

    Conventional heartworm and flea treatment really aren’t an option with Audrey. She is also unvaccinated other then first set of puppy shots (is exempted from rabies for life). She has been given garlic for heartworm and flea tick prevention, giardia, coccidia (I foster), kennel cough plus. I have been giving her garlic her entire life. She will turn 6 the end of June. Not bad for a dog with congenital kd wouldn’t you say :). She is still in the early stages of the disease and symptoms are still just polydipsia and polyuria. Once in a great while she gets halitosis and I up the probiotics and give extra prebiotics (in the form of acacia fiber) for nitrogen trapping. Hasn’t failed her yet. She is unmedicated has never required sub-q fluids etc. She has also been on a raw, high protein diet since weaning.

  7. Kenneth says:

    You might want to edit this article to correct this sentence:

    And as I often remind people, bleach has antibiotic, antifungal, and anti-parasitic properties in the lab, but I would recommend it as a dietary supplement!

    I think you meant to say “I would not recommend it”…

  8. skeptvet says:

    Oops! Thanks for spotting this!

  9. skeptvet says:

    I am glad that you girl is doing well, but I am also confident it is in spite of rather than because of the garlic you give her. Even most propnents of garlic as a supplement would tell you that it is not an effective heartworm preventative, so if you live in a heartworm endemic area you are taing a uge risk in not using a real heartworm preventative. There is absolutely no reason to avoid parasite control or vaccination in a dog with congenital kidney disease, so this puts her at risk for no good reason. And while the connection between protein in the diet and kidney function is weak, there is evidence that hgih protein diets may worsen kidney disease. A raw diet has the advantage of being higher in water content, which is useful for kidney disease, but there are plenty of ways to achieve this without the risks of feeding raw.

    So though I have no doubt you will reject the idea, the fact that her disease is not yet severe enough to make her sick means you and she are fortunate; it is not proof that the methods you are using are protecting her. Good luck to both of you.

  10. skeptvet says:

    “foods that are antiangiogenic — garlic is one of those foods”
    This means that in the lab compounds in these foods might inhibit blood vessel growth. This could be a good thing in cancer, and a bad thing in wound healing, so this is hardly a reason to recommend garlic, especially since it hasn’t been shown to have any clinical benefits from this activity in living animals. Someday it might, but at this point it’s a guess only.

    “Google scholar lists several articles stating garlic induces apoptosis (allicin actually).”
    This means that allicin can cause cells to die spontaneously in the lab. Again, might be a good thing or a bad thing depending on what you’re trying to accomplish, IF it also does so in patients, which isn’t demonstrated.

    “an article on garlics benefit as a mosquito repellant.”
    A topical cream with garlic in it spread on hairless human skin reduces mosquito bites. Are you rubbing garlic on your pets to deter mosquitos? If not, then this has nothing to do with feeding garlic to repel an entirely different type of parasite. I could just as easily say that because rubbing sunscreen on my arm prevents burns from the sun, it is logical to think that eating sunscreen will protect me from being burned by boiling water.

    “allicin/garlic kills the antibiotic resistant hospital super bug MRSA”
    In the test tube. So does bleach.

    I could go on, but the point here is that this list only shows how easy it is for people to draw inappropriate conclusions from research they don’t understand. The best these studies show, which is what I said in the article, is that chemicals in garlic might have some useful properties. If further research is done to show which chemicals, at what does, used in what way, and with what side effects, then we might someday develop useful products based on garlic. But extrapolating from these studies to what you are doing is totally illogical and illustrates how we have managed to spend most of human history, before the discovery of scientifici emthods, employing useless or dangerous medical therappies that never had anything like the impact on our health or life expectancy in thousands of years that science has in only a couple of centuries. It is far too easy to believe what we want in order to feel in control of complex things like health, but it doesn’t lead to good decisions.

  11. v.t. says:

    Shawna, is your vet, by chance, a ‘holistic’ vet?

    Do you have legitimate references to suggest that garlic in any form, is useful for coccidia, giardia, kennel cough?

    Also, since you foster, are you aware of the very high risk of transmission of parasites and other problems between those dogs and your unvaccinated (and otherwise unprotected) dog?

  12. Julie says:

    “A topical cream with garlic in it spread on hairless human skin reduces mosquito bites. Are you rubbing garlic on your pets to deter mosquitos? If not, then this has nothing to do with feeding garlic to repel an entirely different type of parasite. I could just as easily say that because rubbing sunscreen on my arm prevents burns from the sun, it is logical to think that eating sunscreen will protect me from being burned by boiling water. ”


    Heinz body anemia and methemoglobinemia are one of the first things new vets learn about in veterinary nutrition, pathology and clinical pathology classes. Heck, I just had a question on it in my clin path midterm yesterday. How any vet could go, "Well, bunk all that crap I learned in school. Raw garlic for all!" just blows my mind.

  13. rita says:

    “I think you meant to say “I would not recommend it”…” – indeed, good job we’re not Freudian commentators on this blog……..I sincerely hope!

  14. Brent says:

    I’m surprised nobody has commented on how well garlic protects against vampires. I personally prefer a crucifix – I’ve never been bitten by a vampire while wearing one.

    One problem with holistic miracle cures is that even where there is some evidence of some benefit, the purveyors are never able to quantify the effect, define the patient population that will benefit or offer a credible dose range. The true believers are supremely confident of their anecdotes and testimonials and consider the inability of skeptics to PROVE that their anecdotes are NOT due to the ________ (insert your preferred alternative treatment) as ‘evidence’ that their belief is accurate. They get angry because we can’t (or won’t) concur with their conclusions.

  15. skeptvet says:

    Yes, the shifting of the burden of proof from those who claim miracles to those who doubt them is a sly and all-too-effective strategy. It is important that we constantly remind people that if you claim something has healing powers, it is up to you to prove it and answer reasonable questions, not up to everyone else to prove you wrong or believe you unquestioningly.

  16. Catherine says:

    What annoys and confuses me is that well known, large, main stream pet food companies that have been in business since before I was born (and I’m not naming names!) will use garlic oil in their dog food formulas. I’m confused as to why they do it and I’m confused as to whether or not it is safe for the animals to eat without risking Heinz body hemolytic anemia because, even though it’s way down on the ingredients list, the dog would be eating it every single day. Isn’t there a cumulative effect? I trust you, SkepVet. Is it safe to feed a dog food that contains “garlic oil” to a dog? Will it, over time (like a lifetime) harm the dog in any way, or decrease its lifespan?

  17. skeptvet says:

    I believe the toxic effect is acute rather than cumulative because red blood cells are continuously recycled, with a lifespan of only about 120 days. So if a few cells get damaged by oxidative effects from the garlic, they are replaced pretty quickly. It’s only if the dose is high enough to damage a lot of red cells all at once that it creates a real problem, and the research suggests that probably only happens at pretty high doses. However, there are some individual reports of dogs experiencing toxic effects at much lower doses, so while the risk are probably pretty low for this level of exposure, they aren’t zero since some individuals might respond differently than most others. Overall, I don’t think it’s a big concern when used sparingly as a flavoring agent, I’m just not convinced there’s any proven health benefit.

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