In Praise, and Defense, of Garlic

by Greg Tilford

Garlic is an amazing herb, and despite a long time misunderstanding of itspotential hazards, it remains as one of the most widely used botanicals in the pet care industry.  Garlic contains considerable amounts of protein, fiber, potassium, phosphorus, calcium, sodium, vitamin A, thiamin, niacin, taurine, zinc, riboflavin and dozens of other nutritive compounds— and a single clove of fresh garlic may contain as much as 100 sulfur compounds, all of which have been shown to possess medicinal qualities.

Most of us who read the ads and labels surrounding garlic preparations orsupplements at the health food store are continually reminded of allicin—  a volatile oil constituent of garlic.  Once believed to be the definitive factor in garlic’s healing abilities, we now know that allicin represents only a segment of garlic’s complex, medicinally-versatile chemistry.   This is not to say that allicin is


not useful; actually it is one of the most impressive, broad spectrum antimicrobial substances available in nature, with dozens of scientific studies to back up this claim.  Researchers have found that allicin may be more effective against harmful microbes than tetracycline; a frequently prescribed antibiotic drug2.  And, unlike conventional antibiotics, garlic works against many forms of virus, and won’t compromise populations of beneficial flora in the digestive tract when ingested in the appropriate amounts.

Despite its clear value as a healing agent, allicin is not the only healing agent in garlic worth considering.  In fact,  the presence of allicin in garlic preparations is not required at all in many situations where garlic may prove beneficial.  At least 30 other compounds contained in garlic have been shown to be useful for conditions ranging from skin disorders to cancer.

Allicin is a very unstable compound that dissipates very quickly when exposed to air, moisture, or heat.  This means that unless measures are taken to preserve the allicin content by one of several special processes, its presence in many preparations will be nil by the time it reaches the person or animal who needs it.  To confront this dilemma (and admittedly, to make some big money), several  garlic preparations which have been “standardized to allicin” are available on the market.  These extracts, powders, capsules, or tablets have had a certain percentage of allicin added in the laboratory to “guarantee their potency”.  Such formulas are safe and effective when used properly for specific, antimicrobial purposes, but are generally unnecessary and expensive for use in most other instances where garlic is indicated.  And, despite the label claims of many manufacturers,  there is really no way of telling if the allicin content in a standardized preparation still exists at the time of use.  Unless a laboratory analysis is performed after the product has reached store shelves, there’s no way of telling whether or not the allicin has  vanished from the formula.  Before you use a standardized formula, try to find out how the manufacturer can guarantee the allicin content in the product after it leaves their lab.  If their answer meets your satisfaction, then bear in mind that many of garlic’s other medicinal constituents may be absent or overpowered by an unnatural abundance of allicin, and that you will be using garlic in a manner beyond nature’s design.  Regardless of what the manufacturer might say, nature endows garlic with a purposeful limitation of allicin, grouped with hundreds of other compounds that serve unified purposes.  When we isolate a single constituent away from the whole plant, we are no longer working within a natural context, and we limit the healing potential of that plant to the confines of what we know, as opposed to what might be possible.  While science is beginning to understand how single chemical elements and compounds work in or on the body, we still know very little about how they work in a synergistic capacity.  In this realm, just beyond our understanding,  a great many healing secrets are waiting to be discovered.  Any good herbalist will tell you: the whole is always greater than the sum of its parts.

The chemical complexity of garlic is good news for the self-reliant herb user, because in addition to allicin, garlic contains a multitude of compounds that are very stable,  and easy to use for the do-it-yourself herbalist.   However, despite its widespread recognition as a healthy food for humans, garlic demands some added respect, caution, and therapeutic consideration if it is to be used effectively in the care of animals.  Here are three general rules of proper use:

1) The first rule of effective garlic use is to remember that allicin is essential in applications where garlic is to be used as a natural form of antibiotic, but may not be necessary if you are using garlic for general health maintenance or other purposes.

2) The second rule is to remember that if you wish to employ garlic in the capacity of an antibiotic, you will need to use raw garlic, or raw garlic juice  within three hours of chopping or pressing the fresh cloves;  or you will need a good garlic extract from a reputable source.  A properly dried garlic powder may be useful for internal antibiotic applications as well, even though only a residual trace of allicin remains in the powder until it is used.  In this case, two  compounds called alliin  and allinase meet with enzymes to form allicin as they enter the mouth.  The allicin then does its work within the body…  from the inside out!

3) If you decide to use garlic as a topical antibiotic, bear in mind that raw garlic juice is very, very strong, and may cause acute reddening and irritation to skin and mucous membranes if applied in undiluted form.  Cut the juice with some olive oil, vegetable glycerin, or water; at a starting  rate of one part of pure garlic juice to two parts of inert liquid (oil, water, etc.).  If irritation still occurs, dilute it more.  All of this can be avoided by infusing fresh cloves directly into olive oil— more on this in a minute.

4) Rule three—If you are looking to use garlic as a cancer-inhibiting/antioxidant agent, immune-enhancer, blood-thinning agent, cardiovascular tonic, or nutritional supplement, chances are any form of garlic will  bring the desired results.  Perhaps the only exceptions here are preparations of garlic which have been subjected to heat:  pickled, sautéed, boiled, roasted, or otherwise hyper-heated cloves have likely been depleted of their medicinal potential and a considerable percentage of their nutrients.

Used properly and in the correct form, garlic is valuable for treatment of virtually any form of internal or external bacterial, viral, fungal infection, including parasites (such as tapeworms) and protozoan organisms (such as Giardia).   Fresh garlic or properly dried powder (from a reputable market source) can be fed as part of your animal’s diet to fight infections of the mouth, throat, respiratory tract, stomach, or intestines.  In sheep, goats, and cows, it is said to help alleviate mastitis.  Freshly crushed garlic, or juice can be infused or diluted into olive oil for use as a topical antiseptic for minor injuries, ear infections;, or mites.  The rule here is to be sure the garlic is diluted sufficiently— the volatile oils are very strong and can cause burning irritation if applied to the skin in concentrated form.  Never… never  apply essential oil of garlic to any part of the body; it’s too concentrated.   And never use garlic preparations in the eyes.

To use garlic in topical applications, you don’t need much… just enough to impart a mild garlic odor to the oil.  To make a garlic oil, crush two or three cloves of garlic, wet them with vodka to help release the oils, and cover the mixture with four ounces of olive oil.  Shake it vigorously and let it stand in the refrigerator for an hour before using.  The oil should have an obvious garlic odor… but not overpowering.  Within three to twenty-four hours, the allicin will begin dissipating from your oil, and its usefulness for killing microbes will be diminished.  But don’t discard it— it still possesses immune-supporting,  disease-preventative qualities, and it can be added to your pets’ meals, one-half to one level teaspoon per pound of food per feeding; depending on its strength.  Keep your garlic oil in the refrigerator; in a glass jar with a tight-fitting lid.  On the safe side, expect it to keep for no more than a month.  Contrary to what many believe, garlic will not act as its own preservative, and old garlic oil may develop botulism— a bacteria that can be deadly to animals and people (remember – allicin diminishes quickly).  The added alcohol (vodka) is not in enough concentration to prevent botulism either, but 1/4 tsp. of vitamin E oil, added to the oil before it goes into the refrigerator, will help extend shelf life while adding a new element of nutritional support to the formula.  As you can see, all of this justifies the task of making your own oil—  many commercially-prepared oils which are produced from raw garlic afford us no way of knowing how long they will remain fresh and medicinally viable.

Scientific studies have shown that various compounds in garlic stimulate immune functions in the bloodstream at levels of activity that are unparalleled by any other herb (yes – even echinacea!).  Perhaps the most intriguing of these actions is garlic’s effect on the body’s natural killer cells; those which seek out and destroy cancer cells and invading microbes.  In a study conducted with human subjects who had AIDS,  garlic was found to increase killer cell activity three fold3.  Similar animal studies have been conducted with similar results.   Given the fact that these studies were done on subjects with depressed immune functions to start with, it stands to reason that companion animals with healthy immune functions may benefit from the added measure of immune support which is supplied through moderate garlic supplementation in their diets.

A 1988 study found that diallyl sulfide, a garlic constituent, prevented tumor formation in rats4, and several other studies have shown that garlic inhibits various forms of cancer growths in the body.  This may be attributable to the liver strengthening actions of at least six garlic constituents.   In this capacity, garlic gently enhances overall liver function, and triggers enzyme responses to help break down waste materials before they go into the bloodstream. In other words, garlic helps the liver at its job of cleansing the body, and thus helps prevent toxic accumulations that may lead to cancerous growths.  Garlic also helps to lower blood cholesterol and triglyceride levels, making it useful for Miniature Schnauzers,  Beagles, and other breeds that may be predisposed to hyperlipidemia — a condition which may lead to chronic seizures (JAVMA, Vol 206, No. 11, June 1, 1995).

Any form of uncooked garlic will perform these functions, and its use is as simple as sprinkling it onto the animals’ food— 1/8 to 1/4  of a teaspoon (powdered or fresh-chopped) per pound of food fed, once per day is usually a sufficient dose for most animals.

We have seen very good results from the use of garlic against tapeworms.  Used in powdered, fresh, or extract forms, garlic might not kill these persistent parasites, but it will make their living quarters much less desirable.  Intestinal parasites don’t like volatile oils or sulfur compounds, and if fed to an animal in correct dosages over a period of one to two months, garlic will help to drive these nasties out.  In my experiences with dogs, an increase in visible tapeworm segments in the animals’ stool will likely occur after two to three weeks of daily garlic ingestion.  After about two months, populations of these parasites are usually back to acceptable levels, or they may disappear completely.

Garlic works in a similar fashion against protozoan infections, such as Giardia, but in these cases you really need a strong presence of allicin to be effective.  Use fresh garlic, and add  Oregon grape  to your animals’ diet.  This will add an antimicrobial “double punch” against these tough and nauseating organisms.

Garlic is one of the best, all-around cardiovascular tonics in the plant world.  In studies conducted in collaboration with the New York Department of Health, a constituent found in both fresh and dried forms of garlic (called ajoene) was found to be very effective at preventing the formation of blood clots in the vascular system.  In some provinces of France, race horses suffering from blood clots are routinely fed garlic in their grain feed, and as a result, the clots sometimes disappear in a matter of days5.   In fact, many researchers believe that garlic may be as useful as aspirin in this capacity.  This is especially promising for use in cats, as they cannot tolerate the salicylate constituents of aspirin.  Garlic has also been shown to reduce cholesterol levels and the occurrence of atherosclerosis (fat build-up in the arteries) in both animals and humans, thus reducing the possibility of stroke or heart attack6.  All of these attributes, combined with the immune-supportive, liver-strengthening capabilities of garlic, make this herb an excellent multi-system tonic for older animals— especially dogs— who tolerate regular feeding of garlic better than cats do.

Garlic’s effectiveness as a systemic flea or mosquito repellent is the subject of a great deal of debate among those who have tried it.  Some people claim good results, others believe that the usefulness of garlic in this capacity is largely unfounded.  For more on fleas, see “The Fight Against Fleas”.


Cautions & Comments:            Although toxic side effects from consumption of garlic are rare in animals and humans alike, the possibility of harming your dog, cat, or herbivore with garlic does exist, and there is a growing controversy about how much garlic is enough and how much is too much.  At the root of this controversy is a dangerous misconception: the notion that more garlic is always better.   This is seriously  untrue.       Despite all of the grand attributes we have just described to you, moderation ; the cardinal rule of herb use, applies very strictly to garlic— particularly when used in cats.

When misused excessively or over an extended period of time, garlic may cause Heinz-body anemia ; a potentially life-threatening blood disease.  Scientists theorize that two chemical compounds contained in garlic may be attributable to this disorder:  S-methyl cysteine sulfoxide , and/or n-propyldisuldhide.  These compounds are believed to deplete a naturally occurring glucose enzyme called Glucose 6-phosphate dehydrogenase, (or G6PD … don’t worry,  there won’t be a test!).   G6PD has a special function of protecting the cell walls of red blood cells.  Depletion of G6PD  causes oxidative damage to the cells, thus forming “Heinz bodies”, and triggering the body to reject them from the bloodstream (usually via dark-colored urine).  If left unchecked, this process will continue until numbers of red blood cells are lowered to the point that the animal becomes anemic and eventually dies.

Fortunately, this nightmare is easily prevented with some common sense and a few precautions.   And, aside from the worst case scenario I have just described, other side effects of garlic are more predictable and less threatening:  digestive upset and gas when ingested (cut back on the dose and relieve the gas with chamomile); redness and irritation when applied to the skin (usually indicative of a preparation that is too concentrated).

First , it stands to reason that animals with pre-existing anemic conditions should not receive garlic internally; in any  quantity.  And— puppies don’t begin reproducing new red blood cells until after six to eight weeks of age.  Until then, they need every red blood cell they are born with, so a diet which includes garlic is out for young puppies.

In healthy adult animals, it’s important to know that the entire Heinz-body scenario is dose dependent— i.e.., the more garlic fed, the greater the chances of developing a problem.  While the question remains of exactly how much is too much, most recorded instances of Heinz-body anemia in animals involved the ingestion of large quantities of onions and other garlic relatives, many of  which are likely to contain much larger percentages of enzyme-depleting constituents than a typical dose of garlic.   Recorded cases of Allium  poisoning typically involve onion doses exceeding 0.5% of the subject animals’ body weight— this means that a healthy, 60 pound dog would have to eat a whole, five ounce onion, or several cloves of garlic, just to start the Heinz-body process.  And, since red blood cells are regenerated very quickly from the bone marrow, this grotesque overdose would probably have to be repeated several times on a frequent basis to cause permanent harm.   And, in further defense of garlic, several other foods can cause Heinz-body anemia as well— large amounts of turnips, kale, rape, or anything rich in vitamin K may lead to the disorder; especially in herbivores7.

Small doses of garlic added to your companion animals’ food, three or four days per week, perhaps 1/8 of a teaspoon of garlic powder per pound of food fed, is probably going to be of great benefit to the overall health of your pet.   Just don’t over do it!

Cats are much more sensitive to the side effects of garlic than dogs, so they require more caution and attention with its use.     Watch for digestive upset and  behavioral changes.  And if your cat simply doesn’t want any garlic, don’t force it.  Your cat’s behavior may be more than just a finicky attitude— animals know their needs better than we do.

It all boils down to common sense, moderation, and respect for garlic as more than just a table condiment.  Remember that no two animals are alike; one person’s miracle cure is another one’s poison. If you wish to use garlic in a therapeutic capacity, get to know your animal first, then consult a professional (or at least read the label) before you proceed.


  1. Block, Dr. Eric; State University of New York, Albany. “The Organic Chemistry of Garlic Sulfur Compounds”, First World Congress on the Health Significance of Garlic and Garlic Constituents, Washington D.C., 1990.
  2. Shashikanth, K.N., S.C. Basappa, and V. Sreenivasa Murthy. “A comparative study of raw garlic extract and tetracycline on caecal microflora and serum proteins of albino rats”  Folia Microbiol (Praha) 1984;29:348-52.
  3. Abdullah, T.H., D.V. Kirkpatrick, and J. Carter. “Enhancement of natural killer activity in AIDS with garlic” Dtsch Zschr Onkl 1989;21:52-53.
  4. Wargovich, M.J., C. Woods, V.W. Eng, L.C. Stephens, and K. Gray. “Chemoprevention of N-nitrosomethylbenzylamine-induced esophageal cancer in rats by the naturally occurring diallyl sulfide.”  Cancer Res., 1988 Dec. 1;48(23):6872-5.
  5. Block, Dr. Eric; State University of New York, Albany. “The Chemistry of Garlic and Onions”,  Scientific American, 3/25, (252:114).
  6. Jain, R.C., M.D.; Dept. of Pathology at the University of Benghazi, Libya, in his article in The Lancet British Medical Journal, May 31, 1975; p.1240 and Kritchevsky, David; Wistar Institute of Anatomy and Biology, Philadelphia, in his article in Artery, (1:319-23) 1975.

7) The Merck Veterinary Manual, seventh edition, Pg. 20 &25.