If you ask on an internet pet group whether garlic is safe, you will be hit with a mostly negative response, mostly telling you how toxic garlic is and that it causes heinz anemia (hemolytic anemia) and this comes from a widely mis-understood study. The problem with studies and the general pet population is that the average pet parent does not know how to interpret the information so they may mis-interpret the information or they will not be able to validate what someone else is saying - the latter being the most common outcome.
This internet mis-truth about garlic is so wide spread that it is considered truth, even by some Vets. The study that is widely mis-interpreted was released 20 years ago, way back in November 2000 and it was called "Hematologic changes associated with the appearance of eccentrocytes after intragastric administration of garlic extract to dogs" (1). The objective of this study was to determine whether dogs given garlic extract developed hemolytic anemia and to establish the hematologic characteristics induced experimentally by intragastric administration of garlic extract. 8 healthy adult mixed-breed dogs were used. 4 dogs were given 1.25ml of garlic extract/kg of body weight (5g of whole garlic/kg) intragastrically (tubed into the stomach) once a day for 7 days. A clove of garlic weighs around 5g, therefore, for a 20 kilo dog, they SUPER dosed them with the equivalent of around 20 cloves of garlic per day for 7 days. So over the course of 7 days, they fed them 140 cloves of garlic - WOW! It's hard to understand what the point was of this study because this isn't an amount any pet parent would feed their dog or an amount you could probably get a dog to eat even if you tried!
Image: per day representation for a 20 kilo dog.
Even at these extremely high levels, garlic did not cause hemolytic anemia in dogs or cats: "Results: Compared with initial values, erythrocyte count, Hct, and hemoglobin concentration decreased to a minimum value on days 9 to 11 in dogs given garlic extract. Heinz body formation, an increase in erythrocyte-reduced glutathione concentration, and eccentrocytes were also detected in these dogs no dog developed hemolytic anemia".(1) The study is also incredibly vague, only 4 dogs were used and it's a 20 year old study - this sample size isn't a size that would be respected in any study - we don't even know what sized dogs they used in the study. If they were small dogs vs large dogs then this is a variable that would be important to consider - A Great Dane vs Chihuahua or even a standard laboratory dog being a Beagle are likely to have different outcomes. You would think as study to learn about the safe upper limits of garlic would have been more productive.
Let us fast forward to 2018 where another garlic study(4) was conducted, this time on aged garlic. Overview: Plants of Allium spp., including garlic (A. sativum) and onions (A. cepa), are known to be oxidatively toxic to canine erythrocytes resulting in Heinz body hemolytic anemia in dogs. In humans, these plants have been used as medicinal agents for multiple diseases since ancient times. Especially, fresh garlic extracted over a prolonged period produces less irritative and odorless aged garlic extract (AGE), containing unique and beneficial organosulfur compounds that can help prevent many kinds of diseases. In this study, the safety and efficacy of long-term oral administration of AGE is evaluated in dogs. The objectives are to confirm the safe dosage for long-term use and beneficial functions of AGE for dogs and to plan and design a canine health supplement or a preventive agent for multiple diseases based on the data of this study.(4)
Results: Beagles were orally administered AGE (45 or 90 mg/kg body weight once a day) or an equivalent amount of water as control for 12 weeks. In AGE-treated groups, at 12 weeks post-administration at a dose of 90 mg/kg, there were no observable changes in the clinical signs, complete blood count, and serum biochemical parameters. Heinz bodies and eccentrocytes, the markers of oxidative damage in erythrocytes, did not appear in blood smear examination. In order to further evaluate the beneficial effects of AGE on health of dogs, the expressions of nuclear factor erythroid 2-related factor 2 (Nrf2) gene (NFE2L2) and Nrf2-regulated phase II antioxidant enzyme genes (NQO1, GCLM, HMOX1, and SOD2) were determined in whole blood between pre- and post-AGE administration. The expression of NFE2L2 gene was significantly upregulated in the AGE-treated groups [45 (p < 0.05) and 90 mg/kg (p < 0.01), 8 weeks] as compared to in the control group. Among the Nrf2-regulated enzymes examined, the expressions of NQO1 [45 (p < 0.05) and 90 mg/kg (p < 0.01), 8 weeks] and GCLM [45 (p < 0.05) and 90 mg/kg (p < 0.01), 12 weeks] genes were significantly upregulated.
So, in this study, they used significantly less garlic and a different type (AGE) - Dose was 1800mg (1.8g) of aged garlic for a 20 kilo dog. Keep in mind that 1 clove of garlic is around 5g. The conclusion of this study was that: The long-term oral administration of AGE at a dose of 90 mg/kg/day for 12 weeks did not show any adverse effects in dogs. Furthermore, the administration of AGE upregulated the gene expressions of canine Nrf2 and Nrf2-regulated phase II antioxidant enzymes. These results suggest that AGE might safely contribute to the health of dogs provided that the appropriate dosage is used. It is a bit tricky when they refer to the "appropriate dosage" as they didn't test upper limits - the amount tested is quite conservative compared to what is generally recommended by holistic Vets and the likes.
We need to always ask the question "How much" - at what point does the medicine become the poison because pretty much anything can be toxic in higher amounts i.e. Water is being toxic to humans at 90g/kg body weight.
So, how much should you feed your dog?
Well, you can ask 200 different professionals and you'll get 200 different answers and that is because there haven't been any studies to explore the safe upper limit. Here are what some of the main players say:
When using powder, 1/8 of a teaspoon is roughly equivalent to 1 clove. In closing, you do not need to be scared of garlic, garlic is used in pet foods, pet supplements and as medicine by many Holistic Vets. Use in "regular" amounts - a common sense approach. Start off with a very small amount so you do not offend your dog. Check with your Vet first and feel free to share with this this article to help them with their research. --- (1) Lee KW, Yamato O, Tajima M, Kuraoka M, Omae S, Maede Y. Hematologic changes associated with the appearance of eccentrocytes after intragastric administration of garlic extract to dogs. Am J Vet Res. 2000 Nov;61(11):1446-50. doi: 10.2460/ajvr.2000.61.1446. PMID: 11108195.
(2) Mercola.com. 2020. 6 Human Foods That Are Toxic And Potentially Fatal For Pets. [online] Available at: <https://healthypets.mercola.com/sites/healthypets/archive/2017/03/25/foods-that-cause-pet-deaths.aspx?v=1606275965> [Accessed 29 November 2020]. (3) Mercola.com. 2020. Parasites Gone Wild - Draw An Invisible Barrier Around Your Pet. [online] Available at: <https://healthypets.mercola.com/sites/healthypets/archive/2017/05/14/fleas-ticks-seasonal-allergies.aspx> [Accessed 29 November 2020]. (4) Yamato, O., Tsuneyoshi, T., Ushijima, M. et al. Safety and efficacy of aged garlic extract in dogs: upregulation of the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway and Nrf2-regulated phase II antioxidant enzymes. BMC Vet Res 14, 373 (2018). https://doi.org/10.1186/s12917-018-1699-2