The way the garlic supplement works is that when your horse sweats, the horse produces an odor that bugs and flies do not like. So, it becomes a natural fly repellant. As an additional bonus, garlic is a natural decongestant, so it can help with allergies, coughs, and runny noses.
Garlic contains sulphur and selenium as well, which are both necessary for good horse health. Sulphur is helpful in treating and preventing certain equine diseases because it has blood cleansing properties. Selenium is necessary to help protect cell membranes.
While it's yummy, garlic is rich in selenium and sulphur, which are both necessary in small amounts, but can be deadly in excess. Feeding too much garlic can cause anemia in your horse. This is because there is a toxic component called N-propyl disulfide that changes the horse's blood. This toxin changes an enzyme in the blood cells that causes damage by oxidation (there's more chemistry involved, but this is the short version!).
Typically garlic fed in small amounts, so anemia is mild, but if fed in large quantities, it could be dangerous. A 1972 study found that the toxic dose in horses is less than 5 grams per kilogram of body weight.
So what is the deal with these garlic supplements that are on the market?
Garlic supplements that are commercially prepared and sold through supplement companies, typically are safer for horses to eat. This is because it is not usually raw garlic that is sold, unlike what is purchased in grocery stores. Commercially prepared garlic supplements are heat dried, which causes the dangerous active ingredient, allicin, to be destroyed.
If you feed your horse garlic, do your research and fed a reputable garlic supplement and make sure you are consulting a veterinarian.
Keep an eye out in your pasture for wild onion. Wild onion contains the same chemical in garlic that causes anemia and jaundice. Eating just 1 pound of wild onion per day for 11 days caused horse's red blood cell count to decrease nearly 60% of his red blood cells (information from the 1972 study).