I haven't found a recommended amount for regular garlic supplementation, but I can share what I do. I give my dog's about the equivalent of 1/16th of a teaspoon every other day. I do every other day because I alternate with Apple cider vinegar. So I do 1/16th tsp garlic one day, then 1 tablespoon ACV (with mother) the next day, and alternate between the two. Been doing this for a year and a half and haven't had any flea or parasite problems to speak of in all that time. And my dogs' bloodwork has come back normal when we've had them tested to be sure we weren't doing any harm.
My mother in law took my advice to try something like what I do on her own dogs. She does garlic every day, unlike me, and hasn't had any health issues in the approximately 6 months that she's been feeding it. Although she seems to use more garlic than I do and sometimes her dog's don't want to eat their food because it's too strong.
Garlic and onion are from the same family or something like that. Onions are toxic so avoid that. Garlic is toxic in large quantities, but it is actually healthy and beneficial in smaller quantities. Hope that helps.
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~~Val~~