The whole idea behind garlic is that it when in the bloodstream internal parasites don't thrive.
Then as garlic is in the blood external bloodsucking parasites wont be there either.
It works too, the secret is getting it in to the dog.
I feed it raw by chopping in fine and mixing it in their food. Some people buy it concentrated.
I also put it in their water as I do my ferrets, canaries and fowls water.
Mind you everything still gets it's routine worming.
Garlic is not a cure all I just use it in my day to day routine.