Apparently, garlic is toxic to dogs in large doses:

Misc: Onions, especially raw onions, have been shown to trigger hemolytic
anemia in dogs. (Stephen J Ettinger, D.V.M and Edward C. Fieldman, D.V.M. 's
book: Textbook of Veterinary Internal Medicine vol. 2 pg 1884.) Also: "Six
Cases of Heinz Body Haemolytic Anaemia Induced by Onion and/or Garlic
Ingestion" - CM Edwards and CJ Belford Aust.Vet.Prac. 26 (1) March 1996,
18-22.

Also:

Walnuts are poisonous to dogs and should be avoided. In particular, there is
a type of fungus common to walnuts (especially wet deadfall walnuts) that
will cause severe episodes of seizuring. Many nuts are not good for dogs in
general, their high phosporous content is said to possibly lead to bladder
stones.

However, the doses commonly used for fleas, etc. obviously aren't causing
acute toxicity. All the same, I'd keep the doses low.

Susan