Once I read that most metaldehyde-based slug baits have an extremely bitter compound added to deter pets, I stopped worrying.
When one slug eats a poisoned slug, it might get a harmful dose, if the metaldhyde is not metabolized as it works. That's a good thing!
My guess is that, since the iron phosphate bait is so low in toxicity that I almost never see a DEAD slug with it, that slugs eating iron-phosophate-poisoned slugs would probably have to eat several to feel any effect.
However, even if a cat eats a dozen metaldehyde-poisoned slugs, that's very unlikely to affect the cat at all since the cat is hundreds of times larger/heavier than the slugs and any poison works on a "mg per kg" basis.
That said, I know there are many people who just never use any pesticide. Applause to them, if they can make that work in their situation and still have a garden.
