For a healthy dog eating raw or dehydrated foods which still have natural enzymes, I personally don’t think enzymes are necessary (they definitely aren’t going to hurt and if you have the extra money go for it, but I don’t think it’s a necessary expense). For a healthy dog eating quality kibble, I’d probably give half the recommended dose of an enzyme supplement. I think all dogs can benefit from probiotics, but unless the dog is having serious digestion issues I think some plain yogurt or (better yet) kefir or raw green tripe is enough to help keep the gut flora in balance. My dogs eat all raw and get green tripe nearly every day and about 1/3 c. kefir (the kefir I use has 12 strains of probiotics) every other day. I don’t add any supplemental enzymes or probiotics. When Gus was eating kibble I did give him 1/2 the recommended dose of Flying Bassett’s digestive enzymes daily and some plain yogurt or kefir a few times a week, but I quit giving this when I switched to THK and raw. I do like the medizym supplement Sandy posted a link to – GREAT for arthritis.
