Fortunately, we haven't learned this the hard way, but it seems that the artifical sweetener used in sugar-free gum is highly toxic for dogs. I read about this in a rather badly produced newsletter from some or other doggy bunch, and raised my eyebrows. But I have checked several websites on the subject and it seems that it may well be true.
This blog seems to be fairly credible, and, while it is not as sensationalist in its prognosis as some of the sites I encountered, it does sound a sober warning:
Dogs that eat significant amounts can develop a sudden drop in blood sugar, which can cause weakness, lethargy, loss of coordination, collapse and seizures.It goes on to say:
Recent studies also suggest that there is a strong link between xylitol ingestion and the development of liver failure in dogs.No doubt it is also toxic for cats, but cats are not the scavengers that dogs are - and they tend not to eat some of the unspeakable things that dogs consider a legitimate source of nutrition. Unless there is something meaty to be retrieved, a cat is unlikely to be found with its head in the wastepaper basket. And of course, those of us who chew gum after a meal or after coffee, are likely to dispose of it in the wastepaper basket - perhaps wrapped in a tissue or till slip - where it is likely to be found by a nosy dog.
Of course, we are all supposed to teach our dogs not to eat anything we haven't given them permission to eat, but, until they've learned that lesson, it might be a good idea to dispose of gum some other way.