Grapes and raisins have an unknown toxic factor in them that can cause acute renal failure in dogs. In too high doses, they can be fatal. It seems weird that something we humans can enjoy does that, but our pets can’t tolerate Tylenol either, where we can. Our metabolisms are different. And, because we don’t know what it is in the grape that is poisonous, and why one dog will be affected but another one not, it’s confusing. Grapes seem like the perfect bite sized-treats, but they just are not safe. Since raisins are just little grapes, no raisins either. Want more weird? Grapes and raisins don’t seem to be a problem for cats and horses (at least, not that we know of, yet).