If your dog eats a tiny amount over some time, he should be fine. Keep in mind that too much sugar can be toxic for your dog as it may lead to gastrointestinal upset. Some ingredients like xylitol can also be dangerous, even when consumed in small amounts. If your dog falls sick after eating marshmallow fluff, take him to the vet immediately, especially if the symptoms don’t go away after 24 hours. You might also need to keep marshmallow fluff out of your dog's reach, especially if he has a preexisting condition like diabetes, as it can worsen their condition.