While marshmallows are not poisonous to dogs, they are not exactly a health food. A marshmallow is basically just sugar and gelatin, neither of which are particularly good for your dog. In small quantities, marshmallows are not likely to cause any serious problems for your dog. However, if your dog eats a lot of marshmallows, it could end up with a tummy ache or diarrhea. So, while your dog may beg for a taste of your s'more, it's probably best to give him a doggy treat instead.