Ice cream, of course, contains milk. And just like humans, some dogs (not all) are lactose-intolerant. If you feed milk to a lactose-intolerant dog, he may experience diarrhea, painful gas, or vomiting. Of course, ice cream is a treat, not a diet staple, so start by giving your dog no more than a spoonful, and wait for a day to see if he has any negative response to the frozen dessert. As long as he does not, he should be able to enjoy a prudent amount as an occasional treat.