What should I do if my dog ate Lego? If your dog has eaten a single, small Lego piece, it is unlikely to do any harm and you should monitor your dog for the next few days to make sure they are eating and passing poos normally. You may even see the Lego brick come out. If your dog has eaten a larger, more complex Lego piece, or several pieces, these pieces of plastic may cause a blockage. If you are in doubt, its best to see a vet.