My dog just ate half of a large bag of M&Ms! What should I do?!?

If they were dark chocolate I would say call the vet at least. Since its milk chocolate and not nearly enough for a serious medical issue just be prepared for a mess. Either when your dog pukes or it comes blasting out the other end. Make sure he's got plenty of water. If you think he may have eaten any of the bag call your vet and at least speak with them.