My yellow lab was bit on the side of the muzzle last summer by a copperhead, most likely (we have some rattlers around, but copperheads are more common by far). Her head swelled up so much her eyes just looked like slits. I took her to the vet and he just recommended we watch her for signs of trouble breathing, and gave me some steriod pills to help with swelling and an antibiotic. She was a little lethargic for a couple of days and the swelling went down over the course of a week or so, but she never acted like she was in any trouble or pain. You could see where the two fangs penetrated because the hair fell out around each of them.
Unless the fangs penetrate a blood vessel and directly inject the venom into the bloodstream or if your dog has an allergy to one of the components of venom, the vet said that most dogs will recover just fine on their own. You will know if that is the case because the dog will very quickly either die or get into serious distress after the bite. The main thing that the vet was concerned about was cellulitis (an infection that sets up right under the skin and can spread really rapidly), like can occur with dog bites.
