Hopefully not. It is probably digested by now, so you would have seen signs of something by now. Surprising that a dog would swallow a pill on its own, as most dogs eat around pill if shoved in their food when they are supposed to take them. Google search name of prescription and possible side effects. If your dog shows these signs, he may just suffer for a bit, like joint pain, headache, nausea, let it pass. If after 12 hours and your dog is getting worse, get him to a vet. Or call to see what can be done. Make sure he has plenty of water available. Put a dash of salt in his water for him to stay hydrated and hopefully drink more to flush medication out of his system.