What medicine to give a dog for sprung leg?

With sprains, there will also be swelling, so aspirin would be your best choice, since it reduces swelling and relieves pain. The dosage of regular aspirin for dogs is 10 - 35 mg per kg of body weight (1 kg is approximately 2 1/4 pounds). So if your dog weighs 45 pounds, then his weight in kg would be approximately 20 kg. You would then multiply that times the dosage of 10 - 35 mg (one regular aspirin is 325 mg), but it's best to give the lowest effective dose, then give it every 8 - 12 hours with a bit of food. Start out with the lowest dose, then gradually increase it until you reach the dosage that is effective for your dog. So for a dog that weighs 45 pounds, the highest dose you could give him would be two regular aspirin, which would be 650 mg, since they come in 325 mg tablets.