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With the use of aspirin, we generally suggest you not use it for more than a couple days without consulting your regular vet. We also suggest it be given with food to avoid upsetting the stomach.
Most vets do feel rather comfortable prescribing a medication for their patients over the phone, especially if the pain is something that's been a chronic issue or if the dog has been seen recently in the office.
My suggestion would be to call your regular vet in the morning and ask if they think aspirin is the way to treat this injury in your dog.
I hope this helps.