First of all, I could have done a whole lot more for this poor dog had he been brought to me months ago. Arthritis is a relentlessly progressive condition in dogs, and once it occurs, arthritic destruction of cartilage and remodeling of bone cannot be reversed. However, the progression of the disease and the pain that accompanies it can be dramatically reduced in most cases IF TREATED EARLY. Okay, that's not the topic of this blog, so...about that aspirin...
Aspirin in dogs is an effective pain-reliever at the proper dose. In fact, I would prescribe it for some of my patients 20-25 years ago. You can still buy "doggie aspirin" with the dose listed on the bottle. But unfortunately, aspirin has a very high potential to cause more problems than it helps. The number-one risk is gastric ulceration, which can cause death from hemorrhage or stomach perforation. I have seen it happen. Aspirin can also cause kidney failure, especially in older pets (which are the ones most likely to receive it).
Now we have newer drugs which are similar to aspirin, but much safer and more effective. These drugs can also cause serious side-effects, but their risk-profile is muuuuuch safer! The problem is, if our patient has been given aspirin for any recent length of time, it changes the dog's ability to handle the safer drugs. What this means is, if you give your dog more than one dose of aspirin before bringing him to see me, you have effectively tied my hands from giving him something much better and safer.
Dogs who have received previous aspirin therapy should go through a 5-7 day "washout" period of no medication, before we can give the good stuff. During this time, the dog has to suffer with the pain that we could be treating had the dog not been given aspirin.
As a side-note, aspirin is quite toxic in cats, except at super-low doses and prolonged intervals. Never give aspirin (or Tylenol) to a cat without specific instructions from a veterinarian. You know what? Let's just not give any medication to our pets without first checking with the vet. DVMs know what is safe, and at what dose. Use us! That's what we're here for.