Our young boxer had a partial torn ligament in her back knee. Our vet said to give her one buffered aspirin a day, saying she could have that for the rest of her life if necessary. I found more information online about restricting movement, non enteric coating, glucosomine, etc. She gradually improved. Then she injured her knee again, so I started the regiment once again. She didn't handle the buffered aspirin so well that time, so we had to tough it out. :o(
Some dogs don't handle it as well as others, like people.