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Usually I do not recommend the use of human pain medications in dogs for long term pain control. Firstly, there are safer alternatives that are specifically made for dogs that are usually better for pain control, and secondly because aspirin (while o.k. for pain control) can commonly lead to adverse side effects in dogs, especially gastrointestinal ulceration. However, that being said, if you need to use aspirin for pain control until your dog can see a veterinarian so that they can be prescribed a safer alternative, you can give the dose as posted below. The posted dose is that for a 17 pound dog. Also, please make sure to get the ENTERIC COATED aspirin, as this will help to reduce the occurrence of gastrointestinal ulceration.
Aspirin 81mg (enteric coated) - Give 1 tablet by mouth twice daily
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To follow up on your feedback,
I would NOT recommend giving Ibuprofen, but instead using a buffered aspirin for dogs. Ibuprofen is not a well tolerated medication in dogs.
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