thank you for your question!
To better assist you, can you answer the following questions?
What breed of dog do you have?
When did you notice this
Any problem lifting his head?
Any history of trauma?
How much baby aspirin have you given? and how many times?
Let me know, looking forward to assist you.
120 pounds! that is big boy!
bad breath, and a bad tooth, good thinking, one of the possibility, could be also:
-Ear infection, painful ear
-Slip disk in neck, arthritis in neck. Slipped disk in neck tend to lateralize
-Anything else in mouth: foreign body stuck (piece of woods, anything else), tumor (sorry need to mention it)
halitosis (bad breath) can be due to periodontal disease, abscessed tooth, mouth mass, lesions (foreign body stuck), kidney failure.
Best for sure is to check with your primary care veterinarian. They will examine him and let you know if they see what is going on in the mouth and check the neck, ears.
Let me know if you have more questions, or need additional information.
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Thank you for your question.