Thank you for your reply.
It does not sound that the beer was tampered as it would have caused him to have some food poisoning symptoms such as diarrhea or nausea/vomiting. Also, if anyone drinks contaminated beverage or eats contaminated food, it takes hours (sometimes up to 24 hours) before infection sets in.
Drinking beer was coincidental to his abdominal pain, however, beer could have reacted with Suboxone (even though it was only 3 ounces) as it is highly advisable not to drink any alcohol during Suboxone treatment.
Since his pain is quite severe and is disabling, it would be advisable for him to go to a local urgent care center/ER for evaluation (which may include blood work and abdominal x-ray).
Burning sensation in the abdomen is mostly associated with acid reflux, however, possibilities such as stomach ulcers, gallstones, and pancreas inflammation also need to be ruled out.
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Best wishes and good luck,