Trimethoprim
"After several bouts of urinary tract infections while awaiting prolapse surgery - bladder unable to void efficiently - I experienced extremely severe mouth ulcers on each occasion, lasting three weeks. I was told this was probably because I was "run down". However, most recently, half an hour  after taking the first dose of Trimethoprim I experienced intense burning of my lower lip, accompanied by extreme swelling. Recognising this as the start of the severe mouth ulceration I had experienced previously on numerous occasions, I informed my doctor, who immediately prescribed antihistamine and told me to stop taking the Trimethoprim, substituting another antibiotic.
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