For what it's worth, I was on Doxycycline, 100 mg BID, about four or five years ago, for the treatment of adult acne. It worked great and my face cleared up. I was on it for about two years and then out of the blue I developed tumors in my left lung. The condition is called BOOP. I don't recall the whole name to the acronym, but it's known as Organizing Pneumonia. I had surgery and luckily it was beign. I had done extensive reading about it and one of the main causes of this is long term use of antibiotics. I stopped taking the Doxy prior to surgery.
That surgery was a little over two years ago. Last week I had a colonoscopy to try to find the source of pain I had in the lower left quadrant of my abdomen. They found an infection called Clostridium Difficille. I am now on a a very potent antibiotic called Vancocin (250 mg, QID) for 14 days. I am also on Methadone for pain. I have been on the Meth at 180 mg for about two years. Cause of the C-Diff (acronym for my infection) is long term use of antibiotics. Another health issue I have had due to the long term use of Doxy.
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One thing I forgot and that's the use of Differen. Both my wife and I used it and my wife still does. It works great and we've experienced no problems from it.
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I don't mean to scare you, but you might want to research this a little bit. Also, ask your doctor. If he/she doesn't know about it, don't be surprised. Mine had never heard of it either. Scarey!