acne
"I've been on doxycycline for about 10 years at 100 mg, twice a day. It works great at clearing up my cystic acne that I get on my chin.  About six months ago I ended up with ulcers in my stomach and my Digestive Health Specialists thought it was the doxycycline causing it. Stop taking it for 4 weeks & my acne was daily on my chin again. It's a great medicine, but hard on the stomach."