"i&#039;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 &amp; my acne was daily on my chin again. it&#039;s a great medicine, but hard on the stomach."