bacterial infection
"I was prescribed 300mg, 4 times a day for 10 days for a tooth abscess. I too felt the "burn", but my burn was immediately after taking the capsule.  I could feel the burn as it went down my esophagus.  What I found relieved it was drinking at least 8oz of water with the pill, and then topping it off with a few spoonfuls of yogurt.

No more burn as it went down, no more stomach upset."