Azithromycin
"I went to the ER after I coughed up blood a few times thru the day, and I was diagnosed with acute bronchitis. I was prescribed the zpack 5 day regimen. 
After a couple of days I began feeling a little better, but by the 5th or 6th day, I felt worse. I had severe nausea and became anorexic to the point that I lost 14 pounds. It took about another 4-5 days until I could eat more than a bite or so of food at any sitting. 
It's been around a month and a half since this happened, and I'm still having issues with my appetite and weight. I'm still down the 14 lbs, and my energy flags easily. I'll never EVER take that med again due to such an experience. I've had fine success with other antibiotics, without them being this hard on my gut."