Clindamycin
"This medicine gave me SEVERE heartburn immediately after taking the first 2 pills.  I thought it was the spaghetti, but figured out it was the medicine after the heartburn getting worse every time I took it.  I never ever have heartburn or acid reflux, but this medicine caused me unbearable pain and discomfort.  It's just been awful.  It's been a full day since I stopped taking it, but I'm still having some reflux, but that's nothing compared to how sore my throat is.  I can't swallow, eat, or drink without terrible pain. "