Metronidazole
"I took this med along with cipro. First day... I broke out in hives. I knew it was this one cause it was put thru iv and hives came out as soon as it was connected. So, they kept giving me benedryl along with it. Then..on the 3rd night of being in hospital, I was having so much pain (I cried the whole night) in my lower back and legs, thought it was the hospiral bed..so a I went home anyway. Took oral and back pain was unbearable!  I stopped taking it and back pain was completely gone. Will never take that again!!!"