Metronidazole
"I began taking Flagyl on Saturday night to treat diverticulitis.  I could not sleep at all that night or the following three nights.  I lay in bed wide awake, with my leg muscles randomly fasciculating.  I had a headache and neck ache from not being able to sleep.  Occasionally, my entire body would suddenly jerk while I was lying in bed.  I could not concentrate at work, and even though I am a Pharmacist, I kept forgetting the names of medications that I had been taking and the names of the medical conditions with which I had been diagnosed.  I had been taking ibuprofen for a few days the prior week, and I had stopped taking ibuprofen due to heartburn.  Even though I was on Protonix to treat my heartburn, Flagyl significantly worsened my pre-existing heartburn to the point where I had to stand erect throughout the day and night, moaning with pain.  My epigastric pain would begin about an hour after taking Flagyl and be at its worst four hours later.  I also felt as if I constantly had to urinate, but only a small amount of urine would come out each time.  My mouth was extremely dry with a metallic taste and a furry looking tongue.  I ended up in the ER the following Wednesday writhing in epigastric pain which was a “10” on the 1-10 pain scale.  I literally thought that I was going to die, and I spoke with loved ones by phone to say my good-byes.  My blood pressure was moderately elevated and my ECG, which had always been normal, was now “borderline with atrial enlargement”.  My ER doc switched me to Augmentin, and my Flagyl-related adverse reactions rapidly subsided.  Flagyl significantly helped my diverticulitis, but at a horrible cost in terms of lost sleep, lost work, poor work, personal and family anxieties, and time spent at the ER.  If your doctor prescribes Flagyl, my advice is to ask him/her if there is an alternative antibiotic, especially if you have any pre-existing gastroesophageal problems.
All of my adverse events appear in the Flagyl labeling:  epigastric distress, abdominal cramping, incoordination, confusion, insomnia, a sharp, unpleasant metallic taste, dryness of the mouth, furry tongue, glossitis, stomatitis, flattening of the T-wave in electrocardiographic tracings, polyuria, a sense of pelvic pressure, convulsive seizures.  "