Metronidazole
"Works well!!  Second time taking it and I'm getting all but 4 side effects.  Nausea is brutal. Light headed/dizzy.  Abdominal cramps. IBS flare! Moody. Headache, calf pain.....  etc....etc. 
Stopped today after calling the pharmacist.   Side effects are just too severe.  No problem the first time I used it. 
And nooooo booze when taking this! You'll get violently ill.  Seriously."