Metronidazole
"It's only day 2 of this medication (and I haven't taken my morning dose) and I am in tears, feeling panic-ridden, and sick to my stomach. About an hour after taking the pill I have severe headaches and the worst nausea I've ever had. I'm not looking forward to continuing the rest of the pills but from the reviews people generally seem to agree that it works, it's just an awful experience getting through the medication. Will update when complete meds."