Metronidazole
"It's day 3 and I am feeling like death. I woke up feeling like I had the flu; I took a shower because I thought it would help me ease my muscle pains but it mostly just hurt to stand under the water. The first 2 days weren't AS bad, I had mild headaches and slight nausea, fatigue, and diarrhea. But today I have been napping all day. I'm a bit annoyed that my doctor didn't warn me of the side effects ahead of time. I ended up calling and they told me "a lot of people say that the antibiotics are more painful than the thing they treat. It's known to have very severe side effects." Gee, thanks. I just woke up from a nap with an unbearable headache that I almost decided to go to the ER. Overall worst medication I've ever taken."