Flagyl Side Effects: I took the metronidazole for about four days. I went to bed one http://www.medications.com/effect/view/30109 I took the metronidazole for about four days. I went to bed one night and woke up with the worst shoulder pain possible, it feels like I just had surgery on it. it's been this way for about 3 days, so I am stopping the medication. I have muscle neck pain and heart flutters due to this med. I sto Mon, 05 May 2008 20:04:57 -0400 http://www.medications.com/effect/view/30109?item=7966 I have muscle neck pain and heart flutters due to this med. I stopped my medication after 4 days. This is day eight of feeling like this after stopping med. http://www.medications.com/effect/view/30109?item=7966