I am on my second series of flagyl as the symptoms came back. Now my neck is so stiff that I can hardly move it. I saw this listed as a side effect of Flagyl. I'm scared of stopping the flagyl because then I have to take the Vancomycin which is so expensive. I so hate having to return to diarrhea or pay $2000. for Vancomycin, How common is neck pain and will it go away eventually while taking flagyl.