Metronidazole
"I had a very severe reaction to the drug.  It started after two doses (1 day).  My skin erupted in small blisters.  That night the whole of my skin erupted with not only the blisters but also very large welts and extreme reddening of the skin all over my body all of which itched more than I have ever experienced before.  In the middle of the night I took a bath to try to alleviate the itching.  It didn't.  I then took an anti histamine which controlled the itching but only now after a week have the blisters finally started to disappear, even though I only took two doses of the drug.  It was an extremely unpleasant and frightening experience."