nausea/vomiting
"I had a severe migraine, was driven to the hospital when regular pain relievers did not work.  I was given IV Benadryl along with an anti-nausea medicine and fluids. I felt sleepy almost immediately, and my pain was lessened.  The huge down side was that I felt a huge amount of anxiety for 48 hours afterward.  I do not suffer from anxiety normally. I believe this was an adverse reaction of the Benadryl. "