Ranitidine
"I took this after omeprazole (both OTC and prescription) started to lose its efficacy (and after my insurance decided ranitidine was the better/cheaper option).  It gave me pretty severe flu-like symptoms almost immediately both times I took it, including dizziness, nausea, stomach pains, drowsiness, and even confusion.  Eating and drinking still hurt, so this didn't work very well for me."