Sulfamethoxazole / trimethoprim
"I took one for the first time for a urinary tract infection and about 3-4 hours after taking I had rapid heart beat, shaking, diarrhea and nausea.  I took a Xanax because I mistook this for a panic attack but the Xanax did nothing to help and I started to get a strange flushing, burning skin sensation.  The sensation happens in waves over my skin, very strange like nothing that has ever happened to me.  Called Dr.s office and was told it was an allergic reaction, been taking Benadryl all day but still feeling the effects over 2 hours later!"