Metronidazole
"I have been taking this medicine for four days now and have had nausea, and insomnia.  However, last night after taking my medicine in the early evening, my heart started beating rapidly and my chest felt very tight.  I took my emergency inhaler just to be sure my asthma wasn't acting up (hasn't acted up in years). My breathing got a little better afterwards but my heart rate was still up there. I will be consulting my physician today. I'm sure it's a great medicine."