Metronidazole
"I was on 250 mg 3X a day for one week to treat a giardia infection I have likely had for years without detection.  My doctor will also have me on a three-day course in about two weeks, in keeping with the cyst cycles.  

I just finished the course and had no adverse side effects.  I took them in the middle of fairly-sized meals and made sure to drink at least a gallon of water a day, along with eating a clean diet.  Perhaps it was the lower dose or one of those measures, but I did not even feel fatigued during the course of my medication. Know that not everyone has terrible (or any) side effects. I delayed taking these after reading so many accounts of awful experiences, but I imagine there are plenty of others that have no side effects."