Metronidazole
"I'm 4 days into a 7 day treatment for suspected giardia - 400mg 3 x per day. 46 yrs old. No previous giardia or metronidazole. 
Giardia symptoms started improving within 48hrs. Nausea gone. Stomach pain gone. Diarrhea gone. Low fever gone. But these pills really knock me down. About 40 mins to an hour after taking the pill, I get tired, jaded, thick headed for 2-3 hours. Anxiety too. 
I'm really looking forward to finishing the course. And I really hope they work!"