I've taken both Zithromax and Flagyl. Flagyl was probably the most nauseating drug of all of the ones I took, but everyone is different. It did cause me to herx, so it seemed to be doing some good, but I couldn't tolerating the vomiting. There's no way to know how you'll react until you try it. I don't recall what dose I was taking, but it was a pulsing schedule, so the dose was probably high.