I've been prescribed biaxin XL before and had no serious side effects. However, I was recently prescribed this antibiotic on 1000mg once a day for seven days. I've experienced diarrhea, vomitting, pain in the chest, sleeplessness. The worst side effect is the nasty taste in my mouth...and the constant vomitting to the point where I can barely swallow my own spit. Totally disgusted with this medication.