If side effects from flagyl do occur would it happen after the first dose or after a few doses? i'm really scared about side effects happening.

Flagyl side effect: The usual side effect is some nausea/stomach upset, which could happen after the first dose, or later, impossible to predict. Rarely does it cause a true allergic reaction like a rash. Two options for you: if for genital infection, you can get a topical form that is inserted directly into the vagina (voila no side effects). Or ask the doc for nausea medicine if you HAVE to take the pills.