Top 10 Doctor insights on: Cefdinir Vs Amoxicillin For Strep. 1 How long for Cefdinir to work on Strep in a 2 yr old? Amoxicillin ineffective for two days, switched to cefdinir last night, still very fevered & weak. Fever: strep throat is extremely sensitive to antibiotics, weather it is amoxicillin or cefidnir. If your child is not responding, it means that there is something else your child has on top of strep like a viral infection.