Penicillin works for strep but it's important to be patient and finish the whole course. When I was on penicillin for my strep throat, I thought that it was not working because my symptoms of strep throat did not lessen until day four of treatment.
My fever went down by the second day but it took another two days for my throat to start feeling better. By day ten though, I was feeling great. It's a good thing that I didn't stop taking it.