Most effective is a drug called toltrazuril which actually kills coccidia...Corid and sulfadimethoxine set it back until the chicken's immune system can catch up and fight it off. You can get toltrazuril in a powdered form called Endocox at Jedd's Bird Supply (maybe other places, but that's where I found it.) I know about toltrazuril from the dog show/breeding world, where it has revolutionized puppy raising with respect to coccidiosis and was very happy to find it can be used in chickens as well.