Pack includes four replacement brush heads Exclusively designed to fit Oral-B iO electric brush handles Advanced iO technology, with over 4,000 extra-soft bristles, removes 100% more plaque for 100% healthier gums in just one week High density dentist-inspired extra soft bristles provide a gentle and effective clean Optimized bristle patterns adjust to the contours of teeth and gums for ultimate comfort, even for sensitive teeth and gums iO refill technology fades the bristles from blue to white to show when it's time to change the brush head Not suitable for children under age of three Dentists recommend replacing your brush head every three months for the best results