Be sure to change your toothbrush, or toothbrush head (if you're using an electric toothbrush) before the bristles become splayed and frayed. Not only are old toothbrushes ineffective, but they may harbor harmful bacteria that can cause infections such as gingivitis and periodontitis. Toothbrushes should be changed every three to four months. Sick people should change their toothbrush at the beginning of an illness and after they feel better.