Dentist recommend that you change your tooth brush once in 3-4 months. The first signs of fraying of the bristles is an indication to change your brush. If you are using a powered tooth brush, then it is recommended that you change the head once in 3-4 months. Never share you tooth brush with others. You should also remember to change your tooth brush after recovering from an illness or after your routine scaling procedure or after a gum surgery. This is advisable so that the bacteria and germs are not carried to the post recovery phase.