Brushing the teeth , flossing , undergoing dental work , or eating crunchy foods such as potato crisps relatively soon before or after giving fellatio can also increase the risk of transmission , because all of these activities can cause small scratches in the lining of the mouth . Brushing the teeth , flossing , undergoing dental work , or eating crunchy foods such as potato chips relatively soon before or after giving fellatio can also increase the risk of transmission , because all of these activities can cause small scratches in the lining of the mouth .