Once you have whitened your teeth, you may feel some sensitivity. However there are ways to relieve the feeling of sensitivity to your teeth. If you brush your teeth before you whiten your teeth and not after, you would be reducing the amount of sensitivity your teeth feel. This is because whitening your teeth opens up the pores of your teeth, and brushing your teeth straight after whitening causes the pores of your teeth to be opened up even more exposing the dentin which causes sensitivity. Your teeth contain two layers an outer layer called enamel and an inner layer called dentin and thats where the nerves in the teeth are and if exposed will cause sensitivity.