Teeth sensitivity. Increased sensitivity is the most frequent side effect of teeth whitening procedures. The dentin of your teeth is responsible for the sensitivity you feel during the whitening procedure. Dentin is the layer that lies behind your enamel (the white layer covering your teeth). If the intensity of tooth sensitivity persists, contact your dentist. Gum irritation. When the whitening gel comes into touch with your gums, it can cause irritation. This discomfort usually goes away within 24 to 48 hours. It will persist if the gel comes into contact with the gums.