Most discomfort after teeth bleaching starts when peroxide passes through enamel & reaches the inner part of the tooth. That can briefly irritate the nerve & make cold drinks, sweet foods or even air feel sharp. The effect is usually temporary, but it is more noticeable when enamel is already worn, gums have receded or small cracks & untreated decay are present. That is why whitening works best after the mouth is checked as well as existing problems are addressed first.