A dental examination before bleaching is very useful so that any unknown problems with the teeth to be bleached can be identified and treated. Otherwise, the bleaching gel could penetrate the tooth enamel excessively and cause pain. Existing fillings, veneers and crowns should also be assessed, as they could possibly interfere with the later, lighter shade. In general, the teeth are first cleaned before bleaching so that all deposits are removed and the bleaching medication can reach the tooth surface.