Substantially solid dental bleaching or treatment compositions in the form of an insert for placement within a dental tray. The compositions comprise a substantially solid dental bleaching composition that has increased adhesiveness to teeth when moistened with saliva or water. The inserts are horseshoe shaped and may be either flat or contoured. The substantially solid dental bleaching composition becomes more adhesive when moistened with saliva or water, yet remains intact and coherent after the dental bleaching composition is placed within a dental tray and over a person's teeth during bleaching (or other treatment).