Use Retin-A exactly as your doctor has prescribed it for you. Follow all directions on your prescription label. Do not use this medicine in larger or smaller amounts or for longer than recommended. Do not take by mouth. Retin-A is for use only on the skin.
Definition of wall for English Language Learners : a structure of brick, stone, etc., that surrounds an area or separates one area from another : the structure that forms the side of a room or building
Definition of barrier. 1  1a : something material that blocks or is intended to block passage highway barriers a barrier contraceptiveb : a natural formation or structure that prevents or hinders movement or action geographic barriers to species dissemination barrier beaches drugs that cross the placental barrier.