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
barrier noun [ C ] (FENCE) B2 a long pole, fence, wall, or natural feature, such as a mountain or sea, that stops people from going somewhere: Barriers have been erected all along the route the Pope will take. The mountains acted as a natural barrier to the spread of the disease.