The brain is surrounded by a special layer that protects it from infection. This layer is made up of a network of blood vessels with thick walls. It is hard for bacteria and viruses to get through the thick walls. It is also hard for some substances (like anti-cancer drugs) to get past the walls and into the brain.
For more about breast cancer medications and how they work, see Breastcancer.org's Drugs for Treatment and Risk Reduction pages.
Audio contributed by: Shirley Jones