Blood–brain barrier
One way to get medications into the brain is to inject them into the cerebrospinal fluid (CSF).  CSF helps protect the brain and spinal cord.  However, it does not actually touch the brain.  Instead, two of the layers that protect the brain and spinal cord hold the CSF.  Because of this, even if medications are injected right into the CSF, they still do not get into the brain very well.