Why is it so difficult to deliver drugs to the brain via the blood? Certainly blood circulation is an integral part of brain function. For example strokes are so dangerous because the blood supply is not reaching part of the brain.
So what is this Brain Blood Barrier (BBB) that so far prevents delivery of most medications to the brain? Wikipedia has, as often, an excellent entry on the BBB topic . In a nutshell, the “barrier” is more like a filter, filtering out bacteria and other large molecules. The Generic Look encyclopedia has a good layman entry on the BBB .
Certainly the BBB isn’t perfect and some nasty brain bacterial infections, such as meningitis, can occur . How does certain pathogens defeat the BBB is not clear and involves complex physical chemistry. [4,5]. But clearly if the certain bacteria and viruses can traverse the barrier, researchers should be able to find a way to send across medication too.
What if medication were riding on very small “nanoparticle” size entities to penetrate the BBB? ( a nanometer is one billionth of a meter). Such research is ongoing  “…The plant virus, cow pea mosaic virus (CPMV), has recently been explored as a nanoparticle delivery system for therapeutics targeting a number of diseases including cancer and neurodegeneration…”
Other types of nanometer range size vectors, or drug carriers, are being explored, such as polymers 
Additional ways to bypass the BBB for MS treatments include Intranasal Delivery : “”…intranasal delivery to the CNS, with a discussion of pathways from the nasal cavity to the CNS involving the olfactory and trigeminal nerves, the vasculature, the cerebrospinal fluid, and the lymphatic system…” Sounds like going through the backdoor of the brain to me, as opposed to breaking through…Keep in mind that nasal delivery is not as simple as it seems. The OptiNose Company has an interesting video describing a nasal delivery system.
Talk to your Doctor about this exciting ongoing research.
 Blood Brain Barrier; GenericLook Encyclopedia
 Trends in Microbiology; “How do extracellular pathogens cross the blood-brain barrier?” 2002 May;10(5):227-32
 Science ; “”Meningococcal type IV pili recruit the polarity complex to cross the brain endothelium” ; 2009 Jul 3;325(5936):83-7. Epub 2009 Jun 11
 Journal of NeuroImmunology; “Viral nanoparticles associate with regions of inflammation and blood brain barrier disruption during CNS infection”;
 BMC Neuroscience; “Nanotechnology approaches to crossing the blood-brain barrier and drug delivery to the Central Nervous System”
Expert Opinion drug Delivery ” Polymeric nanoparticles for the drug delivery to the central nervous system ”
Journal of Pharmaceutical Sciences; “Intranasal delivery to the central nervous system: mechanisms and experimental considerations”; J Pharm Sci. 2010 Apr;99(4):1654-73