system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the blood-brain barrier so important in medicine?
gpt
The BBB is a *very selective* physical filter within the capillaries that carry blood to the brain.  This filter blocks 99% of all molecules in drugs from reaching the brain, only allowing in the very smallest of molecules.  Creating a drug that does what you need it to in the brain while at the same time being tiny enough to make it through the barrier is very difficult.