system
Given the following post, respond as if you're commenting on Reddit:
human
What prevents certain drugs from permeating the blood brain barrier?

I hear about drugs not being able to reach the brain because they're filtered out by the blood brain barrier. What's going on to prevent this in certain drugs? Is it the size of the chemical or how it binds with proteins in the blood?
gpt
The size of the drug molecule primarily. Also, the BBB is extremely selective and only allows transport of very few molecules, mainly water, glucose, amino acids, and other substances that are critical for the brain.