Great question — and it’s an important distinction.

The BBB (blood–brain barrier) is just the entry point. It tells you whether a drug can physically get from the bloodstream into brain tissue.

The CNS (central nervous system) is the whole territory — brain, spinal cord, and the surrounding fluid spaces.

So:
• BBB penetration = the ability to cross into the brain.
• CNS penetration = how well a drug actually distributes throughout the entire central nervous system once it’s inside that protected compartment.

They’re related but definitely not the same thing.
A drug can cross the BBB but still have limited reach into the spinal cord or CSF. And some mechanisms can influence CNS activity without needing deep penetration into all regions.

Easiest way to think about it:
BBB = getting through the door.
CNS penetration = how far you can move around once you’re inside.

Hope that clears it up.

Kat