[Chris]

Chris asked the Naked Scientists:
   
Medical shows dramatically talk of the brain blood barrier. Is this real as injected drugs seem to slip right past it. Keep up your excellent standards

Chris of Cardiff

What do you think?

[Bored chemist]

Not all drugs cross it.
Some of the newer "non-drowsy" antihistamies used in threating hayfever, for example, are non drowsy simply because they don't reach the brain.
This can be a blessing (as in the hay fever pills) or a nuisance- some antibiotics don't cross the barrier well so they don't work for treating infections of the brain.

As a general guide, very polar molecules won't cross it (with the exception of molecules that the brain's membrane mistakes for glucose- it has to transport glucose in specially as fuel for the brain.

[thedoc]

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