The blood–brain barrier
Why many drugs never reach the brain, how some are designed to get in and others to stay out, and what changes when the barrier is inflamed.
Brain capillaries are lined by endothelial cells sealed together by tight junctions, supported by pericytes and astrocyte end-feet. Together they form the blood–brain barrier (BBB), which protects neurons from toxins and from swings in blood chemistry. It also keeps out most drugs, which matters both for treating brain disease and for avoiding central side effects.
How the barrier is built
Unlike capillaries elsewhere, brain capillaries have no gaps between endothelial cells and very little vesicle transport. Substances the brain needs, such as glucose and amino acids, enter through dedicated carrier proteins. The choroid plexus forms a separate blood–cerebrospinal fluid barrier.
Getting in: physical chemistry and carriers
Passive entry depends on lipid solubility, size, charge and protein binding. A second route is to borrow a nutrient carrier.
- Levodopa enters the brain on an amino acid carrier and is turned into dopamine there; dopamine itself does not cross
- Carbidopa blocks the conversion of levodopa outside the brain, so more reaches the brain and nausea is reduced
- Lipid-soluble anaesthetics and many sedatives cross quickly, which explains their fast onset
Being pushed out: efflux transporters
P-glycoprotein (ABCB1) and related pumps return many drugs from the endothelium to the blood. Loperamide is an opioid that normally stays out of the brain because of this pump; when it is combined with a strong P-glycoprotein inhibitor, or taken in very large amounts, it can cause central opioid effects such as breathing depression. Drug interactions at the barrier are one reason to check transporters (see drug transporters).
Keeping drugs out on purpose
Quaternary (permanently charged) molecules hardly cross the BBB. This gives useful peripheral-only drugs.
- Glycopyrronium and hyoscine butylbromide: antimuscarinic effects with little confusion or sedation, unlike atropine or hyoscine hydrobromide
- Methylnaltrexone: blocks opioid-induced constipation without reversing pain relief
- Pralidoxime: reactivates cholinesterase at peripheral synapses but works poorly in the brain
When the barrier changes
Meningitis, trauma, tumours and stroke disrupt the barrier. Some β-lactams such as ceftriaxone reach useful levels in the cerebrospinal fluid while the meninges are inflamed. Osmotic agents such as mannitol use an intact barrier to draw water out of the brain and lower intracranial pressure. The newborn barrier and the barrier in old age also differ, which helps explain age-related sensitivity to CNS drugs.
Test yourself
Think of the answer, then tap the question to see it.
Why is levodopa given instead of dopamine for Parkinson disease?
Dopamine cannot cross the blood–brain barrier, while levodopa is carried in by an amino acid transporter and converted to dopamine inside the brain.
Why does loperamide normally have no central opioid effects?
P-glycoprotein in the brain endothelium pumps it back into the blood before it can act in the brain.
What property keeps glycopyrronium and methylnaltrexone out of the brain?
They are quaternary amines with a permanent charge, so they cross the blood–brain barrier poorly.
Why do some antibiotics reach the cerebrospinal fluid better early in meningitis?
Inflammation loosens the tight junctions of the barrier, so more drug can pass while the meninges are inflamed.
Example drugs
- Levodopa + carbidopa
- Loperamide
- Methylnaltrexone
- Glycopyrronium
- Atropine
- Hyoscine butylbromide
- Mannitol
- Ceftriaxone
- Pralidoxime
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Levodopa and Carbidopa: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.