Brain concentration

Brain concentration — also called CNS concentration — is the amount of a substance present in brain tissue at a given moment. It is distinct from plasma concentration, the level measured in blood, and the two figures can diverge considerably. A compound may circulate at high blood levels and still penetrate the brain poorly, or it may accumulate in brain tissue well beyond what blood levels suggest.

The brain is where psychoactive effects are actually produced, so brain concentration is the more direct predictor of what a person experiences. Potency measured in a laboratory dish means little unless the compound reaches the brain at a concentration that falls within the active range for its target receptors.

How it works · its role

Entry into the brain is controlled primarily by the blood-brain barrier (BBB), a tightly regulated interface formed by specialised cells lining the brain's capillaries. Several molecular properties determine how readily a substance crosses it.

Lipophilicity — fat-solubility — is the most influential factor: more lipophilic molecules dissolve into the cell membranes of the barrier and diffuse through. Molecular size also matters; smaller molecules generally cross more easily. Only the unbound fraction of a substance circulating in blood is available to cross, so tight binding to plasma proteins reduces brain penetration even for otherwise lipophilic compounds.

Some molecules face an additional obstacle: efflux transporters — particularly P-glycoprotein (P-gp) — actively pump them back out of brain capillaries. A substance can be lipophilic enough to enter the barrier and still achieve low brain concentrations because efflux keeps expelling it. The ratio of brain concentration to plasma concentration, sometimes called the brain-to-plasma ratio, is a standard way to quantify CNS penetration.

Relevance to substances & effects

Most well-known psychoactive substances achieve substantial brain concentrations because lipophilicity is what drives CNS activity. Serotonergic psychedelics such as psilocin and LSD, cannabinoids, and many sedative-hypnotics are fat-soluble enough to cross the BBB efficiently — which helps explain their relatively rapid onset after ingestion or inhalation.

Some substances exploit the barrier indirectly. Heroin (diacetylmorphine) is more lipophilic than morphine and enters the brain faster; once inside, it is converted to morphine, which binds opioid receptors and produces the effect. This two-step sequence — rapid CNS entry followed by local conversion — accounts for its faster, more intense onset compared with morphine at an equivalent dose.

The concept also explains deliberate pharmacological design. Peripherally acting opioids and second-generation antihistamines are engineered for low CNS penetration, limiting sedation or psychoactive effects while preserving peripheral action. Understanding brain concentration helps make sense of why chemically similar compounds within the same class can produce markedly different intensities or onset profiles.