Permeability

Permeability describes how readily a molecule crosses a biological membrane — whether that membrane is the lining of the gut, the wall of a capillary, or the tightly regulated blood-brain barrier. It is a property of the molecule's interaction with a physical barrier, which makes it distinct from solubility (how well a compound dissolves in a fluid) and lipophilicity (how strongly it favours fatty over watery environments), even though all three are connected.

A molecule can be highly lipid-soluble and still cross a membrane poorly — because it is too large, carries too many hydrogen-bonding groups, or becomes electrically charged at body pH. Permeability integrates these factors into one practical question: does the compound get through?

How it works · its role

The most common route across a membrane is passive transcellular diffusion — the molecule dissolves into the lipid bilayer on one side and diffuses out the other. For this to work, the molecule must be small enough, uncharged, and lipophilic enough to partition into the membrane. Most psychoactive substances travel this route.

Ionization complicates the picture. Most drugs are weak acids or weak bases, existing partly in a charged form and partly uncharged depending on local pH. At body pH of 7.4, only the uncharged fraction dissolves readily into the lipid bilayer. The balance between forms — governed by the molecule's pKa — directly determines how much drug can cross at a given site.

Some molecules are actively carried across by transport proteins, or pumped back out by efflux transporters such as P-glycoprotein. Efflux can substantially reduce how much drug reaches its target even when passive permeability looks adequate on paper.

Relevance to substances & effects

Permeability is one of the chief determinants of whether a drug reaches the brain at all, and how quickly. THC is highly lipophilic and crosses the blood-brain barrier readily, which contributes to its rapid onset when inhaled. Heroin crosses faster than morphine because its acetyl groups increase lipophilicity; enzymes inside the brain then cleave those groups to release active morphine.

The prodrug pair psilocybin and psilocin illustrates the same tension. Psilocybin's phosphate group aids water solubility but hinders membrane crossing; gut and plasma enzymes cleave it to yield psilocin, the more permeable form that reaches the CNS.

Route of administration interacts directly with permeability. Inhalation and intravenous injection bypass the gut lining entirely, delivering drug straight into circulation. Oral dosing requires surviving the gut environment and crossing the intestinal epithelium first — a stage where many compounds are lost before they ever reach the bloodstream.