Inhibitor

An inhibitor is any molecule that reduces or blocks the activity of its target — an enzyme, receptor, or transporter — by binding to it. The term covers a wide range of mechanisms and targets, but in pharmacology it most often refers to two categories: enzymes that process drugs in the body, and membrane proteins in the brain that carry signalling molecules between neurons.

Understanding inhibition is practical because it underlies most pharmacokinetic drug interactions. When one substance inhibits an enzyme that another depends on for clearance, the second substance accumulates to higher levels than expected — a mechanistic chain with sometimes significant consequences.

How it works · its role

Inhibition takes several forms depending on where a molecule binds. A competitive inhibitor occupies the same active site as the enzyme's normal substrate, blocking it by direct competition; its effect weakens as the substrate concentration rises. A non-competitive inhibitor binds elsewhere on the enzyme and distorts its shape, reducing activity regardless of how much substrate is present.

Some inhibitors are reversible — the blocking molecule eventually detaches and the enzyme resumes normal function once it clears. Others are irreversible, forming a lasting bond that permanently disables the enzyme until the body synthesises new copies. This distinction matters: irreversible inhibition can outlast the drug itself by days or weeks.

For transporters — proteins that carry neurotransmitters back into the releasing neuron after they fire — inhibition prolongs the time the neurotransmitter spends in the synapse, amplifying its signal.

Relevance to substances & effects

Inhibition is the mechanism behind most of the drug interactions flagged across these pages. CYP enzymes in the liver are the primary clearance route for a wide range of substances. Inhibiting them slows breakdown of anything that shares the same pathway.

Substances cleared by CYP2D6 or CYP3A4 — a list that includes many opioids, stimulants, and antidepressants — can reach higher plasma concentrations when taken alongside a potent inhibitor of those enzymes, even at the same dose. This is why combinations that appear pharmacologically unrelated can produce unexpectedly strong effects.

Reuptake inhibitors act on the brain rather than the liver, prolonging neurotransmitter activity at the synapse. SSRIs inhibit the SERT (SERT); cocaine and many stimulants inhibit the dopamine and norepinephrine transporters. When substances that inhibit the same transporter or enzyme pathway are combined, their effects stack. The interaction data on each substance page reflects where that stacking becomes clinically meaningful.