Pharmacokinetic interaction

A pharmacokinetic interaction occurs when one substance changes how much of another reaches systemic circulation, or how long it persists in the body — by altering its absorption, distribution, metabolism, or excretion. The result is a plasma level that is higher or lower than the dose alone would predict, with a corresponding shift in effect and risk.

This distinguishes it from a pharmacodynamic interaction, where two substances act on the same receptor or physiological system. Both types can operate simultaneously: one substance may alter another's metabolism while also sharing its molecular target, compounding the overall effect in ways that are harder to anticipate.

How it works · its role

Most pharmacokinetic interactions work through metabolism, specifically through the cytochrome P450 family of liver enzymes (CYP enzymes). These proteins break down the majority of psychoactive substances into water-soluble forms the body can excrete.

A CYP inhibitor reduces enzyme activity, slowing the breakdown of any drug the enzyme normally handles. That drug accumulates to higher plasma levels than expected. A CYP inducer does the opposite — it accelerates enzyme production, increasing clearance and lowering plasma levels, sometimes enough to substantially reduce a drug's effect.

Interactions can also occur elsewhere in the chain: at absorption (altered gut motility, transporter competition), at distribution (displacement from plasma proteins changes how much free drug circulates), or at renal excretion (urinary pH shifts alter clearance of some compounds). Enzyme-mediated metabolism remains the most clinically significant route for most psychoactive substances.

Relevance to substances & effects

Several widely used psychoactive substances are potent CYP inhibitors or inducers. Monoamine oxidase inhibitors (MAOIs) block the enzyme that clears many compounds, dramatically raising their effective plasma levels — a well-established source of serious combinations. Fluoxetine and paroxetine are strong CYP2D6 inhibitors that slow the breakdown of co-administered drugs sharing that pathway; both remain pharmacologically active for days to weeks after the last dose.

Cannabidiol (CBD) inhibits CYP3A4 and CYP2C9, which affects the metabolism of a range of medications. St. John's Wort is a potent CYP3A4 inducer, relevant whenever it is taken alongside substances cleared by that route.

Because pharmacokinetic interactions operate invisibly — the administered dose does not change, only what the body does with it — their effects are frequently underestimated. Specific pairings on substance pages record whether one substance raises or lowers the effective level of another; those entries, not this definition, carry the severity assessments and their sources.