Plasma concentration

Plasma concentration is the amount of a drug dissolved in blood plasma — the clear, cell-free fluid that makes up roughly half of blood volume — at a specific point in time. It rises as a substance is absorbed into the bloodstream, peaks, and then declines as the body metabolises and eliminates it. Charting that rise and fall produces a concentration–time curve: the central measurement tool of pharmacokinetics.

Several values are read from that curve. Cmax is the peak concentration reached; Tmax is the time at which it peaks; AUC (area under the curve) captures total drug exposure across a dosing period. Half-life — the time for concentration to halve — determines how quickly a substance clears and whether it accumulates across repeated doses.

How it works · its role

After entering the body, a substance is absorbed into the bloodstream and distributed to tissues. Plasma concentration rises while absorption outpaces elimination, then falls as elimination takes over. The liver and kidneys handle most clearance, breaking down and excreting the drug along a predictable exponential decay.

Plasma concentration is not the same as concentration at the site of action. To affect the central nervous system, a drug must cross the blood–brain barrier, which introduces a delay and a degree of separation between what is measurable in blood and what is happening in the brain. This is why subjective effects can peak after plasma concentration does — and why plasma level and felt intensity don't always move in lockstep.

Relevance to substances & effects

Route of administration shapes the plasma concentration curve more than almost any other variable. Intravenous injection bypasses absorption entirely, producing an immediate, high peak. Inhalation approaches this speed for volatile substances. Oral dosing is the slowest: the drug must survive digestion and first-pass liver metabolism before reaching systemic circulation, which lowers peak concentration and delays its arrival.

The same substance by different routes can produce meaningfully different experiences even at the same total dose. Smoked or insufflated substances reach a sharp, short-lived peak; the oral route produces a lower, broader curve. Subjective intensity tends to track the shape of that curve — not just the area beneath it.

Substances with long half-lives accumulate with repeated dosing, so plasma concentration can climb over days even when each individual dose feels routine. Interactions that slow hepatic clearance — such as inhibition of CYP enzymes — can raise plasma concentration above expected levels without any change in dose. Where that pharmacokinetic evidence exists, the interaction data on these pages reflects it.