Clinical Psychopharmacology · Paper A
Pharmacokinetic order (first-order vs steady state)
Most psychotropic drugs follow first-order kinetics: a constant fraction, not amount, of drug is eliminated per unit time, so plasma concentration approaches a plateau, steady stateThe point at which a drug taken regularly reaches a stable level in the body, because the amount going in equals the amount being cleared.Read more →, after roughly 4 to 5 half-lives. This interval depends only on half-lifeThe time it takes for the amount of a drug in the body to fall by half.Read more →, not on dose size. For example, imagine fluoxetine, whose active metaboliteA breakdown product of a drug that is itself still able to produce an effect in the body.Read more → persists 1 to 2 weeks, being dose-adjusted after only three days. In practice, this means checking the level too early is misleading, since steady state has not yet been reached.
By convention, steady stateThe point at which a drug taken regularly reaches a stable level in the body, because the amount going in equals the amount being cleared.Read more → is reached after approximately 4 to 5 half-lives, at which point roughly 94 to 97% of the eventual plateau concentration has been achieved. A small number of drugs, classically phenytoin, instead show zero-order (saturation) kinetics: hepatic enzymes become saturated, so a constant amount, not fraction, is eliminated per unit time regardless of concentration. Take a patient on phenytoin whose dose is raised by a small, seemingly safe increment. That small increase can cause a disproportionate, unpredictable rise in level and toxicity, since the elimination pathway is already saturated.
When a patient starts a regular dose of most psychotropic medications, the drug does not immediately sit at its final, stable blood level. Instead, the plasma concentration (the amount of drug measured in the blood) climbs with each dose and gradually approaches a plateau, known as steady state (the point at which the amount of drug entering the body with each dose roughly equals the amount being eliminated).
For example, imagine fluoxetine (an antidepressant whose persists in the body for one to two weeks) being dose-adjusted after only three days of treatment. Checking a level, or judging response, that early is misleading, because has not yet been reached; the concentration is still climbing toward its eventual plateau, not reflecting the dose's true eventual effect.
The timing to reach depends only on the drug's half-life (the time it takes for the concentration to fall by half), not on the size of the dose being given. Increasing or decreasing the dose changes where the eventual plateau sits, but not how long it takes to get there: a bigger dose reaches a higher steady-state concentration, but takes the same number of half-lives as a smaller dose of the same drug. Confusing "how high" with "how long" is the single most common error on this topic.
Most psychotropic drugs follow first-order kinetics: a constant fraction, not a constant amount, of the drug is eliminated per unit time. This means the rate of elimination depends on how much drug is currently present: more drug in the body means a larger absolute amount cleared, but the same proportion. By convention, is reached after approximately four to five half-lives, at which point roughly 94 to 97% of the eventual plateau concentration has already been achieved.
Loading doses offer a way to bypass the wait for entirely: a larger initial dose is given to reach a therapeutic concentration immediately, rather than waiting several half-lives for gradual first-order accumulation. This approach is used for drugs such as digoxin, where a rapid clinical effect is necessary and waiting days for accumulation would be clinically unacceptable. or inhibition (drugs, foods, or conditions that speed up or slow down the liver enzymes responsible for metabolising other drugs) shifts the eventual steady-state concentration over days to weeks, layered on top of the ordinary timeline. Suppose a patient who has been stable on a drug for years is then started on an enzyme-inducing medication: their steady-state concentration will drift downward over the following days to weeks, even though their underlying half-life and prescribed dose have not changed at all, because the change comes entirely from faster .
A small number of drugs, classically phenytoin (an anticonvulsant also relevant in psychiatric practice), instead show zero-order, or saturation, kinetics near the therapeutic range. Here, the liver's metabolising enzymes become saturated, meaning they are working at their maximum possible rate, so a constant amount, rather than a constant fraction, of the drug is eliminated per unit time regardless of how much drug is present. This is the key look-alike to distinguish from ordinary first-order behaviour: for example, imagine a patient on phenytoin whose dose is raised by a small, seemingly safe increment: because the elimination pathway is already saturated, that small increase can cause a disproportionate, unpredictable rise in the drug level, and potentially toxicity, in a way that would never happen with an ordinary first-order drug.
In practice, kinetic order is inferred rather than measured directly. The clinical clue for zero-order behaviour is exactly that disproportionate response: a small dose increase producing an unexpectedly large rise in drug level or signs of toxicity, which should immediately raise suspicion of saturation kinetics rather than an ordinary proportional first-order response. Plotted on a graph, a first-order drug's concentration falls off in a curve over time that becomes a straight line when plotted on a logarithmic scale, while a zero-order drug's concentration falls in a straight line even on an ordinary scale, since a fixed amount, not a fixed fraction, is cleared each unit of time. Correctly timing when to check a level matters just as much as recognising which kind of kinetics is at play: requesting a level before roughly four to five half-lives have passed produces a misleadingly low result that does not reflect the eventual steady-state concentration.
These concepts directly shape safe prescribing and drug-level investigation in everyday practice. The BNF (British National Formulary) provides standard dosing intervals and monitoring advice that are built on exactly these kinetic principles, even where the underlying calculation is not spelled out to the prescriber. Ongoing, closer monitoring is especially important for zero-order drugs like phenytoin, precisely because their concentration-dose relationship is unpredictable rather than proportional, so dose changes are made more cautiously and in smaller increments, with levels checked more frequently than for an ordinary first-order drug. Requesting a drug-level investigation before has been reached, or misinterpreting a rising level in a patient on a zero-order drug as a simple, proportional dose effect, are both classic beginner errors that this management approach is designed to avoid.
The high-yield trap is assuming that a bigger dose reaches faster: it does not; only the determines timing, always around four to five half-lives regardless of dose size. The second trap is treating every drug's dose-response relationship as proportional: for phenytoin specifically, a small increment near the therapeutic range can produce a disproportionate, dangerous rise because the elimination pathway is already saturated.
Most psychotropics follow first-order kinetics and reach after four to five half-lives regardless of dose size, but a few drugs like phenytoin follow zero-order kinetics near the therapeutic range, where small dose increases can cause disproportionate, dangerous rises in level: recognising which pattern applies is what actually keeps monitoring and dose titration safe.