Clinical Psychopharmacology · Paper A
Drug half-lives
Dose changes can look safe on paper while timing still makes them risky. Half-lifeThe time it takes for the amount of a drug in the body to fall by half.Read more → is the clock behind two exam decisions: when a drug has settled after a dose change, and when enough has washed out to switch safely. A short half-life creates fast level changes and withdrawal risk; a long half-life creates slow washout and interaction risk. So the first question is not the dose, but how quickly the concentration halves.
For first-order elimination, half-lifeThe time it takes for the amount of a drug in the body to fall by half.Read more → is the time for plasma concentration to fall by 50 percent. It is proportional to volume of distributionA calculated value describing how widely a drug spreads into body tissues rather than staying in the blood.Read more → and inversely proportional to clearanceThe rate at which the body removes a drug from the blood.Read more →, often summarised as half-life = 0.693 x volume of distribution / clearance. Repeated dosing reaches practical 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 about four to five half-lives, and stopping a drug takes a similar number of half-lives for most elimination. Fluoxetine is the classic antidepressant example of a long half-life with an active metaboliteA breakdown product of a drug that is itself still able to produce an effect in the body.Read more →, norfluoxetine, so discontinuation symptoms are less common but washout before an irreversible monoamine oxidase inhibitor must be long. Paroxetine and venlafaxine have shorter half-lives and are more prone to discontinuation symptoms.
questions are really testing whether you can predict two practical risks from a single number: how dangerous it is to switch between drugs, and how bad the symptoms will be if a patient stops abruptly. The same half-life value drives both, which is why the exam returns to it again and again across different drug classes.
Half-life (usually written t½) is the time it takes for a drug's concentration in the blood to fall by half. If a drug has a half-life of 24 hours and you stop taking it, the level is 50% at 24 hours, 25% at 48 hours, 12.5% at 72 hours, and so on: halving each time that fixed interval passes.
Many candidates think half-life only matters for overdose or toxicity. In practice, its two biggest exam uses are switching drugs safely (working out how long to wait before starting a new, potentially interacting drug) and predicting discontinuation symptoms (how abruptly levels fall if a dose is missed or the drug is stopped).
Each half-life removes half of whatever is left, not half of the starting amount. So after one half-life, 50% remains; after two, 50% of that 50% remains, which is 25%; after three, 12.5%; after four, 6.25%; after five, about 3%. Because 3% is clinically negligible for most drugs, five half-lives is treated as the point a drug is essentially "washed out" of the body. The exact same logic runs in reverse for starting a drug: it takes about five half-lives of regular dosing to reach steady state (the point where the amount coming in each dose roughly equals the amount being cleared, so blood levels plateau).
Take fluoxetine (an antidepressant), which has an unusually long of around 4 to 6 days, PLUS an active by-product called norfluoxetine with an even longer half-life of 4 to 16 days. Multiply norfluoxetine's half-life by five and you get a washout period of several weeks: this is why guidance recommends waiting roughly five weeks after stopping fluoxetine before starting an irreversible monoamine oxidase inhibitor, or (an older class of antidepressant that blocks the enzyme monoamine oxidase). Starting the MAOI too soon risks (a dangerous excess of serotonin activity causing agitation, fever, and muscle rigidity), because enough fluoxetine and norfluoxetine can still be circulating.
Now compare paroxetine, an antidepressant with a short of about 24 hours. Take a patient who simply forgets two doses: paroxetine's level falls fast because there is no long-acting active by-product to cushion the drop, and within a day or two she can develop dizziness and electric-shock-like sensations often described as "brain zaps": classic discontinuation symptoms. Fluoxetine's long half-life gives it a built-in, self-tapering effect, so missed doses rarely cause noticeable discontinuation symptoms at all.
Most drugs follow first-order kinetics, clearing a constant FRACTION of what remains per unit time: this is what makes the concept above work cleanly and consistently. A smaller group of drugs follow zero-order kinetics, clearing a constant AMOUNT per unit time regardless of concentration, because the enzyme system that clears them becomes saturated. Alcohol, phenytoin (an anticonvulsant) at high dose, and high-dose aspirin are the classic zero-order examples. In zero-order kinetics, a small increase in dose can cause a disproportionately large, unpredictable jump in blood level, since the machinery simply cannot speed up to keep pace.
Do not confuse half-life with clearance (the volume of blood effectively cleared of drug per unit time): clearance is a rate, half-life is derived partly from it but also from (how widely a drug spreads into body tissue versus staying in blood). For example, a fat-soluble drug can have a long half-life purely because it is stored widely in fat and released slowly, even if the liver clears it quickly the moment it re-enters the blood: the exam occasionally tests which mechanism applies to a named drug.
There is no test that "diagnoses" a half-life problem directly; instead, clinicians recognise it from the timing of symptoms relative to dosing. Discontinuation symptoms appearing within one to three half-lives of a missed dose point to a short-half-life drug; toxicity appearing after starting a new serotonergic drug too soon after stopping a long-half-life one points to inadequate washout. Therapeutic drug monitoring (blood level testing) for drugs like lithium and phenytoin also relies on understanding half-life to time when a sample reflects a true steady-state level.
The Maudsley Prescribing Guidelines recommend explicit cross-tapering or washout schedules based on a drug's whenever switching between antidepressants, and BNF guidance advises the five-week fluoxetine-to- washout described above as standard practice. NICE guidance on depression also recommends warning patients about discontinuation symptoms before stopping short-half-life antidepressants such as paroxetine or venlafaxine, and tapering the dose gradually rather than stopping abruptly.
If a stem describes a drug switch or a patient stopping medication, first ask whether the OLD drug has a long or short . Long half-life predicts a self-taper with a long washout requirement before a new interacting drug; short half-life predicts abrupt, uncomfortable discontinuation symptoms needing a slow taper.
sets both how long a drug takes to reach a steady level and how long it lingers afterward, so a long half-life demands a long washout while a short half-life demands a slow taper.