Pharmacokinetics, pharmacodynamics and individual variation

ADME, half-life, steady state, dose adjustment, age/frailty/genetics/physiology.

Pre-prescribing foundation: this module develops knowledge and judgement for formal prescribing education; it does not authorise prescribing.

• Explain absorption, distribution, metabolism and elimination.
• Relate half-life and steady state to accumulation and monitoring.
• Distinguish loading dose, maintenance dose and dose interval concepts.
• Recognise how age, frailty, organ function and interactions change exposure.
• Use patient variation to challenge standard-dose assumptions.

Absorption, distribution, metabolism and excretion determine exposure, but the clinical meaning depends on organ function, age, body composition, genetics and interacting medicines.

Half-life and time to steady state affect onset, accumulation, washout and monitoring.

Renal and hepatic impairment can increase toxicity or reduce efficacy depending on drug and metabolite handling.

Loading doses, maintenance doses and dose intervals solve different pharmacokinetic problems.

1. Define the clinical problem and whether prescribing is indicated.
2. Gather the patient-specific assessment and diagnostic information needed.
3. Check competence, professional authority and employer/service scope.
4. Compare reasonable options using current authoritative sources.
5. Agree the plan with the patient, including uncertainty and alternatives.
6. Prescribe accurately only when the decision is justified.
7. Document, monitor, review and safety-net.

Module emphasis: Check current organ function, physiology, interacting medicines and the relevant current dosing source before deciding dose or interval.

Scenario 1. Renal function falls after acute illness while renally cleared medicines continue unchanged.

Reasoning prompt: Check current organ function, physiology, interacting medicines and the relevant current dosing source before deciding dose or interval.

Scenario 2. A loading dose is confused with a maintenance dose.

Reasoning prompt: Check current organ function, physiology, interacting medicines and the relevant current dosing source before deciding dose or interval.

Scenario 3. A frail patient develops toxicity despite a “standard” adult dose.

Reasoning prompt: Check current organ function, physiology, interacting medicines and the relevant current dosing source before deciding dose or interval.

• Renal impairment only matters for nephrotoxic medicines.
• Half-life tells me everything about duration of effect.
• Older age automatically means lower doses for every drug.

• What clinical problem am I trying to solve, and is prescribing indicated?
• What information is missing before a safe decision can be made?
• Is this within my current competence, professional authority and employer/service scope?
• Which current authoritative medicine or guideline source should I check?
• What patient-specific factors change benefit, harm, dose, route or monitoring?
• What will I document, monitor, review and safety-net?
• When should I seek supervision, pharmacy advice, specialist input or urgent escalation?

Optional reflection: Without entering identifiable patient or employer information, identify one capability you would need to strengthen before prescribing independently in this area.

• Absorption, distribution, metabolism and excretion determine exposure, but the clinical meaning depends on organ function, age, body composition, genetics and interacting medicines.
• Half-life and time to steady state affect onset, accumulation, washout and monitoring.
• Renal and hepatic impairment can increase toxicity or reduce efficacy depending on drug and metabolite handling.
• Loading doses, maintenance doses and dose intervals solve different pharmacokinetic problems.

Check current versions for time-sensitive decisions.

• BNF-01 - BNF / NICE: British National FormularyOpen official source
• BNFC-01 - BNF for Children / NICE: British National Formulary for ChildrenOpen official source
• RPS-01 - Royal Pharmaceutical Society: A Competency Framework for All Prescribers (2021)Open official source