PT
● MODULE 02 / 20
Pharmacokinetics & Dose Calculations
Follow absorption, distribution, metabolism and elimination to explain exposure and dose adjustment.
Quick revision ↓ Medical Dictionary
FULL CHAPTER EXPLANATION
Deep Learning — Module 02
Read this section in sequence. It moves from drug targets to therapeutic effects, adverse reactions and safe use.
01
Absorption and bioavailability
Bioavailability is the fraction of administered dose reaching systemic circulation unchanged. Oral exposure depends on dissolution, gut absorption and presystemic metabolism. Food, gut motility, transporters and interacting medicines can alter it. The area under a plasma concentration-time curve reflects systemic exposure under comparable conditions. Bioequivalence compares exposure measures; it does not mean two products have identical inactive ingredients or appearances.
02
Distribution and protein binding
Apparent volume of distribution equals the amount of drug in the body divided by plasma concentration. It is a proportionality factor rather than a literal anatomical volume. Extensive tissue binding can produce a large value. Unbound drug is generally available for receptor interaction and glomerular filtration. Changes in albumin can alter measured total concentrations, so a low total concentration does not always mean inadequate free-drug exposure. Redistribution can end a drug effect before elimination is complete.
03
Metabolism and interactions
Oxidation, reduction and hydrolysis are grouped as phase I reactions; conjugation reactions are grouped as phase II. They need not occur in that order. Metabolism can inactivate a drug, activate a prodrug or form a toxic metabolite. Enzyme induction generally develops over time and can lower exposure to susceptible active drugs; inhibition can increase it. The direction differs for prodrugs, so identify both the parent and active metabolite before predicting an interaction.
04
Clearance, half-life and steady state
Clearance describes the volume of plasma cleared of drug per unit time. Under first-order elimination, a constant fraction is removed per unit time and half-life is approximately 0.693 times volume of distribution divided by clearance. Roughly four to five half-lives are needed to approach steady state with a constant regimen. Saturable elimination can make dose increases produce disproportionately large concentration rises. Renal failure may prolong elimination of a drug or its active metabolites.
05
Loading and maintenance calculations
A simplified loading dose is target concentration times volume of distribution divided by bioavailability. Maintenance dose rate is target concentration times clearance divided by bioavailability. For an intravenous teaching example with a target of 10 mg/L and volume of 40 L, loading dose is 400 mg. With clearance of 4 L/hour, maintenance rate is 40 mg/hour. These are model calculations, not patient prescriptions; salt factors, distribution phases and clinical constraints may change actual regimens.
High-Yield Revision
- Loading dose mainly follows distribution volume; maintenance mainly follows clearance.
- Interpret total drug concentration alongside protein binding and sampling time.
- First-order half-life calculations assume stable, linear kinetics.
Common Exam & Concept Pitfalls
- A large apparent volume is not a measured body-fluid volume.
- Increasing the dose does not usually shorten the time to steady state in linear kinetics.
Sources & Further Reading
Curriculum scope checked 23 September 2026. These original study notes summarize established principles; current institutional guidance and product information govern clinical use. Faculty review is pending.
RAHBAR study structure: target → drug action → therapeutic use → adverse effects → monitoring. Use institutional curriculum and faculty guidance for local sequencing, practical requirements and assessment specifications.