Pharmacology & Drug Action mechanisms

Syllabus Unit: Pre & Para Clinical Subjects

Why it matters

Chapter overview

Pharmacology asks how a drug moves through the body and how it produces its effect, and AIIMS PG tests both halves with equal precision — first-order kinetics calculations, receptor-subtype-based drug classification, the exact molecular target of every major antimicrobial class, and the named toxicity/antidote pair for CNS and anaesthetic agents are all asked as exact, mechanism-level facts.

Postgraduate Medical Exam Syllabus Pre & Para Clinical Subjects 50% weightage

Live Animation1 / 4

1AbsorptionDrug enters systemic circulation

2DistributionDrug reaches tissues via blood flow and lipid solubility

3MetabolismHepatic biotransformation, Phase I and II

4ExcretionRenal or biliary elimination

1Pharmacokinetics & Pharmacodynamics

How a drug moves through the body (ADME) and how it produces its effect (dose-response, receptor theory) together set the rational basis for dosing, and are AIIMS PG's most calculation-heavy pharmacology area.

Learning Objectives

Define the four pharmacokinetic processes — absorption, distribution, metabolism, excretion — with one factor affecting each.
Calculate drug elimination using first-order kinetics and state what determines half-life.
Distinguish agonist, antagonist and partial agonist by their effect on a dose-response curve — efficacy versus potency.
Explain zero-order versus first-order kinetics, using ethanol/phenytoin as the classic zero-order example.

Key Terms

Bioavailability (F)The fraction of an administered dose that reaches systemic circulation unchanged; IV drugs have F=1, while oral drugs are reduced by first-pass hepatic metabolism.

Volume of Distribution (Vd)A proportionality constant relating total drug in the body to plasma concentration (Vd = Dose/C0); a large Vd implies extensive tissue distribution/sequestration, not a high blood concentration.

Half-life (t1/2)The time for plasma concentration to fall by 50% under first-order kinetics; t1/2 = 0.693×Vd/Clearance, and roughly 4-5 half-lives are needed to reach steady state or fully eliminate a drug.

Zero-Order KineticsA constant amount (not fraction) of drug is eliminated per unit time because the elimination pathway is saturated; classic examples are ethanol, phenytoin at therapeutic-toxic doses, and aspirin in overdose.

Pharmacokinetics (what the body does to a drug) and pharmacodynamics (what a drug does to the body) are tested through direct calculation and through matching a described dose-response curve to the correct drug-receptor relationship.

The four ADME processes

Absorption — governed by route and first-pass hepatic metabolism for oral drugs.
Distribution — governed by lipid solubility, plasma protein binding and regional blood flow.
Metabolism — mostly hepatic CYP450 (Phase I) followed by conjugation (Phase II).
Excretion — primarily renal (filtration, secretion, reabsorption) or biliary.

t1/2 = 0.693 × Vd / Clearance

Common MistakeZero-order kinetics means a fixed increase in dose can disproportionately raise plasma levels — as seen in phenytoin toxicity — because there is no fixed half-life once the elimination pathway saturates.

Pharmacodynamics

An agonist produces the maximal effect a receptor system can give (full efficacy); a partial agonist binds the same receptor but produces a submaximal effect even at full occupancy; an antagonist binds without producing any effect, blocking the agonist's action. Potency refers to the dose needed for a given effect, while efficacy refers to the maximum effect achievable — the two are independent properties.