Must-know formulas

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your INI CET Pharmacology paper.

Pharmacology is the science of drug action, split into pharmacokinetics (what the body does to a drug — ADME: Absorption, Distribution, Metabolism, Excretion) and pharmacodynamics (what the drug does to the body — receptor interaction, agonist/antagonist activity, dose–response).

Must-know formulas

Half-life: t½ = 0.693 × Vd / CL
Loading dose: LD = (Vd × Ctarget) / F
Maintenance dose: MD = (CL × Ctarget × τ) / F
Therapeutic index: TI = TD50 / ED50 (higher TI = wider safety margin)
Steady-state Css: Css = (F × Dose) / (CL × τ)

High-yield exam pointers

Zero-order kinetics drugs saturate metabolism — phenytoin, ethanol, high-dose aspirin.
Competitive antagonists shift the dose–response curve rightward; Emax is preserved.
CYP450 inducers (rifampicin, phenytoin, carbamazepine) lower plasma levels of co-administered drugs; inhibitors (ketoconazole, erythromycin, cimetidine) raise them.
Only the free (unbound) fraction is pharmacologically active; protein-bound drug is a reservoir.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months of preparation.

Pharmacokinetics — ADME in clinical practice

Absorption depends on route, formulation, and first-pass hepatic loss. Oral bioavailability (F) is reduced for drugs like nitroglycerin, lidocaine, and propranolol because of extensive hepatic first-pass metabolism. Distribution is governed by Vd, lipid solubility, and plasma protein binding (albumin for acidic drugs, α1-acid glycoprotein for basic drugs).

Metabolism occurs in two phases. Phase I (CYP450-mediated oxidation, reduction, hydrolysis) usually activates or detoxifies the parent compound. Phase II conjugates the metabolite with glucuronate, sulfate, or acetate to form water-soluble excretable products. Excretion is mainly renal; drugs with low renal clearance require dose reduction in CKD.

Pharmacodynamics — Receptors and responses

Receptors are classified into four superfamilies: G-protein coupled (β-adrenoceptors, muscarinic M1–M5), ion channel / ionotropic (nicotinic ACh receptor, NMDA), enzyme-linked (insulin receptor, receptor tyrosine kinases), and nuclear / intracellular (steroid, thyroid, vitamin D receptors).

Agonists possess both affinity (Kd) and intrinsic efficacy. Partial agonists produce a submaximal response even at full receptor occupancy. Competitive antagonists increase EC50 but spare Emax; non-competitive antagonists reduce Emax irreversibly.

Order of kinetics

Feature	First-order	Zero-order
Rate	Proportional to concentration	Constant, saturable
t½	Constant	Variable, dose-dependent
Examples	Most drugs at therapeutic doses	Phenytoin, ethanol, high-dose aspirin

Pharmacokinetic calculations — worked pattern

For a target theophylline concentration of 10 mg/L, Vd = 0.45 L/kg in a 70 kg adult, CL = 0.04 L/kg/h, τ = 12 h, F = 1:

Vd = 31.5 L → LD ≈ 315 mg
MD ≈ 168 mg every 12 h

Common exam traps

Confusing TD50 with LD50 in the TI formula.
Omitting F when calculating an oral loading dose.
Assuming CYP450 induction always reduces effect — for prodrugs (codeine → morphine, enalapril → enalaprilat), induction increases active metabolite and can cause toxicity.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for INI CET candidates on a longer study timeline.

Adverse drug reactions and clinical reasoning

Type A (augmented) reactions are dose-dependent and predictable — bleeding with warfarin, hypoglycemia with sulfonylureas. Type B (bizarre) reactions are idiosyncratic and unpredictable — malignant hyperthermia with halothane, Stevens-Johnson syndrome with lamotrigine. Type B reactions carry high mortality but low incidence and dominate the “drug of choice in renal/hepatic failure” vignette style.

Drug interactions — CYP450 map for INI CET

CYP isoform	Substrates	Inhibitors	Inducers
CYP3A4	Statins, calcium channel blockers, cyclosporine	Ketoconazole, erythromycin, ritonavir, grapefruit juice	Rifampicin, carbamazepine, phenytoin, St John’s wort
CYP2D6	Codeine, metoprolol, tricyclics	Quinidine, fluoxetine, paroxetine	— (poorly inducible)
CYP2C9	Warfarin (S-isomer), phenytoin	Fluconazole, amiodarone	Rifampicin

Dose adjustment in organ dysfunction

Renal failure: dose = normal dose × (patient’s CLcr / normal CLcr) for renally cleared drugs (aminoglycosides, digoxin, vancomycin).
Hepatic failure: avoid drugs with high first-pass extraction; titrate warfarin, morphine, and propranolol carefully.
Elderly: reduced hepatic mass, renal blood flow, and lean body mass; start low, go slow.

Connected topics for integrated revision

NSAIDs — COX-1 vs COX-2 selectivity links to gastric mucosal toxicity and cardiovascular risk.
Antimicrobials — bactericidal vs bacteriostatic synergy (β-lactam + aminoglycoside) vs antagonism (β-lactam + tetracycline in meningitis).
Anticancer chemotherapy — cell-cycle specificity (S-phase: methotrexate, 5-FU; M-phase: vincristine, paclitaxel) connects to toxicity profiles.

Common mistakes revisited

Mixing up potency (EC50) with efficacy (Emax).
Forgetting that receptor desensitization explains tachyphylaxis with β2-agonists and nitrates.
Reporting Vd as a real anatomical volume — it is an apparent value that can exceed total body water for lipophilic drugs.

Practice prompts

A 60 kg patient with status epilepticus needs a phenytoin loading dose of 15 mg/kg IV. Vd = 0.65 L/kg, target Cp = 15 mg/L. Calculate the actual IV dose and explain why this drug cannot be given IM or in dextrose.
A patient on warfarin starts rifampicin for TB. Predict the INR change within 2 weeks and the underlying CYP interaction. What INR monitoring schedule is appropriate?

Continue your study

View this topic in your INI CET (AIIMS PG) roadmap — see where “Topic 7” fits in your personalised plan
Build a quick revision plan — 1-day sprint covering highest-weight topics
INI CET (AIIMS PG) exam overview — pattern, eligibility, and syllabus
All Pharmacology notes — browse sibling topics in this subject

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Must-know formulas