Questions
2–4 questions in major B.Pharm semester exams
Difficulty
Medium
Importance
Foundation for all clinical pharmacology topics
Overview
General Pharmacology forms the bedrock of pharmaceutical sciences, governing how drugs interact with the body and how the body handles them. Mastering this topic is essential for understanding drug efficacy, toxicity, and rational therapeutic choices. Aspirants must grasp the relationship between concentration at the site of action and the resulting pharmacological response.
Dose-Response Relationships
This relationship describes the correlation between the concentration of a drug at its receptor site and the magnitude of the resulting effect. It is typically expressed as a sigmoidal curve when plotted against log dose, serving as the basis for calculating potency and efficacy.
- Potency depends on the affinity of the drug for its receptor
- Efficacy represents the maximum response a drug can produce
- Graded dose-response reflects the effect intensity in an individual
- Quantal dose-response indicates the frequency of an effect in a population
- ED50 is the dose producing 50 percent of the maximum effect
Pharmacokinetics: Absorption and Distribution
Absorption is the movement of a drug from the site of administration into the circulation, while distribution describes its reversible transfer between the blood and extravascular tissues. These processes are largely dictated by physicochemical properties like lipid solubility, ionization, and molecular size.
- Bioavailability is the fraction of an administered dose reaching systemic circulation
- First-pass metabolism significantly reduces oral bioavailability
- Henderson-Hasselbalch equation governs drug ionization based on pH
- Vd (Apparent volume of distribution) = Amount of drug in body / Plasma concentration
- Plasma protein binding (e.g., albumin) limits the free active drug fraction
Pharmacokinetics: Metabolism and Excretion
Metabolism (biotransformation) converts drugs into more polar, excretable metabolites, primarily in the liver, whereas excretion involves the removal of substances from the body, mostly via kidneys. Understanding these pathways is crucial for predicting drug interactions and dosage adjustments in renal or hepatic failure.
- Phase I reactions include oxidation, reduction, and hydrolysis
- Cytochrome P450 enzymes catalyze the majority of drug metabolism
- Phase II reactions involve conjugation (e.g., glucuronidation)
- Zero-order kinetics: constant amount of drug eliminated per unit time
- First-order kinetics: constant fraction of drug eliminated per unit time
Formula Sheet
Bioavailability (F) = (AUC oral / AUC IV) * 100
Volume of Distribution (Vd) = Dose / Co
Clearance (Cl) = Rate of elimination / Plasma concentration
Half-life (t1/2) = 0.693 * Vd / Cl
Exam Tip
Always draw the log dose-response curve with the X-axis labeled as 'log dose' and the Y-axis as 'response' to guarantee full marks for conceptual clarity.
Common Mistakes
- Confusing potency with efficacy in exam answers
- Failing to distinguish between first-order and zero-order elimination kinetics
- Ignoring the role of ionization in drug transport across biological membranes
More Revision Notes
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