Pharmacodynamics concepts for RRB, KGMU, KSSSCI, MPESB Pharmacist Exams / GPAT / DPEE / Drugs Inspector Exams / D.Pharm, B.Pharm, and Pharm.D.
o Placebo and nocebo effects. Nocebo is negative thoughts evoked by the pessimistic attitude of the patient. Placebo is opposite of nocebo.
o Drug Tolerance: It is a phenomenon where a drug becomes ineffective upon its repeated administration at the usual therapeutic dose. Progressive increase in the dose is required to produce the desired therapeutic effect.
o Drug dependence (neuroadaptation): It is an altered physiological state in which the user has a strong desire to continue taking the drug either to experience its effect or to avoid the discomfort of its absence. Examples of drugs producing dependence are opioids, barbiturates, alcohol and benzodiazepines.
o Combined effects of drugs: When two or more drugs are given simultaneously or in quick succession, they may not affect action of each other, or they may exhibit additive effect or synergism or antagonism.
o Additive effect: The desirable therapeutic effect of the two or more drugs is in the same direction and simply adds up: Effect of drugs A + B = effect of drug A + effect of drug B. E.g. Ephedrine + Aminophylline as bronchodilator; Aspirin + Paracetamol as analgesic - antipyretic
o Synergistic effect: The desirable therapeutic effect of combination of two or more drugs is greater than the sum of their individual effects: Effect of drug A + B > effect of drug A + effect of drug B. E.g. Cotrimoxazole (sulphamethoxazole + trimethoprim) as bactericidal; Codeine and aspirin as analgesics; Procaine and adrenaline combination increases the duration of action of procaine.
o Antagonism: The phenomenon where one drug decreases or stops the action of another drug then it is said to be antagonism. Effect of drugs A + B < effect of drug A + effect of drug B
o Depending on the mechanism involved, antagonism may be:
1. Chemical Antagonism: The two drugs react chemically and form a pharmacologically inactive or undesirable product. E.g. KMnO4 oxidizes alkaloids. It is used for gastric lavage in poisoning.
Chelating agents (BAL, calcium disodium edetate) form complex with toxic metals (As, Pb).
Drugs may react when mixed in the same syringe or infusion bottle and form undesirable products. E.g. Thiopentone sodium + succinylcholine chloride; Penicillin-G sodium + succinylcholine chloride
2. Physical antagonism: Antagonism due to physical property of antagonist. E.g., Activated charcoal adsorbs alkaloids and can prevent their absorption, used in alkaloidal poisonings.
3. Receptor antagonism: One drug is receptor antagonist. It blocks the receptor action of the other drug which is agonist. Receptor antagonism can be competitive or non-competitive.
(a) Competitive (reversible) antagonism: The antagonist competes with the agonist for the same receptors. This antagonism can be reversed by increasing concentration of the agonist at the receptor site.
E.g. Acetylcholine (agonist) and atropine (antagonist) compete with each other at muscarinic receptors.
(b) Non-competitive (irreversible) antagonism: The antagonist binds to an allosteric site and inactivates the receptor so that the agonist cannot be bound effectively. This antagonism cannot be reversed by increasing concentration of the agonist at the receptor site.
E.g. Phenoxybenzamine binds irreversibly to α-adrenergic receptors; antagonism between diazepam – bicuculline is also non-competitive.
4. Physiological Antagonism: The two drugs act on different receptors or by different mechanisms, and have exactly opposite pharmacological effects.
E.g., Bronchoconstriction produced by histamine in anaphylactic shock is antagonized by adrenaline; effect of glucagon and insulin on blood sugar level is antagonistic.
o Therapeutic Index (Safety Margin): It is defined as the ratio of median lethal dose to median effective dose. A drug with larger therapeutic index is safer than one with smaller therapeutic index. Hence, drug with lesser therapeutic index should be administered cautiously.
MCQs
1. In the presence of this type of antagonist, antagonism cannot be overcome by increasing agonist concentrations.
(a) Competitive
(b) Noncompetitive
(c) Reversible
(d) Nonspecific
2. When the total pharmacological effect of two or more drugs administered together is equal to the sum of their individual pharmacological actions, the effect is called
(a) Synergism
(b) Additive effect
(c) Antagonism
(d) Adverse effect
3. The ratio of median lethal dose to median effective dose is _____.
(a) Bioavailability of a drug
(b) Therapeutic index
(c) Apparent volume of distribution of a drug
(d) Therapeutic window
4. The phenomenon where interaction between two or more drugs produces an effect greater than the sum of their individual effects is _____.
(a) Additive effect
(b) Synergism
(c) Antagonism
(d) None of these
5. Identify the correct statement.
(a) A drug with larger therapeutic index is safer than with one smaller therapeutic index
(b) A drug with larger therapeutic index is more toxic than one with smaller therapeutic index.
(c) Therapeutic index is not associated with safety of drugs.
(d) Therapeutic index shows the potency of a drug
6. The opposite action of two drugs on the same physiological system is called as _____.
(a) Synergism
(b) Antagonism
(c) Drug tolerance
(d) Drug dependence
7. ED50 is the dose of a _____.
(a) test substance that produces desirable effects in 50% of the animals in a dose group
(b) test substance that kills 50% of the animals in a dose group
(c) substance that produces maximum effect in 50% of the animals in a dose group
(d) substance that produces maximum adverse effects in 50% of the animals in a dose group
8. Insulin and glucagon are considered
(a) Pharmacological antagonist
(b) Physiological antagonists
(c) Pharmacodynamic antagonist
(d) Chemical antagonist
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Recommended book for the preparation of GPAT and Drugs Inspector Exams, ESSENTIAL PHARMACY REVIEW FOR DRUGS INSPECTOR EXAMS, NIRALI PRAKASHAN, PUNE
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Recommended book for the preparation of RRB Pharmacist Exam, PHARMACIST RECRUITMENT EXAM, NIRALI PRAKASHAN, PUNE
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Check your Answers:
1. (b)
2. (b)
3. (b)
4. (b)
5. (a)
6. (b)
7. (a)
8. (b)
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