Adverse drug reactions (ADRs) are harmful responses to medications, ranging from mild effects like drowsiness to severe reactions such as anaphylaxis. They are most common in women, the elderly, and young children, with classifications that include type A (predictable) and type B (unpredictable). Understanding the mechanisms and types of ADRs is essential for preventing serious morbidity and mortality associated with drug use.

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2. Adverse Drug Reactions
(ADR)
Harm associated with the use of a given
medications
OR
Unwanted or harmful reaction
experienced after the administration of a
drug or combination of drugs under normal
conditions of use
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3. ADR= significant morbidity & mortality
Range from mild reactions
(drowsiness, nausea, itching& rash);
disappear after discontinuation of drug
OR
Severe reactions (respiratory
depression, neutorpenia, hepatocellualr
injury, hemorrhage, anaphylaxis
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4. ADR most common in
Women
Elderly (>60 y old)
Very young (1-4 y)
Patients taking more than one drug
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5. Classification of ADR
Rawlin & Thompson classification ABCD
Traditional classification A & B
About 80% of ADR----Type A reactions
1) Type A Reactions
a) Related to pharmacological action of drug
Extensions of the principal pharmacological action
of the drug
Cont.
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6. b) Predictable
Relatively easily predicted by preclinical and clinical
pharmacological studies
c) Common
Type A reactions not serious---common
d) Dose-dependent
Usually dose dependent
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7. Type A reactions
(classes)
i) Toxicity of overdose (Drug overdose)
An adverse drug reaction caused by excessive
dosing
e.g., hepatic failure with dose of paracetamol
Headache with antihypertensives
hypoglycemia with sulfonylurea;
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8. ii) Side Effects
Nearly unavoidable secondary drug effect
produced by therapeutic doses
intensity is dose dependent
Occur immediately after initially taking drug or may
not appear until weeks after initiation of drug use
E.g., sedation with antihistamines
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9. iii) Secondary Effects
Secondary pharmacological effect
E.g., development of diarrhea with antibiotic therapy
due to altered GIT bacterial flora
Orthostatic hypotension with a phenothiazine
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10. iv) Drug Interactions
When two drugs taken together & they effect each
other’s response pharmacologically or kinetically
E.g., one drug slow metabolism of 2nd
drug blood
conc.= toxicity
Theophylline toxicity in presence of erythromycin
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11. 2) Type B Reactions
Unrelated to known pharmacological
actions of drug
Unpredictable
Often caused by immunological &
pharmacogenetic mechanisms
Unrelated to dosage
Comparatively rare & cause serious illness
or death cont.
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12. Results (more likely) in withdrawal of
marketing authorization
Often not discovered until after drug is
marketed
Both environmental & genetic factors =
important in this reaction
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13. Type B Reactions (classes)
i) Drug Intolerance
Lower threshold to normal pharmacological action of a
drug
e.g., tinnitus (single average dose of aspirin)
ii) Hypersensitivity (immunological reaction)
Immune mediated response to a drug agent in
sensitized patient
e.g., anaphylaxis with penicillin
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14. iii) Pseudoallergic Reaction
Direct mast cell activation & degranulation by
drugs (opiates, vancomycin & radiocontrast media)
Clinically indistinguishable form Type I
hypersensitivity but not involve IgE (non
immunologic reactions)
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15. iv) Idiosyncratic Reactions
An uncommon & abnormal response to drug
Usually due to genetic abnormality
Affect drug metabolism & receptor sensitivity
Harmful even fatal, appear in low doses
E.g., Anemia (hemolysis) by antioxidant drugs
(G6PD deficiency)
Paralysis due to succinylcholine (enzyme
deficiency)
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16. 3) Type C (chronic) Reactions
Associated with long-term drug therapy
Well known and can be anticipated
Adaptation occurs = discontinuation of
drug=abstinence syndrome
E.g. opoids, alcohol, barbiturates
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17. 4) Type D (delayed) Reactions
Carcinogenic & teratogenic effects
Delayed in onset
Very rare
Carcinogenic Effect
Medication lead to cancer; take >20 y to develop
Teratogenic Effect
Drug- induced birth defects
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18. Sign & Symptoms of ADR
Mild, moderate, severe or lethal
Sign & symptoms manifest soon after 1st
dose or
only after chronic use
e.g., Allergic reactions occur soon after drug is taken
usually 2nd
time ( itching, rash, eruption, upper or lower
airway edema with dyspnea & hypotension)
Idiosyncratic reactions=any unpredicted symptom
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19. Mechanisms of ADR
Type A =non immunological, reversible with
reduction of dose, non serious, extension of
pharmacological effects
Type B
Biochemical mechanism unrelated to
pharmacological
Immunologic = Hypersensitivity (Type I, II, III, IV)
OR
Non immunologic (direct)= Pseudoallergic,
idiosyncratic, intolerance www.mcqsinpharmacology.com

20. Mechanism of Type B
Reactions
i) Often mediated by a chemically
reactive metabolite
Non detoxification of metabolite
Direct cytotoxicity
Direct tissue damage + necrosis
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21. Drug Hypersensitivity
(allergic) Reaction
Common form of adverse response to drugs
Classification (Gell & Coombs)
Type I reactions (IgE-mediated)
Type II reactions (cytotoxic)
Type III reactions (immune complex)
Type IV (delayed, cell mediated)
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