What are the types of Adverse Drug Reactions (ADR)?

Types of Adverse Drug Reactions (ADRs)

Adverse drug reactions are classified into seven main types (A-G), with Type A (Augmented) and Type B (Bizarre) being the most common categories that account for the majority of all ADRs. 1

Primary Classification of ADRs

Type A - Augmented

• Definition: Predictable reactions related to the known pharmacological actions of the drug
• Characteristics:
  • Dose-dependent
  • Account for approximately 80% of all ADRs
  • High morbidity but low mortality
  • Occur in otherwise healthy individuals
• Examples: Hypotension with antihypertensives, bleeding with anticoagulants

Type B - Bizarre

• Definition: Unpredictable, idiosyncratic reactions unrelated to the pharmacological actions of the drug
• Characteristics:
  • Dose-independent
  • Occur only in susceptible individuals
  • Low morbidity but high mortality
  • Often immune-mediated
• Examples: Anaphylaxis, severe cutaneous adverse reactions

Type C - Chronic/Continuing

• Definition: Reactions associated with long-term drug therapy
• Characteristics:
  • Related to cumulative dose
  • Occur after prolonged exposure
• Examples: Hypothalamic-pituitary-adrenal axis suppression with corticosteroids

Type D - Delayed

• Definition: Reactions that become apparent sometime after the use of the drug
• Characteristics:
  • Usually dose-related
  • Occur or become apparent after a delay
• Examples: Teratogenicity, carcinogenicity

Type E - End of use

• Definition: Reactions occurring after drug withdrawal
• Characteristics:
  • Related to drug discontinuation
• Examples: Withdrawal syndromes, rebound hypertension after beta-blocker discontinuation

Type F - Failure of therapy

• Definition: Unexpected failure of therapy
• Characteristics:
  • Often caused by drug interactions
• Examples: Oral contraceptive failure with enzyme-inducing drugs

Type G - Genetic

• Definition: Reactions related to genetic abnormalities
• Characteristics:
  • Determined by genetic variants affecting drug metabolism
• Examples: Glucose-6-phosphate dehydrogenase deficiency reactions

Immune-Mediated ADRs

Immune-mediated ADRs can be further classified based on the Gell and Coombs classification:

• Type I: IgE-mediated immediate hypersensitivity reactions (anaphylaxis)
• Type II: Cytotoxic antibody-mediated reactions
• Type III: Immune complex-mediated reactions
• Type IV: T-cell-mediated delayed hypersensitivity reactions

Factors Affecting ADR Susceptibility

Several factors influence an individual's susceptibility to ADRs:

• Age: Elderly and very young patients are more vulnerable
• Gender: Women have approximately 1.5 times higher risk of developing ADRs than men 2
• Polypharmacy: Increases risk due to potential drug interactions
• Comorbidities: Especially renal or hepatic impairment
• Genetic factors: Pharmacogenetic variations in drug metabolism
• Previous ADR history: Indicates potential for future reactions

Documentation and Reporting

Proper documentation of ADRs is essential and should include:

• Reaction details (symptoms, severity)
• Timing relative to drug administration
• Management approaches used
• Outcome of the reaction

All significant ADRs should be reported to appropriate authorities:

• For medications: FDA's MedWatch program
• For vaccines: Vaccine Adverse Events Reporting System (VAERS)

Clinical Implications

Understanding the type of ADR is crucial for:

1. Prevention: Identifying high-risk patients
2. Management: Appropriate treatment based on mechanism
3. Future prescribing: Avoiding similar drugs or drug classes
4. Patient education: Informing about potential risks

Even mild reactions should be documented as they may predict more severe reactions with continued exposure 1. Proper documentation helps assess true risk for future medication use and contributes to patient safety.

Special Considerations

• Polypharmacy significantly increases ADR risk, especially in older adults 2
• Women experience more ADRs than men, particularly with cardiovascular medications 2
• Genetic factors can predispose individuals to specific ADRs, highlighting the importance of pharmacogenetic testing in high-risk scenarios

Understanding these classifications helps clinicians predict, prevent, diagnose, and manage ADRs more effectively, ultimately improving patient safety and medication outcomes.