Adverse Drug Reactions (ADRs): Definition & Classifications

Adverse Drug Reactions (ADRs): Definition & Classifications

Adverse Drug Reactions (ADRs): Definition and Classifications explain harmful drug effects and their different categories.

Definition

• An adverse drug reaction (ADR) is an unintended, harmful response to a drug occurring at doses used for prevention, diagnosis, treatment, or modification of physiological functions.
• It encompasses any noxious, undesired effect at standard therapeutic doses.

Classifications

• Adverse drug reactions (ADRs) can be categorized based on various mechanisms.
• When it comes to predictability, the categorization can be done as:

1. Predictable ADRs

   • Excessive Effects: Over-intensification of desired effects (e.g., antihypertensives causing low blood pressure).
   • Secondary Effects: Known side effects unrelated to the primary action (e.g., dry mouth from antihistamines).
   • Withdrawal Toxicity: Reactions from abrupt cessation (e.g., antidepressant withdrawal).

2. Non-Predictable ADRs

   • Idiosyncrasy: Unusual, unexpected reactions.
   • Allergic Reactions: Immune-mediated, unpredictable responses.
   • Genetic Toxicity: Reactions due to genetic predispositions, identifiable through genetic testing.

Types of Adverse Drug Reactions (ADRs)

1. Excessive Pharmacological Effects

   • Mechanism: Occurs when the drug’s therapeutic effect becomes excessive, often due to overdose or individual sensitivity.
   • Example: Opioid overdose causing respiratory depression via CNS effects.
   • Management: Requires careful dosing, patient education, and monitoring. In overdose cases, prompt medical intervention and antidotes may be necessary.

2. Secondary Pharmacological Effects

   • Mechanism: Arises from secondary pharmacological effects, separate from the drug’s primary purpose.
   • Example: SSRIs for depression may cause gastrointestinal issues or sexual dysfunction.
   • Management: Inform patients about possible side effects. Consider alternative treatments if side effects significantly impact quality of life.

3. Idiosyncratic Reactions

   • Mechanism: Unpredictable reactions unrelated to the drug’s intended effects, often due to unique genetic factors.
   • Example: Peripheral neuropathy in some patients taking isoniazid due to metabolism issues.
   • Management: Discontinue the drug and provide supportive care if such reactions are identified.

4. Allergic Drug Reactions

   • Mechanism: Immune system reacts to the drug as a foreign substance, causing an allergic response.
   • Example: Penicillin can trigger reactions from mild rashes to severe anaphylaxis.
   • Management: Document any known allergies, discontinue the drug immediately, and provide necessary medical intervention.

5. Genetically Determined Toxicity

   • Mechanism: Genetic factors make some individuals more susceptible to drug toxicity.
   • Example: HLA-B*5701 genetic variant increases hypersensitivity risk to abacavir.
   • Management: Use genetic screening to identify at-risk patients and consider alternative treatments.

6. Toxicity Following Sudden Drug Withdrawal

   • Mechanism: Sudden discontinuation can lead to rebound symptoms or withdrawal.
   • Example: Stopping benzodiazepines abruptly may cause seizures, tremors, and increased anxiety.
   • Management: Taper drugs gradually under medical supervision to prevent withdrawal symptoms.

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