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)
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.
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.
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.
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.
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.
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.