Not all adverse reactions are the same. Some are predictable and dose-related, while others are unpredictable and life-threatening. Understanding the type of ADR is often the first step toward appropriate management.
What is an ADR (Adverse Drug Reaction) :
According to WHO Adverse drug reactions(ADRs) are noxious or unintended responses to medications used for prevention, diagnosis or treatment of disease.
Depending on type of reaction, ADR’s are classified into 6 types :
Type A : Augmented (Dose related)
- ADRs that occur as the result of known pharmacological properties of the drug are called Type A reactions.
- These are common and account for 85% to 90% of ADRs.
These reactions include following categories:
| Category | Examples |
|---|---|
| Drug overdose- overdose occurs when an excessive amount of drug is consumed | Liver failure after paracetamol overdose. Bleeding after a warfarin overdose. |
| Side effects- a predictable or dose dependent effect of a drug that is not the principle effect for which drug is used. | Gastritis after use of NSAIDs, nephrotoxicity after aminoglycoside therapy, phototoxicity due to doxycycline. |
| Drug interactions- Interaction between a drug and other drugs. | Reduction of warfarin’s anticoagulant effect as a result of increased vitamin K intake, respiratory depression as the result of benzodiazepine and opioid use together. |
Type B: Bizarre (Non-dose related)
- This refers to drug effects which are totally unrelated to the dose.
Examples :
- Immunological reactions : Anaphylaxis to penicillin.
- Idiosyncratic reactions : Dapsone-induced hemolysis in a patient with G6PD deficiency
Type C : Chronic (Dose related and time related)
- This refers to drug effects which occur due to dose accumulation or with prolonged use.
Examples :
- Hypothalamic-pituitary-adrenal axis suppression by corticosteroids.
- Osteonecrosis of the jaw with bisphosphonates.
Type D : Delayed (Time related)
- This refers to drug effects occur after prolonged exposure to a drug that does not necessarily accumulate.
Examples :
- Tardive dyskinesia after prolonged use of typical antipsychotics.
- Teratogenesis after use of thalidomide.
Type E : End of use (Withdrawal)
- This refers to the undesired effects of ceasing the drug.
Examples : Withdrawal syndrome with opioids(insomnia, anxiety)
Type F : Failure (Unexpected failure of therapy)
- This refers to an undesirable reduction in the drug’s efficacy.
Examples :
- Inadequate dosage of an oral contraceptive when used with an enzyme inducer.
- Antimicrobial treatment failure due to resistance.
The management of ADRs depends upon the type of reaction and severity. The questions that need to be answered when an ADR is suspected include :
- Is the reaction related to the drug? If so, which one?
- What kind of reaction is it ?
- How severe is the reaction, and what organs are involved?
- How urgently should the reaction be treated?
- Is there an alternative therapy that may be prescribed for the indication of the offending drug?
Why understanding ADRs is crucial?
- Improved patient safety and reduction in morbidity.
- Drug-interaction management
- Pharmacovigilance and reporting
- Individualized patient care.
Key takeaway :
Recognizing different types of ADRs is essential for healthcare professionals. Early detection and appropriate management can prevent serious complications, improve patient outcomes and enhance medication safety. Understanding ADR classification also supports accurate reporting and strengthens pharmacovigilance programs.
Which type of ADR do you think is encountered most frequently in clinical practice?
Have you observed an interesting or unexpected ADR during your clinical practice or training?
MBH/DB