Adverse Drug Reaction (ADR)

Health Canada: Adverse reactions are undesirable effects to health products (including drugs, medical devices and natural health products).

WHO: any noxious and unintended response to a medicinal product.

Why are ADRs so important?


  • 30% of hospitalized patients will experience an ADR.
  • 2-5% of all admissions due to an ADR.


  • A study in UK suggest ADRs cause about 6.5% of all admissions, costing the UK health system $847 million per annum (BMJ 2004;329:15-19).
  • ADRs cause an average 2 day increase in hospital stays.


  • Serious/fatal reactions are between the 4th and 6th leading cause
  • of death (JAMA 1998;279:200-5).

Often preventable.

Risk factors for the development of an ADR include:

  • Predictable reactions.
  • Drug-drug interactions.
  • Is the dose appropriate for hepatic/renal function?
  • Female gender.
  • The greater the number of medications administered, the greater the risk of developing an ADR.
  • Elderly.

Use a systematic approach when evaluating patients with potential ADRs:

  • Clinical diagnosis
  • Analysis of drug exposure
  • Taking accurate drug history, including OTC and herbal medications, is essential
  • Differential diagnosis
  • Literature search
  • Treat the patient
  • Confirmation (or testing)
  • Advice to patient
  • Reporting

Treatment Options:

Discontinue the possible offending agent if:

  • It can be safely stopped.
  • The event is life-threatening or intolerable.
  • There is a reasonable alternative.
  • Continuing the medication will further exacerbate the patient’s condition.

Continue the medication (modified as needed) if:

  • It is medically necessary.
  • There is no reasonable alternative.
  • The problem is mild and will resolve.

Administer appropriate treatment:

  • e.g., dextrose, antihistamines, epinephrine, naloxone, corticosteroids,

Provide supportive or palliative care:

  • e.g., hydration, corticosteroids, analgesics or antipruritics.

Consider rechallenge or desensitization.

Note: Fever is a predictor of a more serious drug reaction.


DRESS = drug reaction eosinophilia systemic.


  • Fever, skin eruption and internal organ involvement (+ eosinophilia).

Generally occurs on first exposure to the drug, with initial symptoms starting 1-8 weeks after starting medication.

Rare reaction: occurs in approximately 1 in 10000 patients.

Genetic factors might play a role.

Specific drug classes that are associated with DRESS include:

  • Anticonvulsants :phenytoin, phenobarbital, carbamazepine and lamotrigine.
  • Antimicrobials: sulfonamide antimicrobials, nitrofurantoin and minocycline.
  • Anti-retrovirals.
  • Allopurinol.

Allergy 2010;65:1357-66

Immunol Allergy Clin N Am 2004;24:425-43

Read also: Tips for Effective Pharmacovigilance Reporting of Adverse Events

Resource Person: Lobna Adi

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