Pharmacovigilance learning module: Adverse drug reactions


Important information

This learning module has now been superseded by a new module developed by BMJ Learning in close collaboration with the MHRA.

Pharmacovigilance – identifying and reporting adverse drug reactions: in association with the MHRA (external link)

Woman with a headacheUnwanted effects of medicines—adverse drug reactions—have important consequences for the individual and have a major impact on public health. A study in two large Merseyside hospitals found that the prevalence of hospital admissions resulting directly from adverse drug reactions was over 5%; adverse drug reactions accounted for 0.15% of fatalities among all hospital admissions.

In a recent 10-year survey, around 1% of hospital admissions in England were for drug-related causes and 4 - 5% of patients admitted to hospital for adverse drug reactions died. Different methodologies and settings for the two studies may go towards explaining the difference in prevalence, but, clearly, adverse reactions to medicines contribute very substantially to serious illness and early death.

Some adverse effects are unavoidable or unpredictable, but with a correct approach to using medicines, the burden of adverse drug reactions can be reduced considerably.

The terms adverse drug reactions and adverse events are not interchangeable. Whereas an adverse drug reaction can be attributed with some confidence to the action of a drug (opens in a new window), an adverse event simply describes an unwanted outcome that occurs, regardless of whether a medicine has been used or is implicated. Clinical trials (opens in a new window) record adverse events, but not all of these will be caused by the study drugs.

Classification of adverse reactions
Adverse drug reactions can be categorised in a number of ways (eg by severity, by body systems affected, or by frequency). The following categorisation is often used:

  • Type A (augmented) reactions are considered to be an exaggeration of the medicine’s normal effect when given at the usual dose. This category includes unwanted reactions that are predictable from the drug’s pharmacology and are usually dose-dependent (eg respiratory depression with opioids and bleeding with warfarin).
  • Type B (bizarre) reactions are effects that are not pharmacologically predictable and can include hypersensitivity reactions (eg anaphylaxis with beta-lactam antibiotics).

Additional categories have been proposed, some to accommodate the time course of the reaction:

  • Type C (continuing) reactions describe those that persist for a relatively long time (eg osteonecrosis of the jaw with bisphosphonates).
  • Type D (delayed) reactions, which become apparent some time after the use of a medicine (eg leucopenia, which can occur up to 6 weeks after a dose of lomustine).
  • Type E (end of use) reactions are associated with the withdrawal of a medicine (eg insomnia, anxiety and perceptual disturbances following withdrawal of benzodiazepines).

The alphabetical categories can be extended even further: type F (failure), type G (genetic/genomic), type H (hypersensitivity)…

The type A/type B classification of adverse drug reactions, while widely cited, does not characterise all reactions effectively. Classification along multiple axes—dose-relatedness, time-relatedness and susceptibility (DoTS)—overcomes many of the difficulties.


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Page last modified: 05 February 2013