Side Effects and Adverse Reaction Mechanisms

A side effect is an unwanted effect of a drug used at normal exposure; an adverse drug reaction is a noxious, unintended response to a drug. This topic concerns why such reactions occur — the pharmacodynamic mechanisms that link a drug's molecular actions to unwanted whole-body effects — and the long-standing classification that separates dose-related, predictable reactions from idiosyncratic ones.

Definition

An adverse drug reaction is an appreciably harmful or unpleasant reaction resulting from an intervention related to the use of a medicinal product at normally used doses; side effects are unintended effects (which may be harmful, neutral, or occasionally beneficial) occurring at usual exposure. Mechanistically, both arise from a drug's pharmacological actions on intended or unintended targets, or from immune and idiosyncratic processes.

Scope

This topic covers the definitions of side effects and adverse drug reactions, the classical type A / type B (and extended) classification, the mechanistic routes by which on-target and off-target actions produce harm, and exemplars such as drug-induced liver injury. It is a reference and educational entry; it does not provide diagnostic, dosing, or treatment advice, nor instructions for managing reactions in individuals.

Core questions

• How are side effects and adverse drug reactions defined and distinguished?
• What is the type A / type B classification, and how has it been extended?
• By what mechanisms do on-target and off-target drug actions cause harm?
• Why are some reactions predictable and dose-related while others are idiosyncratic?
• How are mechanism-specific reactions such as drug-induced liver injury characterised?

Key concepts

• Adverse drug reaction vs. adverse drug event
• Side effect
• Type A (augmented, dose-related) reactions
• Type B (bizarre, idiosyncratic) reactions
• On-target vs. off-target toxicity
• Immune-mediated (hypersensitivity) reactions
• Drug-induced liver injury

Mechanisms

Adverse reactions arise through several routes. Type A reactions are augmented, dose-related extensions of a drug's known pharmacology — either an exaggeration of the intended on-target action or a predictable off-target action — and are common but usually predictable (Rawlins & Thompson, 1991; Edwards & Aronson, 2000). Type B reactions are bizarre, not dose-related in the usual sense, and often immunological or idiosyncratic, including hypersensitivity reactions and host-specific metabolic susceptibilities (Pirmohamed et al., 1998). Mechanistically, on-target toxicity reflects the intended action occurring in the wrong tissue or to excess, whereas off-target toxicity reflects binding to secondary proteins; large-scale work has shown that predicted off-target interactions correspond to observed side effects (Lounkine et al., 2012). Reactive-metabolite formation and immune recognition underlie many serious idiosyncratic reactions, exemplified by drug-induced liver injury, for which standardised case definitions have been developed (Aithal et al., 2011). The classical type A/B scheme has since been extended to additional categories covering chronic, delayed, withdrawal, and failure-of-efficacy reactions.

Clinical relevance

Understanding the mechanism of an adverse reaction underpins how the safety of drugs is appraised and how pharmacovigilance reasons about signals. This entry describes those mechanisms and classifications for reference and education; it is not a basis for diagnosing, preventing, or managing reactions in any individual.

Epidemiology

Adverse drug reactions are a recognised cause of morbidity and of hospital admissions, and surveillance of them (pharmacovigilance) is a continuing part of drug safety; the cited reviews summarise their definitions and burden at a conceptual level rather than providing current incidence figures.

Evidence & guidelines

Definitions and classification draw on widely cited reviews (Edwards & Aronson, 2000; Pirmohamed et al., 1998) and textbook treatments (Rawlins & Thompson, 1991); mechanism-specific standardisation, such as the case definition for drug-induced liver injury (Aithal et al., 2011), and systematic off-target/side-effect mapping (Lounkine et al., 2012) provide the supporting evidence. These are reviews, primary studies, and consensus definitions rather than current treatment guidelines.

History

Systematic thinking about adverse reactions was consolidated in the later twentieth century, after drug-safety crises sharpened attention to harm. Rawlins and Thompson's type A / type B dichotomy gave the field a durable conceptual backbone, distinguishing augmented dose-related reactions from idiosyncratic ones; Edwards and Aronson (2000) refined the definitions, and the scheme was subsequently broadened into additional categories. In parallel, mechanistic study of reactive metabolites, immune recognition, and off-target binding gave specific reactions, such as drug-induced liver injury, a molecular footing.

Debates

Is the type A / type B dichotomy still sufficient?

The original two-category scheme captures dose-related versus idiosyncratic reactions cleanly but leaves chronic, delayed, withdrawal, and failure reactions awkwardly placed; extended classifications were proposed to fill the gaps, and how best to classify reactions for mechanism and pharmacovigilance remains under discussion.

Key figures

• Michael D. Rawlins
• Jeffrey K. Aronson
• Munir Pirmohamed

Seminal works

• rawlins-thompson-1991
• edwards-aronson-2000
• pirmohamed-1998

Frequently asked questions

What is the difference between an adverse drug reaction and an adverse drug event?

An adverse drug reaction is harm judged to be caused by the drug at normal use, implying a causal link, whereas an adverse drug event is any harm that occurs during treatment regardless of cause; not every adverse event is an adverse reaction.

What distinguishes a type A from a type B reaction?

Type A (augmented) reactions are dose-related, predictable extensions of the drug's known pharmacology and are usually reversible, while type B (bizarre) reactions are not dose-related in the usual sense, are often immunological or idiosyncratic, and are harder to predict.