Introduction

An Adverse Drug Reaction (ADR) is an injury caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term differs from the term "side effect" because side effects can be beneficial as well as detrimental. The study of ADRs is the concern of the field known as pharmacovigilance. An Adverse Drug Event (ADE) refers to any injury occurring at the time a drug is used, whether or not it is identified as a cause of the injury. An ADR is a special type of ADE in which a causative relationship can be shown. ADRs are only one type of medication-related harm, as harm can also be caused by omitting to take indicated medications.

Incidence and severity of adverse drug reactions vary by patient characteristics (eg, age, sex, ethnicity, coexisting disorders, genetic or geographic factors) and by drug factors (eg, type of drug, administration route, treatment duration, dosage, and bioavailability). Incidence is higher with advanced age and polypharmacy. ADRs are more severe among older patients (see Drug-Related Problems in Older Adults), although age per se may not be the primary cause. The contribution of prescribing and adherence errors to the incidence of ADRs is unclear.

Types of ADR

Most adverse drug reactions are dose-related; others are allergic or idiosyncratic. Dose-related ADRs are usually predictable; ADRs unrelated to dose are usually unpredictable.

Dose-related ADRs are particularly a concern when drugs have a narrow therapeutic index (eg, hemorrhage with oral anticoagulants). ADRs may result from decreased drug clearance in patients with impaired renal or hepatic function or from drug-drug interactions.

Allergic ADRs are not dose-related and require prior exposure. Allergies develop when a drug acts as an antigen or allergen. After a patient is sensitized, subsequent exposure to the drug produces one of several different types of allergic reaction. Clinical history and appropriate skin tests can sometimes help predict allergic ADRs.

Idiosyncratic ADRs are unexpected ADRs that are not dose-related or allergic. They occur in a small percentage of patients given a drug. Idiosyncrasy is an imprecise term that has been defined as a genetically determined abnormal response to a drug, but not all idiosyncratic reactions have a pharmacogenetic cause. The term may become obsolete as specific mechanisms of ADRs become known.

Drug Reactions

Allergic drug reactions account for only 5 to 10% of all adverse drug reactions. Any medication has the potential to cause an allergic reaction. Skin reactions (e.g. urticaria, erythema) are the most common form of allergic drug reaction, but can also include cough, nausea, vomiting, diarrhea, and headaches, and in severe cases can cause anaphylaxis.

Healthcare professionals therefore have to be vigilant and alert to drug reactions as a causative factor when there is an absence of alternative clinical explanations for the presenting symptoms. Whilst laboratory investigations, are unlikely to confirm an ADR, they can be useful tools to exclude non-drug causes of the presenting symptoms or signs.

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Regards

Mary Wilson

Editorial office

Clinical Pharmacology and Toxicology Research

E-mail: pharmatoxicol@eclinicalsci.com