There are several terms commonly used to describe adverse effects of drug therapy: An adverse drug reaction (ADR) is an unwanted or harmful reaction experienced following the administration of a drug or combination of drugs under normal conditions of use and is suspected to be related to the drug. An ADR will usually require the drug to be discontinued or the dose reduced.An adverse event is harm that occurs while a patient is taking a drug, irrespective of whether the drug is suspected to be the cause. A side-effect is any effect caused by a drug other than the intended therapeutic effect, whether beneficial, neutral or harmful. The term ‘side-effect’ is often used interchangeably with ‘ADR’ although the former usually implies an effect that is less harmful, predictable and may not even require discontinuation of therapy (e.g. ankle oedema with vasodilators). Drug toxicity describes adverse effects of a drug that occur because the dose or plasma concentration has risen above the therapeutic range, either unintentionally or intentionally (drug overdose).Drug abuse is the misuse of recreational or therapeutic drugs that may lead to addiction or dependence, serious physiological injury (such as damage to kidneys, liver, heart), psychological harm (abnormal behavior patterns, hallucinations, memory loss), or death. Adverse drug reactions - basic principles Any drug that is capable of producing beneficial therapeutic effects can also cause unwanted ‘adverse’ effects. Adverse drug reactions (ADRs) are therefore common and constitute an important public health challenge in their own right. A significant proportion of admissions to hospital are caused by ADRs and hospitalised patients frequently experience ADRs that complicate and prolong their stay. Many of these ADRs can be avoided if greater care is taken. All prescribers need to make a judgment about the likelihood that a patient will either gain from the beneficial effects or experience an ADR before prescribing. Some drugs rarely cause ADRs (e.g. paracetamol) while others frequently do so (e.g. cancer chemotherapy). The decision to prescribe these ‘higher risk’ drugs will depend on the extent of the potential benefits. Although prescribers always face the possibility of causing an ADR the risks of doing so can be minimised by (i) recognition of patient and drug factors that increase the susceptibility and (ii) by counselling patients about early indications that an ADR may be developing. Understanding adverse drug reactions and drug allergies Module 1 – Basic principles of adverse drug reactions WHO Medication Safety Curriculum Guide 2019 Adverse drug reactions - categorisation Adverse drug reactions (ADRs) are a significant cause of morbidity and mortality. They have historically been classified as either Type A or Type B.Type A ADRs relate to the mechanism of action (i.e. the known pharmacology) of the medication, and are associated with high morbidity and low mortality.Clinical examples of Type A ADRs are common and physicians often need to reduce the dosages of the medicine so that the patients can tolerate the treatment. Patients can be empowered to be aware of the ADRs and reduce the incidence of unnecessary Type A ADRs.In contrast, Type B ADRs are idiosyncratic and cannot be predicted from the known pharmacology of a drug. These reactions are associated with low morbidity and high mortality. Formal recording of the incidence of Type B reactions can be life-saving.Additional categories of ADRs are described in an article by Kaufman that was published in 2016 (PMID: 27507394). Classifying_Adverse_Drug_Reactions.pdf Module 2 – Categorisation Adverse drug reactions - drug allergy 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. Medscape - Stevens-Johnson Syndrome. Author: C Stephen Foster (2016) NICE: Drug allergy: diagnosis and management Module 3 – Drug allergy classification Adverse drug reactions - diagnosis and management ADRs can be difficult to diagnose for many reasons. They may mimic the symptoms of a common illness, they may be rare or unexpected, or in patients with multiple co-morbidities or being treated with several drugs, the drug-related symptoms are not obvious. 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. Once a diagnosis of ADR is made, management will depend on the category of ADR. Type A ADRs generally respond to a reduction in dosage, whilst Type B ADRs indicate immediate discontinuation of the offending drug. Module 4 – Diagnosis, interpretation and management Adverse drug reactions - avoiding adverse drug reactions The risk of developing ADRs arises from a combination of patient-related factors and the drug or drugs being prescribed. The level of risk is minimised by changing prescribing, providing appropriate warnings and undertaking relevant monitoring. Module 5 – Avoiding adverse drug reactions eMC (Electronic Medicines Compendium) Adverse drug reactions - pharmacovigilance Pharmacovigilance (or drug safety) is the practice of monitoring the effects of medical drugs after they have been licensed for use. One important goal of this activity is to detect, collate, assess and monitor previously unreported adverse reactions, with the aim of preventing adverse effects and reducing the considerable economic and clinical costs associated with adverse reactions. Clinical and economic burden of adverse drug reactions. Module 6 – Pharmacovigilance This article was published on 2025-01-28
