Adverse reactions: Introduction

Last updated: Monday, January 22, 2018

NB: See learning outcomes for this tutorial mapped to competencies, a PDF of the whole text, and a one-page summary.

☞ Why this subject matters...

Amoxicillin-induced rash  
Courtesy of Skoch3, Wikimedia Commons
Wherever you work in pharmacy, you will be asked about adverse drug reactions (ADRs), as well as a large number of related patient safety issues (e.g. contra-indications). Statistics from hospital medicines helplines for patients show that a high proportion of calls are concerned with side effects. ADRs occur in 10-20% of hospitalised inpatients and may be responsible for 1 in 1000 deaths on medical wards. Studies suggest about 5% of all hospital admissions are due to ADRs. Although it has been studied less well, as many as 2% of GP consultations may be due to suspected ADRs.

In practice, pharmacists encounter many clinical problems concerning adverse reactions such as:
  • Can drug X cause symptom Y? How common is this side effect? Is it reversible?
  • Which of drugs A, B or C is the most likely cause of symptom Y?
  • How should I manage a suspected ADR in this patient?
  • This patient has experienced a suspected reaction to drug X. What can I use instead?


ADRs are great mimickers of disease, causing diagnostic confusion and resulting in unnecessary referral and investigation. Certain disorders are commonly drug-induced and a thorough assessment of drug exposure is particularly important in these cases. Examples of signs and symptoms include rashes, electrolyte disturbances, postural hypotension, confusion, constipation, renal or liver dysfunction, and gastrointestinal haemorrhage. Less common examples include blood dyscrasias.
Quick clinical question 
What's the difference between a sign and a symptom?    Consider, then click for answer.
A symptom is something that a patient notices and complains of (e.g. headache, thirst); it is subjective evidence. A sign is something objective picked up by examination or investigation (e.g. increased plasma creatinine, splenomegaly).

Shortly, we'll be looking at causality – how to assess whether a medicine is responsible for your patient's symptoms rather than something else. But first, we need to be sure that we are all talking about the same thing when we describe an ADR. What are the essential features of an adverse drug reaction?

What is an ADR?

ADRs are also known as side effects, and have these characteristics:

– There is some evidence that at least one medicine is responsible.
– The effect is unintended.
– It is harmful, or potentially harmful.
– The reaction is seen at normal doses used clinically (to distinguish ADRs from ‘toxicity’ which describes symptoms of overdose or poisoning).

Some patients have an increased susceptibility to ADRs including the elderly, neonates and children, patients with multiple disease states (e.g. ICU patients, patients with HIV), those with impaired kidney or liver function, and patients receiving polypharmacy. Ignoring manufacturers’ contra-indications or failing to monitor the patient adequately are also common methods of increasing the likelihood of known ADRs.