Adverse reactions: Introduction

Last updated: Thursday, November 18, 2021

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
It is estimated that 1 in 16 hospital admissions are due to an adverse drug reaction (ADR), and they are responsible for 4% of hospital bed capacity. ADRs themselves are also thought to occur in 10-20% of hospitalised inpatients, and one study found that over 2% of patients admitted with an ADR died, approximately 0.15% of all patients admitted.

Statistics from hospital medicines helplines for patients show that a high proportion of calls are concerned with side effects.

In practice, pharmacists encounter many clinical problems concerning adverse reactions such as:
  • Can drug X cause symptom Y? 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 signs and symptoms of overdose or poisoning).

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