01Principles and purposeThe professional or clinical skill and the decisions it supports.
An adverse drug reaction is harmful, unintended medicine-related harm. It may be an exaggerated expected effect, an immune reaction, organ toxicity or a consequence emerging after prolonged exposure or withdrawal. The label describes a possible causal relationship; it does not require certainty before action. A medication error concerns the process and may or may not cause harm. An adverse event occurs during treatment but may be unrelated. Keeping these concepts separate prevents two opposite mistakes: attributing every new symptom to a drug, and dismissing a plausible drug contribution because the person also has an illness.
Clinical reasoning runs on two tracks. One protects the person from immediate deterioration; the other tests the causal explanation. Do not let uncertainty about attribution postpone emergency care. Equally, do not convert a provisional suspicion into a permanent class-wide allergy label without recording what actually happened. The most useful record preserves the medicine, dose, timing, reaction features, objective findings, alternative causes and response after withdrawal. Such detail allows another clinician to reassess risk later. A single word such as allergy loses most of the information needed to select future treatment.
Key points
- A reaction can follow a new medicine, dose increase, interaction or altered clearance of long-established treatment.
- Severity describes intensity; seriousness describes outcomes such as hospitalisation, disability or threat to life.
- An adverse drug reaction is not automatically an allergy; record the actual phenotype and its timing.
- Stop suspected causative medicines when serious toxicity is plausible, while protecting essential treatment through an alternative plan.
- Report serious suspected reactions to established medicines and all suspected reactions to black-triangle medicines through Yellow Card.
- Update the prescribing record and explain future avoidance or specialist assessment before the patient leaves care.
02Situations and prioritiesThe context, relevant information and actions that matter most.
A reaction can be related to the intended mechanism: excessive anticoagulation leads to bleeding, and excessive blood-pressure reduction produces dizziness or falls. These patterns often become more likely with greater exposure or susceptibility. Search for dose changes, impaired elimination and additive drugs. The fact that an effect is predictable does not make it harmless or remove the need for reporting when serious.
Urticaria, angioedema and respiratory compromise suggest an immediate hypersensitivity pattern. Delayed reactions may begin after days or weeks and can involve skin plus internal organs. Skin pain, blistering and mucosal lesions are more concerning than an isolated mild itch. Widespread rash with fever, facial swelling or organ abnormalities warrants urgent assessment rather than a routine antihistamine prescription.
New toxicity may follow dehydration, an intercurrent infection, cessation of an enzyme-inducing exposure, a changed formulation or a new purchased medicine. Ask what changed in the patient and their environment, not merely whether a prescription was added. A stable printed dose does not guarantee stable drug concentration or biological effect.
Symptoms after reduction or cessation can arise from physiological adaptation or recurrence of the treated disorder. Timing, new symptom qualities and the pace of dose reduction help distinguish them. Withdrawal symptoms should not be automatically relabelled as proof that lifelong treatment is necessary, but neither should severe symptoms be dismissed as anxiety about stopping.
03Assessment and interpretationHow to gather information, assess the situation and recognise uncertainty.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Exposure timeline - Why
- Relate medicine use to the onset and evolution of harm.
- Interpretation and limitations
- Include first and last doses, changes in strength, intermittent products, injections, patches and medicines stopped recently. Plot the symptom onset against each exposure. A plausible sequence supports causality, but temporal association alone is insufficient: the illness prompting treatment may produce the same findings.
- 02
Physiological and organ assessment - Why
- Identify instability and quantify the suspected toxicity.
- Interpretation and limitations
- Use observations and examination to determine immediate risk, then choose focused tests such as blood count, renal profile, liver tests, ECG or drug concentration according to the suspected syndrome. A broad untargeted panel may create incidental abnormalities without resolving the key question. Serial change can be more informative than one result.
- 03
Alternative explanations - Why
- Check whether another process better explains the event.
- Interpretation and limitations
- Consider infection, progression of disease, dehydration and non-medicine exposures. A drug can contribute even when another cause is present: diarrhoea may both represent illness and reduce clearance of a medicine. Reason in terms of interacting causes rather than insisting on a single explanation for every finding.
- 04
Allergy-focused documentation and referral - Why
- Preserve the reaction phenotype for future treatment selection.
- Interpretation and limitations
- For suspected allergy, record signs, severity, timing, route and the number of doses before onset. Routine serum specific-IgE testing in a non-specialist setting does not resolve most drug-allergy questions. Severe immediate or severe delayed reactions need specialist allergy assessment, with the acute record available to support interpretation.
04Worked approachesCases with ordered reasoning, an action and a check of the outcome.
01Worked caseEvaluate a possible pulmonary drug reactionA 68-year-old taking long-term nitrofurantoin for recurrent urinary infections develops progressive dry cough and breathlessness. Oxygen saturation is 91% on air; there is no recent urinary illness and the repeat medicine has not been reviewed for a year.+
- 1Prioritise the respiratory impairment and arrange urgent assessment. Establish observations and examine for infection, heart failure and other causes; do not assume a familiar long-term preventive medicine is irrelevant merely because it predates the symptoms.
- 2Reconstruct the duration of nitrofurantoin exposure and any previous respiratory or hepatic symptoms. The temporal pattern is compatible with a delayed medicine reaction, but alternative pathology remains possible and requires appropriate investigation.
- 3Stop nitrofurantoin when pulmonary toxicity is suspected and document the reason. Assess the ongoing need for urinary prophylaxis separately after the acute problem is addressed; replacing one preventive prescription should not distract from current hypoxaemia.
- 4Arrange focused respiratory evaluation and review any relevant liver abnormalities. Explain the suspected relationship, the uncertainty and what symptoms should prompt immediate help. Record the medicine as a suspected serious reaction with its actual manifestations.
- 5Verify the outcome through follow-up of respiratory findings and investigation results, update the repeat system and communicate with the regular prescriber. Submit a Yellow Card on suspicion; improvement after withdrawal adds evidence without proving that every symptom was drug-induced.
02Causality assessmentBuild and revise a defensible explanationA stable patient develops a new symptom that could plausibly reflect a medicine effect.+
- 1Write the competing explanations before changing treatment. Compare latency, known pharmacology and objective findings. A reaction occurring after each dose is suggestive, while a symptom beginning before exposure weakens that particular causal claim.
- 2Choose a proportionate dechallenge when safe: stop, reduce or substitute the most plausible agent and define the observation interval. Account for its persistence in the body; symptoms may not resolve immediately after the final dose.
- 3Avoid deliberate re-exposure after a severe allergic or organ-toxic reaction simply to establish certainty. If future use could be essential, obtain specialist advice and preserve the original evidence rather than performing an informal challenge.
- 4Reassess using the subsequent course. If symptoms worsen despite stopping the medicine, revisit alternative diagnoses and the possibility of persistent injury. Do not allow a medication label to close further investigation prematurely.
03Aftercare and reportingPrevent the next avoidable exposureAcute treatment is complete and the person is ready to move to another setting.+
- 1Enter a specific, visible reaction record and distinguish suspected allergy from other adverse effects. Include the event date, responsible medicines, seriousness and any uncertainty; keep the prescribing and discharge records consistent.
- 2Explain what should be avoided now and whether that applies to one medicine or a wider group. Give the person written information that they can show when another professional proposes treatment.
- 3Report the appropriate suspected reaction through Yellow Card with the clinically useful sequence and findings. Record the brand and batch when relevant, especially for biological products, and complete the local incident pathway if an error also occurred.
- 4Name the team responsible for investigation results, allergy referral and recovery monitoring. Check that an automatically generated repeat has not reintroduced the suspected culprit after the clinical team intended it to be stopped.
05Relevant medicines and safetySpecific regimens and precautions when the skill involves prescribing.
Adrenaline for adult anaphylaxis
Give 500 micrograms intramuscularly: 0.5 mL of 1 mg/mL adrenaline into the anterolateral thigh; repeat after 5 minutes for persisting airway, breathing or circulation problems.Confirm concentration and intramuscular route; intravenous adrenaline requires experienced specialist management. Antihistamines do not treat airway compromise or shock.
06Feedback, follow-up and evidenceReview outcomes, seek feedback and identify what to improve.
- Assess organ recovery using the same clinically meaningful markers that established injury. A falling abnormal result can be reassuring, but persistent functional impairment may still need specialist follow-up and a revised treatment plan.
- Review whether withdrawal of the culprit has left an untreated disease. Arrange a safe alternative when necessary, and distinguish urgent replacement of essential therapy from restarting a non-essential preventive medicine.
- Check that the reaction is visible wherever further prescriptions will be generated. The patient-held description and the clinical record should identify the same suspected medicine and manifestations.
- Update the causal assessment when new information becomes available. Record specialist conclusions, including successful exclusion of allergy, so that an inaccurate provisional label does not continue restricting future treatment.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
Seriousness is an outcome
A symptom described as mild can be serious if it precipitates a fall and hospitalisation. Conversely, an intense but brief symptom may not meet a regulatory seriousness criterion. Record both the symptom severity and its consequences rather than assuming they are interchangeable.
Dechallenge has limitations
Improvement after stopping a medicine may reflect simultaneous treatment of infection, correction of dehydration or the natural course of disease. It increases confidence when the timing and mechanism fit, but it should be interpreted alongside the entire clinical sequence.
Nocebo does not exclude harm
Expectations can influence symptom experience, yet a worried patient can still have genuine toxicity. Validate the concern, investigate objective warning features and agree a transparent plan. Prematurely attributing symptoms to expectation can delay recognition of an important reaction.
A useful pharmacovigilance signal
Spontaneous reports help identify possible safety problems, especially unusual patterns that trials cannot characterise fully. A report is an observation requiring assessment; report counts alone cannot establish the incidence of a reaction because the number exposed and reporting behaviour are uncertain.
08Common pitfallsFrequent interpretation and management errors.
- 01
Recording gastrointestinal intolerance as anaphylaxis creates an inaccurate future risk assessment; document the actual features even when the person prefers to avoid the medicine.
- 02
Waiting for proof before reporting a suspected serious reaction defeats the purpose of a surveillance system designed to investigate uncertainty.
- 03
Changing several plausible culprit medicines simultaneously may be necessary in severe illness but makes subsequent attribution harder; document why each was stopped.
- 04
Assuming no reaction can occur after years of treatment overlooks delayed toxicity and changes in clearance, interacting medicines or susceptibility.