Synopsis
Recognise suspected medicine-related harm, distinguish allergic from non-allergic reactions, manage immediate risk and document evidence that prevents re-exposure.
- 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.
Key red flags
Painful skin with blistering, mucosal erosions or systemic illness after a new medicine requires immediate withdrawal of suspected culprits and urgent hospital assessment.
Reasoning priorities
Relate medicine use to the onset and evolution of harm.
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.
Worked reasoning
A 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.
- Prioritise 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.
- Reconstruct 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.
- Stop 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.
- Arrange 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.
- Verify 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.
A stable patient develops a new symptom that could plausibly reflect a medicine effect.