Adverse drug reactions, interactions, allergies and pharmacovigilance

MED7 — Adverse drug reactions, interactions, allergies and pharmacovigilance

• Distinguish adverse drug reactions, side effects, intolerance, allergy, interaction and medication error.
• Take a structured approach to suspected medicine-related harm.
• Improve the accuracy and usefulness of allergy and adverse-reaction documentation.
• Recognise clinically important interaction risk without relying solely on software alerts.
• Use the MHRA Yellow Card scheme appropriately as part of pharmacovigilance.

“Reaction to a medicine” can describe several different problems. A predictable pharmacological adverse effect, an immune-mediated allergy, a dose-related toxicity, a withdrawal effect, a drug-drug interaction and a medication error have different implications for future treatment. Labelling all of them simply as “allergy” can deny useful treatments; failing to record a true serious allergy can expose the patient to preventable harm.

Start with chronology: what medicine was started, stopped or changed, when did symptoms begin, and what happened after withdrawal or dose adjustment? Consider dose, route, renal and hepatic function, concurrent illness, other medicines, supplements, alcohol or substance use, and alternative diagnoses. The question is usually not “Can this medicine ever cause this symptom?” but “How plausible is this medicine as the cause in this patient at this time?” Severe reactions require immediate clinical management before detailed causality analysis.

An allergy record should describe the medicine, the nature and severity of the reaction, approximate timing where known, and the source or certainty of the information. “Penicillin allergy” without a reaction history may represent rash, gastrointestinal intolerance, childhood hearsay or a genuine severe hypersensitivity reaction. The distinction matters. Do not remove or challenge a serious allergy label casually; where clarification is clinically important, use appropriate specialist or local pathways.

Interaction checkers are valuable but cannot determine significance in isolation. The same combination may be acceptable with monitoring in one patient and unsafe in another because of frailty, renal impairment, QT risk, bleeding risk, CNS depression, narrow therapeutic margins or concurrent acute illness. When an interaction alert is overridden, the rationale and any monitoring or mitigation should be clear.

A new symptom after a medicine change can be mistaken for a new disease and treated with another medicine. This creates a prescribing cascade. Before adding treatment, consider whether the symptom could be medicine-related. Examples include dizziness, confusion, constipation, oedema, bleeding, postural symptoms and electrolyte disturbance. The correct response may be dose adjustment, substitution or withdrawal rather than adding a second medicine.

The MHRA Yellow Card scheme collects suspected safety concerns involving medicines and other healthcare products. For established medicines and vaccines, serious suspected adverse drug reactions should be reported even when the reaction is already recognised. Reporting is based on suspicion; certainty is not required. Reports are more useful when they include chronology, relevant history, concomitant medicines, treatment dates and outcomes.

Learning from reactions

The immediate clinical response should be followed by system learning. Update the medication and allergy records, explain the event to the patient, communicate relevant information across care settings and consider whether others may be at risk from the same process. If the event also involved an error or near miss, local patient-safety reporting may be required in addition to pharmacovigilance reporting.

Practical reasoning framework

When harm is suspected ask: What exactly happened? What changed beforehand? Is there a plausible temporal and pharmacological relationship? What alternative explanations exist? What must be stopped, treated or monitored now? What should the patient avoid in future? What needs recording and communicating? Is a Yellow Card or local incident report indicated?

Sources for this module

• MHRA - The Yellow Card scheme: guidance for healthcare professionals, patients and the public (updated July 2025).
• GMC - Reporting adverse drug reactions, medical device incidents and other patient safety incidents.
• NICE NG5 - Medicines optimisation.
• Current BNF/product information and local allergy guidance.

Educational note: medicine-specific doses, thresholds, contraindications and rapidly changing regulatory requirements must be checked against current authoritative guidance and local policy at the point of care.