01Principles and purposeThe professional or clinical skill and the decisions it supports.
Adverse drug reactions range from common dose-related effects to rare severe immune-mediated syndromes. The first distinction is clinical urgency. A patient with airway compromise needs treatment while the team is still identifying the exact antibiotic, whereas a stable patient with a mild symptom may allow a more deliberate assessment of timing, mechanism and alternatives. Do not let uncertainty about causality delay action on a credible serious reaction. Equally, do not assign every new symptom to the most recent prescription without evaluating infection, disease progression and other plausible explanations. Good assessment combines rapid safety decisions with a clear account of what is known and uncertain.
Causality is strengthened by a plausible time relationship, a known mechanism, dose or exposure change, improvement after withdrawal and lack of a better explanation. None is infallible. A medicine taken uneventfully for years can cause problems after kidney function declines or an interacting drug is added, and some reactions occur after a delay. Deliberate re-exposure solely to prove a serious allergy is inappropriate outside a properly indicated specialist process. The aim is to make a defensible clinical decision and prevent further harm, not to create false certainty. Documentation should preserve the evidence so that future treatment choices remain both safe and appropriately broad.
Key points
- Assess and treat the patient before completing a causality score or reporting form. The next action is determined by physiological severity as well as the suspected mechanism.
- For adult anaphylaxis, give adrenaline 500 micrograms intramuscularly into the anterolateral thigh, using 0.5 mL of 1 mg/mL solution; repeat after 5 minutes if airway, breathing or circulation problems persist.
- Stop the suspected infusion or exposure when feasible, call for help and use an ABCDE approach. Antihistamines and corticosteroids do not replace adrenaline for life-threatening anaphylaxis.
- An adverse drug reaction is not automatically an allergy. Record the actual symptoms, timing, severity, medicine and route so future clinicians can distinguish intolerance, pharmacological toxicity and suspected immune-mediated reactions.
- Consider new medicines, dose increases, interactions, organ deterioration and withdrawal as possible explanations for a change in condition, while assessing competing diagnoses.
- Report suspected clinically important reactions through the appropriate MHRA Yellow Card route; proof of causality is not required before reporting a reasonable suspicion.
- After immediate care, correct the prescription, arrange monitoring and communicate the reaction and future avoidance or specialist assessment plan across the patient’s care team.
02Situations and prioritiesThe context, relevant information and actions that matter most.
A rapidly evolving illness with airway, breathing or circulation problems after exposure raises anaphylaxis; skin or mucosal features may support the diagnosis but are not required in every case. Wheeze, stridor, hypotension and collapse demand immediate assessment. A rash alone does not describe the same emergency, and waiting for every classic feature can delay treatment.
A new rash accompanied by fever, mucosal lesions, skin pain, blistering, facial swelling or systemic illness needs urgent assessment. Ask about recently started medicines and the full exposure timeline. A delayed reaction should not be dismissed because the last dose was tolerated immediately or because the suspected medicine has already been stopped.
Bleeding with antithrombotics, hypoglycaemia with insulin or sulfonylureas, and bradycardia with rate-limiting treatment illustrate mechanisms that may become excessive. Check dose, administration, interactions and organ function. These effects require accurate adverse-reaction documentation and management, but labelling all of them as allergy can distort future prescribing.
Ask what changed before the symptoms began, including over-the-counter medicines, dehydration, smoking status or a new comorbidity. A stable prescribed dose does not guarantee stable exposure. An interaction may increase the concentration of one drug or combine similar physiological effects without changing either measured concentration.
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
Immediate observations and targeted bedside tests - Why
- Assess severity and identify threats requiring treatment now.
- Interpretation and limitations
- Use airway assessment, respiratory observations, pulse, pressure, consciousness and other indicated bedside tests. Do not wait for a laboratory result to treat anaphylaxis. In a suspected pharmacological reaction, glucose or ECG assessment may rapidly identify the mechanism of collapse or confusion.
- 02
A detailed medicine and symptom timeline - Why
- Evaluate causality and identify the most plausible culprit exposure.
- Interpretation and limitations
- Record start dates, recent dose changes, route, actual administrations and the time symptoms appeared. Include medicines taken intermittently, supplements and recent courses that have ended. A precise timeline may distinguish a delayed reaction from an immediate infusion-related event.
- 03
Organ function and reaction-specific investigations - Why
- Assess consequences and alternative explanations of the adverse event.
- Interpretation and limitations
- Select tests according to the presentation, such as blood count and liver tests for systemic delayed reactions or renal function and electrolytes for accumulation. The purpose is to assess the patient and guide treatment, not to obtain a generic panel that proves all adverse drug reactions.
- 04
Appropriately timed allergy investigations after stabilisation - Why
- Support subsequent specialist assessment without delaying emergency care.
- Interpretation and limitations
- After suspected anaphylaxis, follow the relevant pathway for tryptase sampling and allergy referral, documenting timings. A test result must be interpreted with the clinical event. An apparently normal result does not retrospectively justify withholding adrenaline from a patient with convincing life-threatening features.
04Worked approachesCases with ordered reasoning, an action and a check of the outcome.
01Worked exampleTreat an immediate severe antibiotic reactionA 42-year-old receiving an IV co-amoxiclav dose develops widespread urticaria, wheeze and throat tightness within minutes. Blood pressure is 78/46 mmHg and oxygen saturation 90%. Adrenaline 1 mg/mL is available, and no adrenaline has yet been given.+
- 1Recognise airway/breathing and circulatory compromise consistent with anaphylaxis. Stop the suspected infusion, call the emergency team and assess ABCDE. Position safely, usually lying flat with legs raised if tolerated; adapt for breathing difficulty and do not allow sudden standing.
- 2Calculate the intramuscular adrenaline volume: 500 micrograms equals 0.5 mg; 0.5 mg divided by 1 mg/mL equals 0.5 mL. Administer 500 micrograms IM into the anterolateral thigh promptly, alongside oxygen and the required resuscitation support.
- 3Reassess airway, breathing and circulation and repeat the IM dose after 5 minutes if problems persist. Obtain senior help, IV access and further anaphylaxis-pathway treatment as needed; antihistamines are not the first treatment for this hypotensive wheezing patient.
- 4Give the final immediate action as IM adrenaline and continuing emergency reassessment. Independently check the ampoule concentration, microgram-to-milligram conversion and 0.5 mL volume, then record administration time and response. After stabilisation, arrange observation, reaction documentation, indicated tests and specialist follow-up.
02Causality assessmentInvestigate a possible non-emergency adverse effectA stable patient reports a new symptom after a medicine or regimen change.+
- 1Describe the symptom, severity and functional effect, and assess alternative causes. Establish whether the medicine was newly started, increased, combined with an interacting drug or continued through organ deterioration before deciding how strong the relationship is.
- 2Check current product information and relevant guidance for the suspected effect. Compare the mechanism and expected timing with the actual case, recognising that an unlisted reaction may still be possible and that a listed effect is not proof of causation.
- 3Agree whether to continue with support, reduce, substitute or stop treatment, taking account of withdrawal and the consequence of losing control of the underlying condition. Give a specific plan for the next dose rather than leaving the patient uncertain while investigations proceed.
- 4Review the response to the change and update the causality assessment. Report the suspected reaction when appropriate and retain the clinical details, including uncertainty, so future prescribers do not inherit an inaccurate definitive allergy label.
03PreventionMake an adverse-reaction record clinically usefulA suspected medicine reaction has been assessed and an immediate plan agreed.+
- 1Record the drug and formulation, indication, route, dose exposure, onset, symptoms, severity and treatment required. Separate the observed facts from the working conclusion about mechanism or culprit, especially when several medicines were introduced together.
- 2Update the appropriate allergy or adverse-reaction field and the narrative record. State which medicines should be avoided pending assessment and whether a specialist referral is needed; avoid an unnecessarily broad class prohibition unsupported by the event.
- 3Explain the reaction and interim medicine plan to the patient, supplying accessible written information. Check that they know what to tell a pharmacist or clinician in another setting and what to do if symptoms recur.
- 4Complete relevant pharmacovigilance and local incident reporting without delaying care. Communicate the final plan to the usual prescriber and pharmacy, and confirm that a stopped culprit has not remained active on an automatic repeat list.
05Relevant medicines and safetySpecific regimens and precautions when the skill involves prescribing.
Adrenaline for adult anaphylaxis
500 micrograms intramuscularly in the anterolateral thigh, equivalent to 0.5 mL of 1 mg/mL solution; repeat after 5 minutes if airway, breathing or circulation problems persist.Check concentration and route carefully. IV adrenaline requires experienced specialist management and is not the routine initial route. Pregnancy does not justify withholding indicated IM treatment; positioning and further resuscitation require the clinical context.
06Feedback, follow-up and evidenceReview outcomes, seek feedback and identify what to improve.
- After anaphylaxis, continue physiological observation and use a risk-based observation and discharge plan through the relevant emergency pathway. Initial improvement does not automatically establish that recurrence or further treatment is unlikely.
- Following a suspected dose-related reaction, monitor the parameter connected to the mechanism, such as glucose, pulse, bleeding or organ function. Confirm that the intervention improved the clinical problem without causing avoidable loss of therapeutic benefit.
- Review the accuracy of the adverse-reaction record when specialist results become available. Removing an unsupported allergy label or refining the implicated medicine can improve future treatment options while preserving essential avoidance information.
- Check the patient’s understanding and the receiving team’s medication list after discharge. A clearly documented reaction is only protective if it informs the next prescription and the patient can communicate it when records are unavailable.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
Severity and certainty are separate axes
A serious plausible reaction may justify immediate withdrawal and treatment before causality is certain. A mild uncertain symptom may allow continued treatment during assessment. Do not demand the same evidential threshold for these different safety decisions.
Absence of rash does not exclude anaphylaxis
The dangerous component is airway, breathing or circulatory compromise. Skin findings can be absent or delayed, so the clinical assessment should not require a visible rash before initiating emergency treatment.
Reporting does not require proof
Pharmacovigilance depends on clinicians and patients reporting reasonable suspicions that may reveal patterns across many cases. State the evidence and uncertainties accurately rather than withholding a report until an impossible level of certainty is achieved.
A label can create future harm
Recording nausea as a severe antibiotic allergy can unnecessarily restrict later treatment, while recording anaphylaxis only as an unspecified side effect can understate risk. Precise event description helps future clinicians make the right distinction.
08Common pitfallsFrequent interpretation and management errors.
- 01
Giving antihistamine first to a patient with hypotension and respiratory compromise can delay the adrenaline needed for life-threatening anaphylaxis.
- 02
Calling every adverse effect an allergy can obscure the mechanism and create unnecessarily restricted future treatment choices.
- 03
Deliberately re-exposing a patient to a suspected serious culprit solely to prove causality can cause preventable severe harm.
- 04
Completing a reporting form while leaving the unsafe prescription active does not prevent another dose from reaching the patient.