Anaphylaxis in the waiting room after the first dose of amoxicillin

Diagnosis: anaphylaxis (Ring and Messmer grade III) to amoxicillin. Airway, breathing and circulation are all involved; this is a time-critical emergency.

1. Stop the trigger - no further amoxicillin, and remove any remaining tablets from her possession. Note the exact time of the dose.
2. Call emergency services immediately (999/112) and state "anaphylaxis, adrenaline given" - do this early, before the situation deteriorates further.
3. Position - she is breathless and hypotensive, so let her sit upright with legs extended; do not make her stand or walk (sudden upright posture in a hypovolaemic patient can precipitate cardiac arrest). Lay her flat with legs raised if she loses consciousness.
4. Adrenaline 0.5 mg intramuscularly (0.5 mL of 1:1000) into the anterolateral thigh - this is the first and most important drug. Repeat every 5 minutes if there is no improvement. Do not delay it while giving anything else.
5. High-flow oxygen, 15 L/min via a non-rebreathe mask.
6. IV access and a crystalloid bolus (500–1000 mL) for persistent hypotension.
7. Nebulised salbutamol 5 mg as an adjunct for bronchospasm - never as a substitute for adrenaline.
8. Monitor pulse, blood pressure, SpO2, respiratory rate and ECG continuously; reassess after each adrenaline dose.
9. Second-line drugs once adrenaline has been given: chlorphenamine 10 mg and hydrocortisone 200 mg by slow intramuscular or intravenous injection.
10. If refractory: repeat adrenaline, consider a nebulised adrenaline solution, and prepare for airway support; the crew will take over. Start CPR if she arrests, with prolonged resuscitation.

Why not first: antihistamines block H1 receptors but do nothing to reverse established laryngopharyngeal oedema or the profound vasodilatation and capillary leak that cause the hypotension; their onset is far too slow to matter in the first minutes. Corticosteroids act over hours, not minutes, and there is no evidence that they prevent biphasic reactions - they are given for their delayed anti-inflammatory effect, if at all. Giving chlorphenamine rapidly intravenously can itself cause hypotension. Adrenaline is the only drug that reverses the pathophysiology: alpha-1-mediated vasoconstriction raises blood pressure and reduces mucosal oedema, beta-2 stimulation bronchodilates, and beta-2 activity on mast cells reduces further mediator release. Every minute of delay in adrenaline is associated with worse outcomes.

Monitoring and disposition:

• Continuous observation with vital signs every 5–10 minutes until the ambulance arrives, and transfer to hospital by ambulance even if she looks completely recovered - she should not drive or be sent home with a relative.
• Warn the receiving team about a possible biphasic reaction, which occurs in a significant minority of cases and can appear hours after apparent resolution; observation for at least 6–12 hours is typical, longer for severe reactions.
• Document the serial observations, the adrenaline doses and times, and the response to each.
• Ensure the ambulance crew receive a clear handover covering the drug, dose, route, time and the treatment given.

Record: the drug, dose (1 g), route (oral) and exact time; the time of onset and the sequence of symptoms; every observation with a timestamp; each adrenaline dose with time, site and response; all other drugs; who was called and when; and the handover to the ambulance crew. Write it contemporaneously and sign it.

Communicate:

• Tell the patient clearly, in writing, that she has had anaphylaxis to a penicillin and must avoid amoxicillin, ampicillin and (unless testing shows otherwise) all beta-lactams, including phenoxymethylpenicillin.
• Give her a written allergy record and advise a medical alert card or bracelet; ask her to tell every future prescriber, dentist and pharmacist.
• Refer her to a specialist allergy clinic for skin-prick and specific IgE testing - the label "penicillin allergy" has lifelong consequences and should be confirmed or refuted properly.
• Write to her general medical practitioner the same day, and flag the allergy prominently in the dental record and on the prescription screen.

Future prescribing: the odontogenic infection still needs treating. Choose a non-beta-lactam according to local resistance patterns and guidance - for example metronidazole 400 mg three times daily, or clindamycin 300 mg four times daily where indicated, with the caveat about Clostridioides difficile. Review the tooth and proceed with definitive treatment (extraction or root canal treatment) rather than relying on antibiotics.

Practice changes: restock the emergency kit with in-date adrenaline, consider autoinjectors as a backup alongside ampoules, run a team debrief and a simulated anaphylaxis drill, verify that every team member can locate the kit and draw up adrenaline, and check that the anaphylaxis algorithm is displayed in every surgery. Consider whether the first dose of any new antibiotic should be taken in the practice under observation only when there is a clear indication.