# Anaphylaxis > Anaphylaxis for NEET-PG Medicine: diagnostic criteria, intramuscular adrenaline 0.5 mg into thigh, positioning, fluids and biphasic reactions. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/anaphylaxis - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Anaphylaxis", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/anaphylaxis ## Direct answer Intramuscular adrenaline into the thigh, before anything else, is what saves the patient with anaphylaxis — a severe, rapid-onset systemic hypersensitivity reaction involving airway, breathing or circulation, usually with skin or mucosal changes, triggered by foods, drugs (especially antibiotics and NSAIDs), venoms and contrast media. The adult dose is 0.5 mg (0.5 mL of 1 in 1,000) into the mid-anterolateral thigh, repeated every 5 to 15 minutes, with the patient laid flat with legs raised, high-flow oxygen and aggressive intravenous crystalloid. Antihistamines and corticosteroids are adjuncts only and must never delay adrenaline. ## What you must remember - Diagnostic criteria: acute onset of skin or mucosal involvement (urticaria, angio-oedema, flushing) plus respiratory compromise or hypotension; or two or more systems after likely allergen exposure; or hypotension after a known allergen. - Adrenaline: 0.5 mg intramuscularly (0.5 mL of 1 mg per mL) mid-thigh in adults; children 0.01 mg per kg up to 0.5 mg; repeat every 5 to 15 minutes — most patients respond to one or two doses. - Position: supine with legs raised (autoresuscitation); sitting up can cause empty-ventricle cardiac arrest; pregnant patients lie on their left side. - Refractory anaphylaxis: start an intravenous adrenaline infusion in monitored care; glucagon (1 to 5 mg intravenously) bypasses beta-blockade; bronchodilator nebulisers for persistent wheeze. - Fluids: a rapid bolus of 500 to 1,000 mL of crystalloid through a large-bore cannula for shock, repeated as needed — massive volume shift can need several litres. - Adjuncts only after adrenaline: nebulised salbutamol, antihistamine for urticaria, hydrocortisone (its benefit is debated); none prevents the immediate reaction. - Observe for biphasic reactions (recurrence 1 to 72 hours later, most within 8 to 12 hours); discharge with an adrenaline autoinjector, a written action plan and specialist allergy referral. - Common triggers in India: drugs (beta-lactams, NSAIDs, neuromuscular blockers), foods (peanuts, tree nuts, shellfish, eggs), hymenoptera stings, latex and radiocontrast. ## A reaction unfolds in the emergency room Ten minutes after a diclofenac injection, a 45-year-old develops generalized urticaria, a swollen tongue, stridor and a blood pressure of 80/50. The criteria are already met — acute skin involvement plus respiratory compromise plus hypotension after a likely allergen — so treatment starts without waiting for anything. Adrenaline 0.5 mg intramuscularly goes into the mid-anterolateral thigh now, drawn up as 0.5 mL of the 1-in-1,000 solution; the vastus lateralis absorbs faster and reaches higher peaks than the deltoid. Lay the patient supine with legs raised — sitting up has caused empty-ventricle cardiac arrest, and late pregnancy tilts the patient to the left. Give high-flow oxygen and run in 500 to 1,000 mL of crystalloid through a large-bore cannula, repeating as needed, because massive capillary leak can consume several litres. Reassess continuously and repeat adrenaline every 5 to 15 minutes; most patients respond to one or two doses. If shock persists despite two or three doses, start an intravenous adrenaline infusion in monitored care — a specialist decision, unlike the intramuscular dose. If the patient takes beta-blockers and responds poorly, glucagon 1 to 5 mg intravenously bypasses the blocked receptor. Only after all this do the adjuncts earn their place: Observe for biphasic recurrence — most within 8 to 12 hours, though the window extends to 72 — and discharge with an autoinjector, an action plan and allergy referral. ## Where students slip Three slips account for most lost marks. The mimic: vasovagal syncope shows bradycardia, pallor and prompt recovery lying flat with no urticaria — and hereditary angio-oedema presents with angio-oedema but no urticaria, a poor adrenaline response and a low complement C4. The sequencing: a question offering "hydrocortisone first" is testing whether you know adrenaline intramuscularly into the thigh answers nearly every anaphylaxis question. The route panic: intravenous adrenaline belongs to refractory shock in monitored hands, and choosing it as the first step is the planted wrong option as surely as choosing it never is. ## Frequently asked questions ### What is the first-line drug, dose and route in adult anaphylaxis? Adrenaline 0.5 mg intramuscularly (0.5 mL of 1 in 1,000) into the mid-anterolateral thigh, repeated every 5 to 15 minutes as needed. ### Why is the thigh preferred over the arm? The vastus lateralis site gives faster absorption and higher peak blood levels than deltoid injection. ### What is given when the patient takes beta blockers? Intravenous glucagon, which activates cardiac adenylate cyclase independent of beta receptors; atropine may help bradycardia. ### What are biphasic reactions? Recurrence of symptoms hours after initial resolution — most within 8 to 12 hours — which is why patients are observed before discharge. ### Which patients need an adrenaline autoinjector? Anyone with a systemic reaction to an unavoidable allergen, significant asthma, or a reaction to a trace exposure — plus education and allergy referral. ### How is anaphylaxis distinguished from vasovagal syncope? Syncope shows bradycardia, pallor and prompt recovery on lying flat without urticaria, bronchospasm or angio-oedema.