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Intramuscular epinephrine 0.3–0.5 mg of a 1:1000 solution is universally recommended as the first-line treatment for anaphylaxis because it rapidly reverses airway obstruction and shock; IV epinephrine is reserved for refractory cases, while adenosine and atropine have no role in this setting [Ellis & Day, 2003, PMID 12542987; Simons & Simons, 2010, PMID 20519893].