From uptodate:
Adjunctive agents — Epinephrine is first-line treatment for anaphylaxis, and there is no known equivalent substitute. Once epinephrine has been administered, adjunctive therapies may be given for residual symptoms. There is no direct evidence supporting the benefit of antihistamines or glucocorticoids in anaphylaxis, although they remain in common use, and they should not be considered part of first-line therapy. Patients treated with epinephrine generally do not need any additional adjunctive agents. If symptoms or signs persist, more epinephrine should be administered. When a patient's anaphylaxis manifestations have cleared after epinephrine but the patient continues to report itching or urticaria, an antihistamine may be helpful in relieving the symptom. Antihistamines do not relieve upper or lower airway obstruction or shock.