"Agents that may be given as adjunctive therapies to epinephrine in the treatment of anaphylaxis include H1 antihistamines, H2 antihistamines, bronchodilators, and glucocorticoids (e.g. prednisone). None of these medications should be used as initial treatment or as sole treatment because they do not relieve upper or lower respiratory tract obstruction, hypotension, or shock and are not lifesaving.
Epinephrine is first-line treatment for anaphylaxis and there is no known equivalent substitute. A systematic review of the literature has failed to retrieve any randomized controlled trials that meet current standards and support the use of H1 antihistamines in anaphylaxis.
Despite this, H1 antihistamines are the most commonly administered medications in the treatment of anaphylaxis. This suggests overreliance on these agents, which should be considered adjunctive to epinephrine for the purpose of relieving itching and hives."
Up-To-Date, Anaphylaxis: Rapid recognition and treatment, Simons, F., Camargo, C.
"Other medicines that are used in the treatment of allergic reactions can help with some of the symptoms, but only epinephrine treats the entire reaction. Antihistamines (such as diphenhydramine and others) help with itching and hives, and asthma inhalers (such as albuterol) can help with coughing and wheezing, but these medications do not treat the dangerous symptoms of throat swelling and low blood pressure. In addition, antihistamines taken by mouth are too slow-acting to be effective in a rapidly-developing episode of anaphylaxis. In contrast, injected epinephrine works within a few minutes. In short, antihistamines and asthma inhalers are useful in the treatment of anaphylaxis, but they are NOT substitutes for epinephrine. There is no substitute for epinephrine in anaphylaxis."
Up-To-Date, Anaphylaxis: Rapid recognition and treatment, Simons, F., Camargo, C.