Antihistamines have no role in treating anaphylaxis.  The medical literature supports only the use of epinephrine and perhaps steroids. 

Diphenhydramine and other antihistamines are indicated only for itch relief.  

Diphenhydramine itself has now been shown to cause significant problems in assessing pediatric patients due to its sedative effects (making kids look sicker than they are, as well as behaviorally dyscontrolled), so pediatricians are now strongly against its use. They support later-gen nonsedating antihistamines. 

Diphenhydramine is also contraindicated in the elderly due to its anticholinergic effects (delirium, dysrhythmias, urinary retention).  

So, it is a med only for itch, with bad side effects— no indication in EMS (pruritis doesn’t kill). And should not be given in anaphylaxis since it doesn’t help. 

Nonsedating antihistamines are the drugs of choice for itch and are now in the STP.