I suspect that you may be as knowlegeable as the pros on this one, since this is largely a matter of pharmacology rather than established medical practice.
KWIM?
Yes, diphenhydramine is sedating. Second gen antihistamines aren't, and some (fexofenadine and cetirizine in particular) are great at treating cutaneous symptoms in particular... BUT... they don't hit both receptor subtypes, which means that they don't put any kind of damper on GI stuff the way the sedating ones do. Oh-- and for that reason, benadryl works a TREAT for treating nausea and other GI stuff even if it's viral.
Can it "stop" progression during a major reaction, though?
The jury is still out on this one. The pharmacology suggests not, but there is
puhhhhhlenty that isn't known about food anaphylaxis in particular. There are a LOT of mechanstic feedback points in this particular IgE-mediated second messenger cascade. The problem is that because some of those connection points are relatively unexplored, the answer is far from clear as to what impact receptor blockade has. Certainly benadryl won't STOP anaphylaxis
in progress. Not once it has developed CNS features, anyway, and those are what tends to kill. For that reason, there's NO WAY that it's a lifesaving medication.
On the other hand, is there harm in administering it promptly during a suspected/possible reaction?
I don't think that there is. It's what we've done for years.
Has it "prevented" more severe reactions? Probably not, but maybe it has prevented us from a few ER visits which were borderline (grade II to grade III without airway or cardiovascular symptoms). <shrug> Remember,
most anaphylaxis self-resolves. How much is "most?" Well, nobody knows. Obviously there is a vexing problem with collecting good data on that one.
Has it "masked" more seriouc symptoms?
Absolutely NOT. The dosing window is
so wide for benadryl that even though it has sedating effects, most of us aren't going to err significantly enough to harm our kids by giving it... and honestly, like any of us are going to let our kids out of arms reach once we've given a dose or two of benadryl, right? So I'm not too worried about her being sleepy. I'll be alert enough for
both of us, basically. LOL. BTDT, spent a NUMBER of nights watching over a benadryl-dosed, sleeping child. Even up until she was 12.
DD has been given enormous doses of Zyrtec by her allergist, but only because they kept samples on hand, and didn't for OTC benadryl.
I know that we have a couple of threads down in RXNs about the pharmacological mechanistics of all of this. One is about diphenhydramine and the other is about epinephrine.