Important to clear up something.
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Antihistamines won't help much with a systemic reaction.
But they should help moderate local swelling for those with a normal (if strong) local reaction.
Subject to the usual caveats contained in the "Patient Information Leaflet", it shouldn't be harmful to take an antihistamine before you have been stung, but it won't help enough to be any use if you are one of those that have an extreme systemic response!
Hmmm!
Not sure that I agree entirely with what you've written.
There's a big difference between older generation (systemic) antihistamines and newer generation (peripheral) antihistamines.
One of the primary keys to surviving anaphylaxis is protecting the airway and taking a first generation antihistamine such as chlorpheniramine (Piriton) will have an anti-inflammatory response which may well be just enough to prevent a fatality in a 'first aid' rescue situation (i.e. where prescribed adrenaline is not available).
Yes, there is an issue about delayed response but this is as much about recognising early warning signs and taking the anti-histamine early as it is about the type of formulation - so Piriton liquid works faster than tablets and chewing tablets before swallowing works faster than swallowing tablets whole. Agreed that anti-histamines are not a substitute for adrenaline but your posting could be interpreted as a recommendation not to take anti-histamines during anaphylaxis and I think on balance that that is not necessarily wise counsel.
There is quite some debate (professionally) about the merits of antihistamine use in anaphylaxis. Cochrane couldn't find any evidence for their use because there have been no formal clinical trials. However, this is not the same as saying there is no evidence of value for their use.
The fact that very few people actually die from anaphylaxis in my view suggests that existing treatment is by and large effective and this is generally adrenaline followed by IV Piriton and steroids with intravenous fluids. Taking anti-histamines won't sufficiently interrupt the cascade mechanism which results in the catastrophic flood of histamine, but they will interfere with the binding of histamine to systemic receptors responsible for the swelling that threatens closure of the airway. Bearing in mind that anaphylaxis varies in speed of onset, then the anti-histamine may well have more time to work in 'delayed' onset cases.
There are other complications such as Kounis syndrome which compound the picture further and Kounis does require treatment with antihistamines and steroids.