Anaphylactic shock, the most serious of allergic reactions, is a life-threatening medical emergency because of rapid constriction of the airway, often within minutes of onset.
In addition to insect stings people may have reactions to eating nuts, nut oils, shellfish and pork.

Note: Sierra Club Medical kits do NOT contain EpiPens, so participants are responsible for bringing their own.

If you have any reason to be concerned that anaphylaxis will occur even before you see signs (i.e., patient with a known history or anaphylactic reactions to bee stings) administer Benadryl ASAP after exposure to the allergen.

If patient shows signs and symptoms of anaphylaxis (swollen face, lips and tongue; difficulty swallowing; systemic hives; respiratory distress; inability to speak in more than one or two word clusters; signs and symptoms of shock), use an EpiPen to administer epinephrine (adrenaline) via a shot in the thigh.
Epinephrine generally lasts for 20 minutes (you may be able to stretch it to 40 min. between shots) so additional injections or a dose of liquid antihistamine (Benadryl®, Zyrtec®) would generally be needed to get to a place where a participant could be evacuated via helicopter.

Give Oral antihistamines if patient is able to swallow (e.g. oral diphenhydramine [Benadryl®] :50-100mg every 4-6 hours).

Two EpiPens are recommended, because of the time¹ to get evacuated or possible malfunction (this is quite rare, but it does happen occasionally, whether because of a manufacturing defect or because of exposure to temperature extremes), or one EpiPen might not be enough.
Repetitive administration of epinephrine can cause tachycardia (rapid heartbeat) and occasionally ventricular tachycardia with heart rates potentially reaching 240 beats per minute, which can also be fatal.

EpiPens have a relatively short shelf-life of about 1-2 years.

1. Many of the places we go on day hikes and Introduction to Backpacking do not have cell phone service, so it may take 15 minutes to get up to a ridge where we can call 911, then 30 - 40 minutes to respond. Helicopter landing spots are also an issue, so the patient may have to be transported to an open area. All of which could take up to 1 hr.
The Benadryl will usually kick in by the time the epipen wares off, but a second epipen is a good idea.

See:
EpiPen information at: EpiPen.com, Anaphylaxis here

Sierra Club Leader Notes: Certification to Carry an EpiPen and Anaphylaxis Protocol at ClubHouse