Generally speaking, the stimulant vasoconstrictors don't do much for the allergic response itself; they buy you time for antihistamines and steroids to take effect.
Epi-pens are a life-saver in cases of rapidly developing anaphylaxis, like when somebody says they feel itchy and immediately pass out on the floor. Other than that, it feels gnarly, wears off quickly, and has a high risk of heart attack and stroke.
If an allergic reaction develops over an hour or so and you notice it quickly (pay attention to itching, hives, swelling etc), you can get by with oral drugs.
In the field, this is what I've used to deal with fairly severe anaphylaxis:
- Benadryl 50mg - chew it up at first signs
- Prednisone 60mg - chew it up at first signs
- Albuterol inhaler - If throat swells or fluid in lungs
- Epi-pen - Keep one for absolute emergencies.
You can re-dose the benadryl every 4 hours or so if necessary, and you can re-dose everything if vomiting occurs. Do try to stay below 100mg benadryl and 120mg prednisone in a short timespan.
No significant interactions between these except albuterol + epinephrine, but if you need epi you're fucked anyway.
Albuterol inhalers are easy to get if you say you've experienced "reactive airway syndrome". This is idiopathic asthma which often happens in healthy people that exercise outside in cold dry air and then go inside a warm humid space.
Epi-pens are also easy to get if you have a doctor, if you say you have an allergy to bee stings etc. You can also get them for your first aid kit if you lead group wilderness events.
Prednisone you might be lucky enough to have left over from a sinus infection, a really nasty case of poison ivy, a deceased pet with an autoimmune condition, etc. I've also been able to get it prescribed for use as needed managing allergies.