So, when I think an austere environment, I am not just thinking 3 miles from the trailhead in Nevada. I am thinking anywhere on Earth (so who needs epi?) Here’s my 5:
Ciprofloxacin: If I can only pick one abx, I am picking a nuclear option like a flouroquinolone. Cipro seems like a good pick to me against most gram positive and negative pathogens particularly for UTI and GI bugs. Sure it is rough, but I need a multitool and this is a good one… unless I’ll be treating infants, peds, pregnant ladies… then I’d better choose something else.
Loperimide: Not that I particularly like immodium as a drug, particularly because you can have some bad adverse and rebound effects... but, I can make up some ORT solution pretty easily, and immodium might help keep the patient alive until we can start ORT or evac of patients suffering from G/I maladies that I can’t take out with the Cipro. The cipro would work well for vibrio or shig (where loperamide won’t work well anyway) but a rotavirus or other viral diarrhea is an issue the loperamide can help with. An alternative GI med would be Bismuth subsalicylate… really a great choice because it will help with N/V/indigestion too which keeps people going.
Ibuprofen: I probably want an antipyretic and anti-inflammatory more than I want a narcotic. It buys a lot more types of maladies to treat and buys a lot mor time than a narc to allow for self-transport to definitive care since we need patient participation in a truly austere environment and a narced out tibial plateu fracture is not going to walk any faster, but someone with a fever might be able to stumble along with enough antipyretic. These will also keep some injuries in the game instead of stopping an expedition. Narcs also make me a target. I’d be really hard pressed to select morphine or Oxy/APAP over ibuprofen.
Diphenhydramine: Allergies and anaphylaxis! 1st generation antihistamine crosses the blood brain barrier so it is marginally effective as a sleep aid and for motion sickness. It’s like a multitool! Need I say more?
#5: Artemisin… or… Good antimalarial with limited resistance that is also quite useful as an antiparasitic against helminthes, trematodes, and notably maybe effective against
Trypanosoma cruzi. Malaria is amazingly widespread and I don’t want it (nor trypanosoma)! In the #5 slot there are so many things I could choose… I could choose a combivent, but I expect people with emphysema, asthma, etc to have their own supply when traveling in austere regions. The same would go for something like glucagon or insulin... lifesavers for common stuff, but I expect people to have their own. #5 could be an antifungal, another antibiotic, a narcotic, an altitude med, or an antimalarial. My key reason for asking on the region is that in the tropical areas an antifungal and often antimalarials becomes more important vs say having an altitude illness med for operating above 3500m with acclimated persons and above 4500m with acclimated persons. But areas above 4000m realistically make up a small portion of the austere areas of the world, although if speaking to expeditions, they are a preferred destination. If I was in the Darien Gap, perhaps I want some HemCon bandages and a liter of DEET
Obviously, I ‘m going to improvise a lot of care (improvised ORT) and try to prevent most other major issues through proper water treatment, bug netting and repellent, clean socks, ascending slowly, proper nutrition, and being generally careful. 
