Nitrous Oxide, or a blend holding it, is still used an analgesic, typically the 'Entonox' brand.
I believe it is used in birth-pain easing but I am no expert on that kind of thing! (Her Majesty Queen Victoria used chloroform.)
However, I recall being taught how to use Entonox, and a Warm Air Breathing kit, in a cave-rescue "workshop" session. With these the casualty holds the face-mask, and tends to drop it when sufficiently dosed, so it is inherently safe when used like that, under supervision. A demand-valve prevents waste of gas. It big advantage is that used properly, it soon wears off and does not leave residues that could interfere with the subsequent hospital treatment.
Similarly if the air-heater, using a CO2 + soda-lime reaction in a heat-exchanger, becomes too warm. (If the casualty is unconscious one of the first-aiders monitors the temperature, if necessary personally taking a cautious breath at intervals.) It is used to ward off exposure and its resulting hypothermia.
I have also used Entonox for real, when I slashed my hand on a badly-opened food tin and needed stitches.
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What of the "disposable" cylinders from small DIY-type MIG welders? The label says dispose of as scrap metal in the local "recycling centre" but I am wary of that. Ordinary aerosol cans are a different matter. I vent their residual propellant completely then knock the valves in with a punch and hammer. The welding-gas cartridges seem more problematical and it is not easy to determine if they are empty even in use. ('Specially with my steel-splodging!)