If the dex that is injected is similar to the liquid stuff my son takes every day, then it is good for about a year, while the label on our solu-cortef states that it is good for only 3 days after it has been mixed (sorry, not the 24 hours I stated above, though not much better). I would assume that that is why the dex is available in prefilled syringes, while the solu-cortef isn't, because of this big difference in shelf life, once in a liquid state. I am not familiar with cortisone acetate, but would guess that it is similar to solu-cortef, which is also a form of hydrocortisone. As to the bigger issue of why one medicine lasts so much longer than another, I'd guess it would have to do with basic molecular makeup.  To that degree, that is probably a better question for a chemist.