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All the information I have found to date says that, once mixed, powder form Caverject / Viridal (Alprostadil) must be used within 24 hours, and not refrigerated.

My prescription is for 20mcg, but I only need 5mcg.
I would like make use of the rest of the ‘mix’ at a later date.

Can anyone tell me how quickly reconstituted Alprostadil actually loses its effectiveness?

I won't answer your question directly as I have no experience with the Caverject / Viridal. I would suggest you follow doctors or pharmacist directions.

I do have experience with Edex cartridges. They are cartridges that contain two compartments in a glass cartridge separated by a rubber plug. One compartment contains saline and the other dry alprostadil. You place the cartridge in it's dispenser, attach a needle and depress a plunger to mix the compartments. At this point, in normal use, you would inject the contents. As the cartridges are designed to be used immediately after reconstituting the saline may contain no preservatives.

Contrary to the package insert instructions I have combined the ingredients of a Edex cartridge contents, removed the needle from the cartridge applicator, and then removed the cartridge from the applicator. I then used small diabetic syringes and drew up multiple doses from the cartridge through where the cartridge needle pierced the rubber plug. I then used one dose. I then drew air into the unused syringe containing the other dose, capped it, and put it in the freezer needle up and froze it. When I used the other dose days later it worked just fine.

I'll assume the Caverject cartridge is similar to the Edex cartridge. If I wanted 5mcg doses from the 20mcg cartridge, I'd draw four doses drawn into the appropriate sized diabetic syringes, as described above. Three of syringes I'd pull air into and freeze with the needle up. Make sure you expel the air before use.

What I posted above is what I've done in the past with Edex cartridges which are also alprostadil. It's certainly contrary to instructions and so clearly against medical advice. So don't do that!

Maybe that made sense.