A few months back I used a multi use vial after not having used it for an unknown time.
I have sense come to understand that once the vial is open one should either use it up or throw it away. I use 90 days as my maximum length of time to use a vial once it is opened or mixed. I take tape and write the date I want to throw it away.
What happened is I developed redness, tenderness and minor swelling. I assumed it was a one off until I had 3 +- injection sites that were like this. I stopped using that vial.
I keep my vials wrapped in tin foil and check the needle for discoloration. I took the tinfoil off the vial and there were 'clots' in the bottom of the vial that disappeared when I shook the bottle. Same thing the next day.
I had planned on writing about this experience one it was over and the redness etc. resolved. Then forgot.
It took about three days to resolve. I do not know if it was that bacteria had grown or that the cobalamin had come out of solution so I was injecting a very high amount. I expect my 90 day maximum use has eliminated the problem.