Please help with this question. When I was putting on new pump tonight, I only had a little insulin in Bottle left, so I drew that, withdrew the syringe it had about 80 units in it? Then I put that syringe in a bottle that was almost full, it might have been a new bottle unused not sure. But the cap was off. So I might have used it once. When I was drawing more insulin from this new bottle into the syringe with the 80 units in it, when I got to how much I needed and then let go of the plunger, insulin went back up into the new bottle from syringe because it was inverted upside down obviously because I was drawing it. The question is since insulin from the old bottle went back up into the new bottle from the letting go of plunger, I’m not sure how much went back up into bottle, maybe 50 units? WOULD this EXTRA insulin going up into the new bottle change the DOSAGE/ CONCENTRATION of the insulin in the new bottle? Because now there could have been more insulin in the new bottle than a new bottle has in it when it’s unopened. Or even if it was used a little, could 50 extra units that go into it and fill it past the starting point of unopened bottle could that change the concentration? So that now when I drew from basically a mixed insulin bottle/ VIAL would it be the same concentration as a regular bottle. I think I heard you’re not supposed to do this? Now I am very nervous to use the pump supplies that I just replaced.

Adding insulin from another vial into a new vial does not change the concentration of the insulin in the new vial. If it is U100 insulin, it still contains 100 units of insulin per ml.

I would generally not allow insulin to be mixed from one vial to another but I don’t think it’s particularly harmful, just not a desirable practice. I would not hesitate to use the insulin that has been mixed from two separate vials. It’s a lot more important that storage practice has not allowed freezing or storing too hot.

1 Like

You won’t notice a thing, I promise. So long as the old vial was stored appropriately, i.e. it didn’t sit for more than a month a room temp, it’s guaranteed to still be full potency. So, as Terry pointed out, you’re not changing the concentration of your insulin any.

But insulin does degrade with time. Old insulin doesn’t work quite as well as new, fresh insulin. It doesn’t just up and quit on us, though. It just doesn’t work quite as well as it did when it was new. It’s a slow process where you need to use just a little bit more to get the same effect.

So, let’s assume the absolute worst. Let’s say that old bottle got cooked and the insulin inside was worthless, with an efficacy of 0. It’s really simple math. You still have 1000 good units of insulin in a vial that now contains the volume of 1050 units. 1000 ÷ 1050 = 95%

That’s the absolute worst scenario possible, that the new vial will work 95% as well as it would have at the time you opened it. You would seriously never notice that. It’s just one more drop in the crazy bucket of living with unpredictable and ever-changing diabetes. BUT… Since you were still using that old vial, and you know it was still working well for you, you can assume that the true efficacy is going to be even closer to 100%.


Thanks for your answer. I was more concerned with an INCREASE of the potency a lot in the bottle, not decrease as you said, and I wouldn’t be able to tell what the dosage/ concentration was in the new mix insulin that I had used in the syringe. If you guys are sure it wouldn’t change the concentration if the old insulin went into the new bottle whether it was brand new with a thousand units and added to that to make it over a thousand, or if the new bottle was slightly used and the old insulin was pushed up into it to equal less than a thousand in the new bottle, either way it wouldn’t change the concentration? I was going to call a pharmacist but thought I’d ask it here. If you guys are sure I’ll listen to you hopefully it’ll be okay thanks for your answers appreciate it.

Insulin never gets stronger. It’s in a constant state of degradation and we take so many steps to try and preserve it’s effectiveness for as long as we can. There is ZERO chance mixing old with new will make the newer insulin stronger.

And a pharmacist couldn’t tell you that. They’re bound to dispense medication according to manufacturer directions.


If you ever are in the situation again, it’s best to combine the insulin in the syringe instead of in the bottle.
I started off on my diabetes journey mixing different kinds of insulin in one syringe, so it was a lesson I learned early.
All you do is inject some air in the full bottle, then inject air in the second almost empty one. Then pull from the full bottle to get the full syringe. This way you don’t contaminate the new bottle in case something was wrong with one of them.

If you have a tandem, it’s even easier, you can fill the cartridge with one shot and add a second shot from the new bottle.

I would be very surprised that there is any difference between them as long as neither was left in a hot car or something.

1 Like