changed my pump out, and didn’t have enough insulin in bottle to completely draw as much as i usually do. So added the rest from a brand new just opened bottle of insulin. Some of the old insulin from syringe got INTO the new bottle of insulin, by sliding up bc of the pressure? Not sure how much insulin…the QUESTION is, will the ADDED insulin to the new bottle, (BC its adding more insulin and volume, especially to a totally filled brand new bottle and now surpassing the original amount of insulin in a new bottle, a 1000 unit bottle?) WILL THAT CHANGE the CONCENTRATION of the insulin in the new bottle and make it much MORE POTENT bc there’s more insulin that is surpassing the maximum amount a bottle can hold/is manufactured with!!! IS it SAFE to USE the pump/insulin now after drawing the rest from this new bottle with extra insulin in it??? ANy info is helpful, as when i call Tandem, they don’t know much. Very scary.


Hi. Don’t panic. The insulin in the new vial is fine*. How many days does a vial last you?

*I’m assuming the insulin in both vials is the same brand, product and concentration e.g. U-100.


I often have gotten old insulin mixed with new. I will shoot too much of a draw back into the bottle, or suction sucks it back into the bottle some. It’s not been a problem. I wouldn’t make a continual habit of it because I suppose eventually you could end up with a vestige of years old insulin in the new insulin. Insulin is measured by units/as long as it’s the same insulin it doesn’t become more potent just because there is more in the bottle.


You’re fine, I promise. You’re overthinking this. I do exactly what you just described every single time I switch a vial. I try to be cautious about not getting any old insulin into the vial, but surely it happens. I know it happens, and I’m fine with it.

If you want more specifics…

You didn’t change the concentration of insulin as you describe. The old insulin you added is the same concentration as the new insulin. All you did was change the volume of insulin in the vial.

You can’t hurt the vial by adding volume to it. You could certainly explode the vial by forcing too much insulin into it, but that would be exceedingly difficult to do. It wasn’t even pressurized to start with, so it’s certainly not going to shatter. If anything, the exact opposite was true and there was a vacuum in the vial. Liquids move from high pressure to low pressure. That’s physics for you. You actually helped make the vial safer by equalizing the negative pressure that existed before.

TECHNICALLY, you may have changed the efficacy (how well the insulin works) of that new vial a tiny bit, but I promise you’ll never notice it. We know that insulin stored at room temperature for 28 days has nearly the same exact efficacy as when it started. Approximately 99% percent, but extreme examples can go as low as 95% efficacy. We even know from this heat stability study done in tropical Kenya that insulin stored unrefrigerated in extreme heat still retains nearly all it’s efficacy for 28 days: Heat-stability study of various insulin types in tropical temperature conditions: New insights towards improving diabetes care - PMC

So seriously, the old insulin you mixed in with the new stuff was still in nearly pristine condition and had no effect whatever on the efficacy of the new vial.But let’s assume the absolute worst scenario and do the math…

You probably don’t know how much insulin got sucked into the vial, but I’m going to guess it was less than 10 units, simply from personal experience. If takes a long time for that much insulin to transfer and I can’t believe you’d have just sat for that long and not done anything. But let’s error on the side of caution and use 20 units in this worst case scenario. The new vial started out with 1000 units of insulin suspended in 10 ml of solution = 100 units per ml. Let’s say that worst case 20 units had absolutely no usable insulin in it whatever, which we’ve already established is patently false. It’s completely degraded, and just adds to the volume but no usable units of insulin. You now have 1000 units of insulin suspended in 10.2 ml = 98.05 units per ml. That’s still 98% efficacy in the absolute worst case scenario I can imagine. You wouldn’t ever notice that 2% difference. The FDA standardized that 90% efficacy is the benchmark by which we judge insulin.