Thanks, all, for the great info!Usually the vials start to lose efficacy somewhere around 4-6 months, but we've heard of them lasting much longer. The only way to know is to watch the numbers....if they start to go up and there's no other reason, it's time to suspect the insulin.
Most of us use the pens so we don't end up throwing half a vial away. A vial holds 1000 units which most cats won't use before it starts to go bad. A pen only holds 300 units in each pen so we can use every drop.
The other pens (that aren't currently in use) stay safely in the refrigerator until needed and they are good until the expiration date on the box....usually about 2 years away.
Most of us buy our Lantus from Marks Marine pharmacy in Canada because they're so much cheaper than anywhere in the US

I did really love the convenience of the pen and the easy way to measure and shoot etc
I think most people keep the pen they're using in the fridge, too, assuming they're on a small enough dose that it won't be used up within a month. I also use Mark's Marine.And the ability to use just one pen and keep the others in the fridge.

Thank you @Chris & China. That makes sense and I understand why you do it. Due to a bad experience with a previous vet, it probably won't work for us. But I'm really glad to know about Mark's Marine! (what a funny name for a pharmacy?) Agree that syringes are so much more precise.We don't use the "dial-a-dose" mechanism on the pens.....we use the regular insulin syringes and pull the insulin out of the pen, just like as if it were a "mini-vial"
The pens don't give the option of making the small changes in doses like we recommend
HA!Maybe Pretty Girl likes the vials better because they're like crystal--or diamond!--but the pens are only common plastic.![]()