Why do pens last less long than vials?

it seems like i can have a vial at room temperature for a month, and then start using it and have it stay good for a month while i use it, but a pen seems to be less effective after just a couple of days out of the fridge. both are humalog. any idea why?

what i found with pens in general is if you don't press and hold for a little bit of time, that the whole bolus doesn't go through and you get higher blood sugars. If you are just using a syringe with a bottle of humalog the shot does have a better affect.

Are you keeping your pen in your pocket or someplace warmer than if you just leave a bottle of humalog out on a table lets say? They do need to be kept at room temp. so maybe where you keep your pen is in a higher temp than your bottle?

According to the packaging Humalog expires 28 days after you first inject air into the bottle. Maintaining constant temperature is suppose to help maintain the life. I think it has something to do with being made from actual dna rather than a synthesized (man made) formula like Novalog.

My Humalog seems to start declining in efficency after 16-20 days. Actually it doesn't really decline in efficiancy it just takes longer to start acting.

I don't know anything about vials.