Storing insulin correctly
Insulin is a fragile protein: too hot or too cold and it quietly stops working, usually with nothing to see. Where to keep it, and what ruins it.
Insulin is a protein, and proteins are fragile. Too hot or too cold and it quietly stops working properly, with no reliable way to tell by looking. Most unexplained runs of highs that turn out to have a cause turn out to have this one.
- Spare insulin lives in the fridge, not the freezer and not against the back wall where things freeze.
- Insulin in use stays at room temperature, for a limited number of days that differs by product, so check yours.
- Frozen insulin is ruined and cannot be rescued by thawing, even if it looks normal.
- Heat damages it too: cars, windowsills, poolside bags, radiators, a coat pocket in strong sun.
- Damaged insulin usually looks completely fine, so where it has been matters more than what it looks like.
Spare in the fridge, in-use at room temperature
Unopened insulin you are not using yet keeps in a fridge, at ordinary fridge temperature. The part people get wrong is where in the fridge: the back wall and the coldest shelf can dip below freezing, and a salad drawer or a door shelf is usually steadier. The freezer is not a slightly colder fridge, it is a different outcome entirely.
Once a pen, cartridge or vial is in use, it comes out and stays at room temperature. Cold insulin injected straight from the fridge stings more and absorbs less predictably, so this is not just about convenience. In-use insulin has a limited life once started, commonly around a month but genuinely different between products, so the number that matters is the one for yours rather than a general figure.
That in-use clock is worth taking seriously, because it runs quietly. A pen that has been on the go for six weeks may look identical to a fresh one and be doing noticeably less, and it is one of the more common invisible causes of insulin that seems to have stopped working.
The places insulin gets damaged without anyone noticing
Freezing is the absolute one. Insulin that has frozen is finished, whether or not it has thawed and whether or not it looks normal, and it should be replaced rather than used. The usual culprits are the back of a fridge, a checked bag in an aircraft hold, and a cold car overnight in winter.
Heat is the more common problem and it is cumulative. A car in summer, a windowsill, a bag on a poolside, a radiator, a coat pocket in strong sun: none of those feel dramatic, and all of them degrade insulin. It does not have to reach an extreme to lose potency; it just has to be warm for long enough, repeatedly.
Light matters a little too, which is why insulin comes in a box. Keeping it in its packaging until use costs nothing and removes one more variable.
Why looking at it is not a reliable test
Sometimes there are signs. A clear insulin that has gone cloudy, visible clumps or crystals, or a change in colour all mean it should not be used. Cloudy insulins that are meant to be cloudy should mix evenly, and clumping or a residue that will not resuspend is the same signal.
But most damaged insulin looks completely normal, and that is the important part. You cannot check a pen and reassure yourself. What you can do is think about where it has been: a hot weekend, a bag left in a car, a fridge that froze something else, a pen that has been in use much longer than it should have been.
So when insulin stops behaving, replacing it with a fresh pen is a genuinely reasonable diagnostic step, and a cheap one compared with days of unexplained highs. If a fresh pen fixes it, that was your answer.
What changes the risk
Travel raises every one of these at once, which is why insulin goes in hand luggage rather than the hold: holds get cold enough to freeze, and a bag can also go missing. In hot climates, a cool bag designed for insulin is worth having, and insulin should not sit directly against ice or a freezer block. And the fridge itself is worth knowing: plenty run colder than their dial suggests.
Where it goes wrong
Examples, not instructions or doses.
Spare pens left in a glovebox on a warm weekend, then a fortnight of stubborn highs afterwards. Nothing looked wrong with them, and nothing about the doses had changed.
A box pushed against the back wall and quietly frozen. Thawed it looks perfectly normal, and it is finished, which is why frozen insulin gets replaced rather than judged by eye.
In use for six weeks rather than the few weeks it is rated for. Not obviously different, just doing less, which is easy to read as your needs having changed.