Medical review by Elizabeth Gomez, MSN, FNP-BC
Summer is coming on fast in the Northern Hemisphere and with it, the annual worry of people with insulin-treated diabetes: How do I keep my insulin cool when I’m out of the house?
In the past, we’ve reviewed some of the many products designed to keep vials and pens of insulin cool. And our Diabetes Daily forums are full of opinions on the best way to keep insulin pumps cool when out in hot weather.
But plans are laid to waste, mistakes are made, and sometimes even the best of us realize that we’ve let our insulin languish in the hot summer sun. And then we’re stuck wondering: Is my insulin still good? Should I throw it away? For those of us that struggle to pay for our insulin, these questions can be very stressful.
It is my opinion that insulin is more resilient to heat than most people in the diabetes community realize. And I fear that a great deal of perfectly potent insulin gets thrown in the trash unnecessarily.
Let me be clear: I am not a doctor; I am not a chemist. I’m just a guy. I’m not giving advice. This article has been reviewed by an expert, to make sure I haven’t made any egregious amateur errors in interpreting the professional literature, but it does not count as expert advice.
In this article, I’ll share my thinking. You can draw your own conclusions — or, even better, discuss your thoughts with your doctor.
The Official Word on Insulin Storage
Specifications differ slightly from one type of insulin to another. But generally speaking, here are the guidelines for insulin storage as outlined by manufacturers:
Insulin should be refrigerated before it’s used.
Once opened, insulin can be stored at “room temperature.”
Once opened or removed from refrigeration, insulin should be used within four weeks (up to six weeks for some types).
Insulin can be exposed to a maximum temperature of 77 degrees F (up to 86 degrees for some types).
What happens if your insulin exceeds the maximum temperature? The instructions that come with my basal insulin put it clearly:
“If a vial has been frozen or overheated, throw it away.”
So that means that if I go on a walk on a 90 degree day and neglect to bring some device to keep my insulin cool, I should throw it in the trash. Is that reasonable?
The Dagahaley Refugee Camp Study
I’m aware of one especially compelling study on insulin’s effectiveness in very hot climates. In 2021, the journal PLOS ONE published the results of a study facilitated by the legendary nonprofit Doctors Without Borders. This study was performed with a humanitarian imperative. If they could show that insulin was more resilient than the manufacturer’s specs suggested, it could significantly alleviate the burden faced by the many people with diabetes living in hot climates without access to refrigeration.
The study was conducted in Kenya’s Dagahaley refugee camp, part of what may be the world’s largest refugee complex. Dagahaley is almost exactly on the equator. It’s hot. The study’s scientists measured a daily maximum temperature of 98.6 degrees F and an overnight low temperature of 77 degrees F. According to the official guidelines, no insulin would be safe outside climate-controlled storage.
Unfortunately, refrigerators are scarce in the Dagahaley camp, and few diabetes patients are able to keep their insulin at home. Instead, they need to travel to a treatment clinic multiple times per day in order to receive their life-giving injections. (Just consider, for a moment, how difficult that would make your day and your blood sugar management.)
The scientists wanted to know how much potency insulin retained when exposed to high temperatures such as these. And so, researchers stored one batch of insulin in ambient conditions in Kenya and another in an incubator programmed to oscillate between 77 degrees F and 98.6 degrees F. A third batch was kept cool as a control.
I’ll cut to the chase: After four weeks, all of those insulins were fully and equally potent. The insulin that warmed up to nearly 100 degrees F — not just once, but every day for a month — was just fine!
Just wait, it gets even better. Impressed by the above results, the researchers extended the experiment to a total of 12 weeks. The result held: All of the insulins, even the ones exposed to sweltering temperatures daily, were still potent. The insulin that had been warmed to nearly 100 degrees F a total of 84 times, and never once allowed to cool down to room temperature, remained exactly as effective as it was the day that it was manufactured.
It does appear that regular overnight cooling was critical to preserving insulin’s potency. Further experimentation showed that insulins stored continuously at 87.8 degrees F or higher “degrade rapidly and no longer conform with [pharmaceutical standards] after four weeks.” There certainly are places in the world where the overnight temperature is hot enough to degrade insulin, but even in these conditions it may be days or weeks before insulin loses potency.
What about even higher temperatures? The study was not designed to evaluate them, but there’s one tidbit that might prove interesting. The researchers wanted a “negative control” of heat-degraded insulin to use for their study, so they cooked a batch of insulin for 30 minutes at 176 degrees F. This insulin did lose potency — but shockingly little: “a decrease in insulin content of 9 to 14%.” If I’m reading the study properly, the scientists decided that a 30-minute exposure to 176 degrees F was actually not hot enough to sufficiently denature the insulin, and so they had to cook some more insulin at an even higher temperature, 239 degrees F.
The researchers hope that their work proves that people in tropical environments without access to refrigeration can keep their insulin at home. It wasn’t their intention, but it might also prove that the insulin you take on a summer camping trip will be similarly safe.
More Data on Insulin Durability
This was not, by any means, the first academic study on insulin’s potency after its exposure to higher temperatures.
A study from way back in 1972 found that insulin retained sufficient potency for “10 years at 15 degrees C [59 degrees F], 20 months at 25 degrees C [77° F], three months at 35 degrees C [95° F], or 10 days at 45 degrees C [113 degrees F].”
A 2009 study proposed that insulin stored at 25 degrees C (77 degrees F) had an estimated shelf life of 199 days.
A 2012 study found that insulin in a pump can retain potency for 14 days when exposed to a continuous temperature of 37 degrees C (98.6 degrees F).
None of these studies is definitive, and none of them tells us exactly what we want to know about insulin left in a hot car for a few hours. It’s also worth noting that insulin formulations have changed over the years and will change again. The older studies may be of limited relevance today.
Nevertheless, the results are striking. In controlled experiments, insulin consistently outperforms the manufacturer’s specifications for both shelf life and resiliency to heat. I didn’t cherry-pick the above studies; I was unable to find any that proved otherwise.
To reiterate, I’m not giving anybody advice. I’m just explaining my amateur understanding of the topic. And it’s worth noting that we don’t have much evidence regarding the potency of insulin exposed to extended extreme temperatures.
A car parked in the sun on a hot day can get very hot, much hotter than 100 degrees F. If you were to leave your insulin in a closed car in the Southwestern United States on a sunny summer’s day, for example, it could easily be exposed to hours of temperatures exceeding 120 degrees F. None of the studies above clarify how long insulin might retain potency in such conditions.
But I think a picture is emerging here — that insulin is more durable than the manufacturers ever suggest it is, and that it can be expected to retain potency in most nonextreme situations. It’s unsurprising that pharmaceutical businesses want to err on the side of caution, but given global problems with insulin access and scarcity, things might be better if we had a more realistic understanding of insulin-handling parameters.