Does Insulin Really Go Bad After 28 Days?

Medically reviewed by Anna Goldman, MD.

*DISCLAIMER: This is not medical advice. The author is not a doctor, pharmacist, or chemist. This article is not a guide to managing insulin but a look at different opinions, research, and guidelines regarding insulin expiration dates and temperature limits.

Despite being known as one of the “most powerful” hormones in the human body, insulin can also be pretty darn fragile. Like Goldilocks rummaging through the bears’ house, the fine print on your insulin vial or pen says it can’t be too hot, too cold, or too old.

Time and temperature can both play a big role in the potency of your insulin, and insulin that isn’t stored in perfect conditions must be thrown away — according to the manufacturers. But some independent research and anecdotal patient experiences say otherwise! The truth is that many members of the diabetes community regularly use insulin beyond the 30-day limit, although opinions differ on the wisdom of this approach.

Insulin can be catastrophically expensive, which can make throwing a partially used vial or pen into the trash a tragic event. For some people, especially those in the developing world, using insulin beyond its expiration date may almost be a medical necessity. 

Does an opened vial of insulin really go bad after 30 days? Does a few hours in the hot sun or the bitter cold really kill insulin whether it’s been opened or not? Let’s take a closer look at the official guidelines vs. the real-life experience and independent research.

What the paperwork says: 28 to 30 days for opened insulin

According to most manufacturer’s official guidelines, insulin starts to degrade — and should be disposed of — 28 to 30 days after it’s been opened.

It certainly makes sense. Insulin is a protein. It eventually “spoils” — although it doesn’t lose its potency all at once. Instead, it gradually becomes weaker and weaker. Actually, insulin starts degrading immediately upon opening but the degradation is very slow in the first 28 to 30 days. You really won’t notice it.

When you pass that 28 to 30-day window, it degrades more quickly. 

If you usually finish a vial or pen of insulin within 30 days, you don’t have to pay much attention to how many days it’s been since you opened it.
If you don’t usually finish a vial or pen within 30 days, don’t be surprised if days 30+ leave you feeling frustrated with stubborn highs that won’t budge. 

Most likely, the more sensitive you are to insulin, the more likely you are to notice when its potency is fading. 

How Long Does Insulin Really Last?

What happens if you ignore that 28 to 30-day timeframe? I’ve been talking to people about diabetes for 20 years, and I can tell you that there’s no consensus. Some people say nothing happens — insulin continues to work just fine! Others say they see a noticeable difference — you’ll find a broad mix of opinions when you look at this topic in the Diabetes Daily forums!

Should you toss your insulin after 30 days? Perhaps not — here’s why: It’s not going to go from 100 to 0 potency. Instead, it’s a gradual decline. Chances are you could get more out of it.

Independent research says insulin could last potentially four times longer than the medication guidelines suggest. In 2023, an international group of scientists tested a variety of insulins in real-world unrefrigerated conditions in India, during a season in which temperatures regularly reached as high as 95° Fahrenheit. All of the insulins retained at least 90 percent potency after four months, and most retained more than 95 percent potency. The insulins fared even better when placed into clay pots to keep cool.

A spokesman for the researchers said, “If our results can be confirmed in larger studies, it may drive a change in the requirement to discard insulin kept outside a fridge after one month. The period when insulin may still be used can potentially, in that case, be extended to three or perhaps even four months.”

Extreme Heat and Cold 

Also in the manufacturer’s medication paperwork are plenty of warnings about temperature limits. The paperwork says too cold and too hot temperatures can destroy the potency of your insulin in just a few hours of exposure.

Ideally, insulin should be stored in the refrigerator when it’s not being used — and the temperature in your fridge should be between 36 to 46 degrees Fahrenheit. 

The guidelines say insulin can tolerate temperatures up to 77 degrees and as cold as 36 degrees, but ideally not for long. The longer your insulin is at a temperature above 46 degrees, the more likely it’s degrading. And insulin, just like water, freezes solid at 32 degrees — even after defrosting, it may be radically altered and unreliable. 

Is it really too hot for your insulin?

Some research suggests that high temperatures are rarely a serious problem for insulin safety and potency.

In many parts of the world, it’s very challenging to protect insulin from temperatures over 77 degrees. A 2021 study led by Doctors Without Borders set out to test the accuracy of these heat limits in real-world conditions at Kenya’s Dagahaley refugee camp, where refrigeration is scarce.

In fact, the temperature in this area hits a typical daily high of around 98.6 degrees and rarely falls below 77 degrees, even at night. As Diabetes Daily editor Ross Wollen explains in his review of this study, according to the manufacturer’s instructions, no insulin should be able to endure these conditions for long. 

After one month, though, “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!” They kept the study going for a total of 12 weeks, and the results held up: all the insulin was still potent.

My Take

Insulin doesn’t go bad immediately when it reaches its expiration date If your vial is only half-empty, and it’ll take another month before you use it up, you might notice it’s not as effective during those last couple of weeks. And if that happens? Well, then toss it

Most likely, your insulin is still going to be relatively effective, but it will continue to degrade. The more sensitive you are to insulin, the more likely you’ll notice it losing its potency.

What about heat? Personally, I’ve left a pack of 3 cartridges of Afrezza inhaled insulin in my car for several weeks during a hot July summer. I did this on purpose and wanted to see how it impacted the potency. I used a cartridge and found no change in its efficacy.

So maybe heat doesn’t matter quite as much as we’ve been told? Although, it’s still worth stashing your insulin kit in the shade while you’re hanging at the beach. (And if you like hot tubs: do not let your insulin pump or patch dip into the water of a hot tub! Yikes! No need to boil insulin.)

But I have experienced the dangerous consequences of damaged insulin from extreme cold.

In high school, I worked at the local movie theater. This is December in Hanover, NH — the temperature outside was well below freezing, and I was tasked with stringing up Christmas lights on the marquee letterboard out front. I also had to update the marquee with the movies coming that Friday.

I was outside with my insulin pump in my jacket pocket for at least an hour.

Little did I know, the freezing temps were destroying my insulin quickly. My blood sugars seemed perfectly fine through the evening. It didn’t even cross my mind to be worried. Mind you, this was around the year 2004, before diabetes blogs or continuous glucose monitors (CGM).

I woke up the next morning in diabetic ketoacidosis (DKA) and my blood sugar was in the 500s. All night, I’d been getting damaged, weak insulin. I puked 15 times before finally going to the emergency room. 

And that, my friends, is a little old story about insulin temperature. So, I am pretty darn careful with my insulin in extreme cold because DKA is not fun.

Okay, so maybe we can meet in the middle. Maybe you don’t toss your insulin out after 30 days but you simply keep a closer eye on its efficacy. If you’re experiencing stubborn highs, maybe your insulin has lost some of its potency. 

If you’re worried about it, here are a few simple methods of keeping an eye on the calendar:

Use a permanent marker to write the date you opened it directly on the vial or pen. Do this the moment you open that vial or pen. Just get in the habit of it.
Mark your kitchen calendar with an “Insulin is done!” sort of note. (Yes, some people do still have calendars pinned to the wall!)  
Set a reminder on your phone or other “Smart” devices. “Hey Alexa, tell me on June 30th at 7 a.m. that my insulin is 28 days old.” Personally, I use my Amazon Echo to remind me of just about everything because I’m getting old.
Put an old-school Post-It note in the ‘fridge compartment where you store your insulin and write the “opened” date. Then just keep updating that Post-It note.

Ah, insulin. We rely on it so desperately to keep us alive and yet it’s so darn finicky. Maybe you shouldn’t toss that expensive stuff into the trash just because of the calendar or hot temps. Be careful and be safe, and look closely at how your blood sugars are doing.

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