Insulin can freeze when stored at 32 degrees F or colder, just like water, says Jovan Milosavljevic, MD, an assistant professor of medicine at Montefiore Einstein in New York.
“When this happens, its protein structure becomes damaged,” he says. “Even if the insulin later thaws, it won’t work the way it should.”
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Extra-cold temperatures also may cause glass vials to crack and contaminate the insulin inside.[2]
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“To prevent freezing, never keep unopened insulin in the freezer or near the back wall of the refrigerator,” says Hess-Fischl, adding that insulated bags are available to keep insulin at an appropriate temperature.
In the winter, you should be especially careful about how you carry your insulin. Leaving a vial or pen outdoors in a backpack, purse, or car could easily expose it to freezing temperatures. If you’re spending the day skiing or sledding or even just shopping, stay mindful of the insulin you keep on your person so that it doesn’t freeze.
“Keep insulin close to your body, like inside a jacket, and consider an insulated pouch when traveling or spending long periods outdoors so the temperature stays safely above about 40 degrees F,” Dr. Milosavljevic says.
