Ice packs freeze insulin. Wrap it first.

The trick to keeping insulin cool in a lunch box is simple: keep it cold, but never let it touch the ice pack. Direct contact with a frozen pack will freeze the insulin, and frozen insulin is ruined even after it thaws. So the fix is dumb easy — wrap the pen or vial in a folded paper towel, a cloth napkin, or even a clean sock before it goes in the bag.

If you want to be fancy about it, use a small insulated pouch or put the insulin in a thermos with cold water. Water stays above freezing while still keeping things cold, so the danger of freezing basically disappears. There are also Frio packs that cool by evaporation instead of ice — they physically can’t freeze your insulin, and people with diabetes swear by them for exactly this reason.

One more thing: depending on the situation, you might not need ice at all. Most opened insulin pens and vials are stable at room temperature up to about 86°F for 28 days. If it’s a normal workday in an air-conditioned office, just leaving the pen on your desk is perfectly fine. If it’s summer, or it’s sitting in a hot car, then the wrap trick matters.

Frozen insulin is worthless insulin, and the fix costs about three cents.