Because the insulin is no longer any good after that time. It is not refrigerated so the preservatives only hold up for 28 days. Once the device is opened, it is supposed to be kept at room temp.
How should I store Lantus?
Store new unopened SoloStar® disposable insulin pen in a refrigerator (not the freezer) between 36°F to 46°F (2°C to 8°C). Do not freeze Lantus. Keep Lantus out of direct heat and light. If a diposable insulin pen has been frozen or overheated, throw it away.
Open (In-Use) SoloStar®:
Once SoloStar® is opened (in-use), SoloStar® should NOT be refrigerated but should be kept at room temperature (below 86°F [30°C]) away from direct heat and light. The opened (in-use) SoloStar® kept at room temperature must be discarded after 28 days.
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