How long does insulin last once it's been opened?

As a general rule, most bottles of insulin are good for 28 days once they're opened. Of course, how quickly a person goes through a vial is highly individual. Some may go through a bottle in a week or two. Others, on a lower dosage, may not use all the insulin within four weeks. But the drug's stability and potency is only guaranteed for 28 days.

Opened insulin pens typically last 14 days, though some last only 10 days. If you're uncertain, check with your mother's pharmacist to find out how long her insulin should last. When either of you opens a new vial or pen, make a note on the calendar -- and note the date when you'll need to throw out any remaining insulin.

It's best to store an opened bottle of insulin at room temperature, even though manufacturers often recommend refrigeration for opened containers. It's usually less painful to inject insulin when it's at room temperature than when it's cold.

Store unopened insulin vials and pen cartridges in the fridge, though, where they should last until their expiration date. Insulin shouldn't be exposed to extreme temperatures, so don't leave it in the car, next to the stove, in the freezer, or in the bathroom. If the bottle freezes, it must be discarded.

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Two typical signs that insulin has gone bad: poor performance and unusual appearance. If your mother is following her treatment plan and her glucose levels stay stubbornly, inexplicably high, her insulin may have lost its potency. Insulin that's cloudy when it's supposed to be clear or that contains particles, crystals, or small clumps is no longer safe or effective and should be discarded.

Even if you or your parent are only mildly concerned that the insulin has gone bad, don't take any chances: Throw the bottle away and open a new one. If the bottle is unopened and the expiration date hasn't passed, you may be able to return it to the pharmacist for an exchange or refund.