Basically the iodine from the tabs is absorbed into the thyroid, making sure that any radioactive iodine from a nuclear accident doesn't get into the thyroid, thus protecting it from the radioactivity.

If you do a google of potassium-iodide or 'Iosat', you'll find the expiration-date. I think it was 7 years or so..
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''It's time for Plan B...'' ''We have a Plan B?'' ''No, but it's time for one.'' -Stargate SG-1