This article explains how to use potassium iodide after a nuclear strike and addresses some frequently asked questions.

At the end, there's a link to a downloadable format of this article that you can print out to keep with your emergency supplies. I'm not a doctor – this article is based on research done on the FDA and CDC websites. Sources are cited at the end.

Why you need potassium iodide after a nuclear emergency

Aside from the immediate threats of a nuclear blast, the thyroid gland is the most susceptible organ to damage from radiation. Potassium iodide is a stable form of iodine (stable meaning it isn't radioactive.) If the thyroid gland is loaded with stable iodine, it can't absorb radioactive iodine. Radioactive iodine can cause cancer. Here's how the CDC explains it:

The thyroid gland cannot tell the difference between stable and radioactive iodine. It will absorb both. KI (potassium iodide) blocks radioactive iodine from entering the thyroid. When a person takes KI, the stable iodine in the medicine gets absorbed by the thyroid. Because KI contains so much stable iodine, the thyroid gland becomes "full" and cannot absorb any more iodine—either stable or radioactive—for the next 24 hours.

This doesn't protect your body from any other type of radioactive isotopes – only radioactive iodine. It won't undo the damage done by radioactive iodine, so you must begin taking it immediately for protection. If there is no radioactive threat, you should not take KI, as it can be harmful.