Potassium Fluoride

A small amount of fluoride may prevent tooth decay in children. The maximum recommended fluoride level is 0.7 milligrams per liter of water. Excessive fluoride consumption may cause bone fractures in adults. - 231

The U.S. Centers for Disease Control says that potassium iodide pills will keep the thyroid gland from absorbing radioactive iodine.
It cannot protect other parts of the body from radiation.
Side effects can include nausea, upset stomach, rashes, inflammation of the salivary glands and possibly severe allergic reactions. - 482