Potassium iodide (KI): what it does, who should take it and when

What potassium iodide does

Some nuclear emergencies, such as a reactor accident or a nuclear detonation, release radioactive iodine. The thyroid absorbs iodine and can't tell the radioactive kind from the ordinary kind. The FDA explains that KI floods the thyroid with non-radioactive iodine and prevents the uptake of the radioactive molecules, which are then excreted in the urine. Taken in the recommended dose, the FDA says, KI is effective in reducing the risk of thyroid cancer in people at risk of breathing in or swallowing radioactive iodine.

That's all it does. The FDA says KI works only to prevent the uptake of radioactive iodine into the thyroid gland and is not a general radioprotective agent. CDC adds that it does not protect other parts of the body and cannot reverse damage already done to the thyroid.

Only take it when you're told to

CDC's first rule is not to take KI unless instructed by public health or emergency response officials or a healthcare provider, and it warns that KI can have harmful effects when used incorrectly. Officials know whether radioactive iodine has been released, where it's going and who needs to take KI, which is why the decision isn't left to individuals.

Timing matters. CDC says KI must be taken within 24 hours before or 4 hours after exposure to be most effective, and the FDA says that since KI protects for approximately 24 hours, it should be dosed daily until the risk no longer exists. Officials will tell you when to start and stop.

Who it's for

CDC says KI is recommended only for people under 40 and women who are pregnant or breastfeeding. Dosing depends on age: CDC publishes a single-dose table by age group, from infants (who take the oral solution) to adults. Follow officials' instructions and the package.

CDC notes that KI can cause side effects, including allergic reactions. If you have an iodine allergy or thyroid disease, ask your doctor ahead of time whether KI is right for you.

Which products to buy, and what isn't a substitute

CDC says to use only KI products approved by the FDA. The FDA lists these as approved and available without a prescription: iOSAT 130 mg and 65 mg tablets from Anbex, ThyroSafe 65 mg tablets, and a 65 mg/mL potassium iodide oral solution from Mission Pharmacal. Many 'potassium iodide' tablets sold online are supplements, not approved products. Our potassium iodide tablets page lists only the approved ones.

CDC says not to use table salt or foods that contain iodine as a substitute for KI: they don't help, and eating large amounts could be harmful. It also says dietary supplements containing iodine may not work.

KI is not the main protection

Ready.gov's advice for any radiation emergency is to get inside, stay inside and stay tuned. Shelter in a basement or the center of a large brick or concrete building protects your whole body from fallout; KI protects one gland from one material. Our fallout shelter guide covers where to go and for how long.