Potassium iodide (KI): when to take it and who needs it

Short answer. Potassium iodide (KI) protects only the thyroid, and only against radioactive iodine ¹². Take it only when public health or emergency officials tell you to ². The FDA single dose is 130 mg for adults, 65 mg for ages 3 to 18, 32 mg from 1 month to 3 years and 16 mg for newborns ¹. One dose protects for about 24 hours ¹.

What does potassium iodide actually protect against?

Your thyroid takes up iodine and cannot tell stable iodine from radioactive iodine. A dose of stable iodine taken just before or at the start of exposure fills the gland, so it absorbs less of the radioactive form ³. The Canadian Nuclear Safety Commission says that by blocking radioiodine absorption, KI is effective in reducing the threat of thyroid cancer ⁵.

That is all it does. The limits are clear:

• Thyroid only. "KI protects only the thyroid. KI does not protect other parts of the body." ²
• Radioactive iodine only. It "does not protect against other types of radioactive materials" ². The FDA calls it "not a general radioprotective agent" and says it does not protect against radiation such as that from a dirty bomb ¹.
• Not a cure. It "cannot reverse damage already done to the thyroid" ².
• Not 100%. KI "may not give a person 100% thyroid protection from radioactive iodine" ².
• Not a substitute for other protective actions. The UK government says stable iodine "does not protect against health effects from other sources of radiation" and lists it alongside sheltering, evacuation and food and water restrictions ⁴. The FDA describes KI as an adjunct to other public health protective measures ¹, and WHO pairs it with control of food and drinking water ³.

Is KI useful after a nuclear detonation?

The CDC says "KI will not help in a nuclear bomb emergency." Radiation from fallout is the biggest health threat after a detonation, and taking in enough radioactive iodine for KI to matter would mean you had already been exposed to deadly levels of external radiation ². The CDC also warns that leaving shelter to look for KI could expose you to dangerous radiation; the protection that matters is to get inside, stay inside and stay tuned ² (see our guide to the fallout 7-10 rule). The UK government links stable iodine advice to emergencies at reactors that are operating or were shut down within the last 90 days ⁴.

When should you take KI?

Only on official instruction. "Do not take KI unless you are instructed by public health or emergency response officials or a healthcare provider" ². The FDA adds that KI "should not be taken as a preventative before radiation exposure" ¹. Switzerland and Germany give the same instruction: take tablets only when the authorities say so ⁶⁷⁸.

Timing matters because the thyroid absorbs iodine quickly. Agencies give slightly different time windows:

• CDC: take KI within 24 hours before or 4 hours after exposure to be most effective ².
• UK government: most effective between 24 hours before and 2 hours after exposure. It can still help up to 8 hours after exposure. It must not be taken for the first time more than 24 hours after exposure ⁴.
• WHO: stable iodine works when given before or at the start of exposure, and its 2017 guideline covers timing and repeat doses during a continuing release ³.

One dose protects for about 24 hours. Officials will say if another dose is needed ¹².

How much KI should each person take?

This is the FDA dosing table, which the CDC repeats ¹². It is a single daily dose.

Source: FDA ¹; CDC ².

Adolescents approaching adult size (over 70 kg or 150 lb) should take the full adult dose of 130 mg ¹². Take exactly the dose officials give: the CDC warns of "severe illness or death in people who take more KI than recommended" ².

Who needs KI the most, and who should avoid it?

Highest priority. The FDA puts infants, children, and pregnant and breastfeeding women first. They are treated at the lowest predicted thyroid exposure ¹.

Pregnant women and newborns. KI can suppress thyroid function in a fetus or newborn. The FDA advises against repeat doses for these two groups unless no other protective measure is available, and gives them priority for evacuation and sheltering ¹. The CDC notes that infants under one month who get more than one dose can develop hypothyroidism ².

Adults over 40. The CDC says adults over 40 have a much lower thyroid cancer risk from radioactive iodine and are more likely to have conditions that raise the risk of harm from KI ². The FDA says adults over 40 should take KI only when predicted exposure is high enough to destroy the thyroid and cause lifelong hypothyroidism ¹. The CDC says adults over 40 "should only take KI when recommended by officials" ².

People who should not take it. The FDA lists known iodine sensitivity, dermatitis herpetiformis and hypocomplementemic vasculitis. People with nodular thyroid disease combined with heart disease should not take KI. People with multinodular goiter, Graves' disease or autoimmune thyroiditis need caution, especially for use beyond a few days ¹. If you are in any of these groups, ask your doctor before an emergency.

Side effects. These include stomach upset, rashes, inflammation of the salivary glands and allergic reactions ².

Do not swap in other iodine. The UK government says iodised salt and other iodine sources should not be taken in place of stable iodine tablets ⁴. The CDC says the same of table salt and iodine-rich foods ².

What official agencies recommend

Agencies agree on the core point: KI protects only the thyroid and is taken only on instruction. They differ on age limits, timing and how tablets reach people.

United States. KI is sold without a prescription. As of March 2022, the FDA listed four approved products: 130 mg and 65 mg tablets and a 65 mg/mL oral solution ¹. The age threshold is 40 ¹². The timing window is 24 hours before to 4 hours after exposure ².

Canada. CNSC rules require nuclear power plant licensees to make sure KI pills are "pre-distributed to everyone within 10 km of all power plants" ⁵. Around the Pickering station, the KI Pill Working Group chaired by the CNSC is studying whether to pre-distribute pills to all schools in the ingestion planning zone, which reaches 50 km ⁵.

United Kingdom. Stable iodine is advised for emergencies at reactors that are operating or recently shut down. Officials tell you how to get the tablets, when to take them and how much ⁴. The timing window is 24 hours before to 2 hours after exposure, with some benefit up to 8 hours ⁴.

Switzerland. Households, businesses and schools within 50 km of a Swiss nuclear power plant receive iodine tablets every ten years; beyond 50 km, the cantons stock them ⁶⁷. Each tablet contains 65 mg of potassium iodide ⁷. The Federal Commission for Radiation Protection recommends an age limit of 45. Pregnant and breastfeeding women are exempt from that limit ⁶.

Germany. The BfS advises people over 45 not to take iodine tablets for thyroid blocking. The federal states are responsible for storing and distributing high-dose tablets, and more than 180 million are stored at sites across the country ⁸. Tablets are taken only on explicit instruction from civil protection authorities ⁸.

Where the agencies differ: the age cut-off is 40 in the United States ¹ and 45 in Switzerland and Germany ⁶⁸. The latest useful time after exposure is 4 hours for the CDC ² and 2 hours for the UK, with some benefit up to 8 hours ⁴. Follow your own country's instructions.

Frequently asked questions

Should I take potassium iodide now as a precaution?

No. The FDA says KI should not be taken as a preventative before radiation exposure ¹. A dose protects for about 24 hours ¹, and Swiss authorities note that tablets taken too early lose their protective effect ⁶. Wait for an official instruction ².

Does KI protect against radiation from a nuclear bomb?

No. The CDC says KI will not help in a nuclear bomb emergency: fallout is the main threat, and the protection that matters is to get inside and stay inside ².

Can I use iodised salt or iodine supplements instead?

No. The UK government says iodised salt and other iodine sources should not be taken in place of stable iodine tablets ⁴, and the CDC says only FDA-approved KI products should be used in the United States ². Take the dose officials give ²⁴.

How long do KI tablets last?

Check the expiry date on the package. Swiss authorities redistribute tablets every ten years, and the expiry date is printed on each box ⁶⁷.

My child cannot swallow a tablet. What do I do?

The FDA dosing table lists an oral solution of 65 mg/mL for children under 3: 0.5 mL from 1 month to 3 years and 0.25 mL from birth to 1 month ¹. Ask a pharmacist or doctor how to prepare a child's dose before an emergency.

Is KI part of your household kit?

Whether you should keep KI at home depends on where you live. Some countries hand it out to people near nuclear plants, the United States sells it without a prescription, and Germany holds large public stocks ¹⁵⁶⁸. Check what your own authorities advise before adding KI to a household kit, and ask your doctor if you have a thyroid condition ⁸.

Sources

1. U.S. Food and Drug Administration (FDA) — Frequently Asked Questions on Potassium Iodide (KI) — https://www.fda.gov/drugs/bioterrorism-and-drug-preparedness/frequently-asked-questions-potassium-iodide-ki (accessed 2026-10-20)
2. U.S. Centers for Disease Control and Prevention (CDC) — Potassium Iodide (KI) | Radiation Emergencies — https://www.cdc.gov/radiation-emergencies/treatment/potassium-iodide.html (accessed 2026-10-20)
3. World Health Organization (WHO) — Iodine thyroid blocking: Guidelines for use in planning and responding to radiological and nuclear emergencies — https://www.who.int/publications/i/item/9789241550185 (accessed 2026-10-20)
4. UK Government (GOV.UK) — What to do in a radiation emergency — https://www.gov.uk/government/publications/radiation-emergencies-information-for-the-public/what-to-do-in-a-radiation-emergency (accessed 2026-10-20)
5. Canadian Nuclear Safety Commission (CNSC) — Potassium iodide (KI) Pill Working Group — https://www.cnsc-ccsn.gc.ca/eng/resources/emergency-management-and-safety/potassium-iodide-pill-working-group (accessed 2026-10-20)
6. Swiss Federal Office of Public Health (FOPH/OFSP) — Comprimés d'iode : une mesure de protection préventive — https://www.bag.admin.ch/fr/comprimes-diode-une-mesure-de-protection-preventive (accessed 2026-10-20)
7. Swiss Armed Forces Pharmacy — FAQ | Kaliumiodid — https://jodtabletten.ch/en/faq (accessed 2026-10-20)
8. German Federal Office for Radiation Protection (BfS) — Jodtabletten - Einnahme und Wirkung von Jodtabletten — https://www.bfs.de/DE/themen/ion/notfallschutz/verhalten/jodblockade/jodblockade.html (accessed 2026-10-20)