Potassium iodide tablets: when to take them, dosage and who should not

This is not medical advice. Take potassium iodide only when the authorities in your country instruct you to, in the dose and at the time they announce. If you have a thyroid condition, take regular medication or are unsure, ask a doctor or pharmacist, ideally before any emergency happens.

Potassium iodide tablets protect against one specific danger: radioactive iodine being taken up by the thyroid gland after a nuclear accident. They do not protect against radiation in general, and they are not something to take “just in case”. According to WHO, you take them only when public health authorities explicitly tell you to.

Below we explain how potassium iodide works, when to take it, the official doses by age, who should not take it and why salt, supplements or iodine tincture cannot replace it. Everything here comes from the WHO, CDC and FDA sources listed at the bottom of the page.

What potassium iodide is and how it works

Potassium iodide (KI) is a salt of stable, meaning non-radioactive, iodine. The thyroid uses iodine to make hormones and takes it from the blood. It cannot tell the difference between ordinary iodine and radioactive iodine.

WHO describes the principle simply: if the thyroid is already saturated with stable iodine, radioactive iodine can no longer build up in it and the body excretes it. This is called iodine thyroid blocking. The protection matters most for children, whose thyroids are the most sensitive to radioactive iodine.

What potassium iodide does NOT do

This is the most important part of this guide. According to WHO and CDC, potassium iodide:

protects only the thyroid, not other organs,
works only against radioactive iodine, not against other radioactive substances that may be released,
does not protect against external radiation, for example from contamination on the ground or on surfaces,
is not a treatment. CDC notes it cannot reverse thyroid damage that has already happened.

A tablet is one protective measure among several, not a replacement for them. WHO says it should be combined with other actions the authorities may order, such as sheltering indoors, evacuation and avoiding contaminated food and water. Following official instructions matters more than the tablet itself.

When to take potassium iodide

Only on official instruction

WHO is direct: take potassium iodide only when public health authorities explicitly instruct you to. FDA states that it should not be taken as a preventive before radiation exposure. Having tablets at home is not a reason to take them.

You will hear the instruction through radio, television, official websites and phone alerts, depending on how your country warns the public. During a power cut, a battery or hand-crank radio keeps you informed. Our blackout preparedness guide covers what to keep at home for that.

Timing

Timing matters, but the authorities decide it. For orientation, the sources say:

WHO (2017 guideline): the best window is less than 24 hours before and up to 2 hours after the expected start of exposure. Taking it up to 8 hours after is still reasonable. Starting more than 24 hours after exposure may do more harm than good.
CDC: KI is most effective when taken within 24 hours before or 4 hours after exposure.
FDA: KI works best within 3 to 4 hours of exposure.

One dose protects for about 24 hours. Usually a single dose is enough. WHO says repeat doses may be needed only if exposure lasts longer than 24 hours, if contaminated food or water cannot be avoided, or if evacuation is not possible. Officials will say if another dose is needed. A larger dose does not protect better: FDA warns that taking more, or more often, than recommended can cause severe illness or death.

Potassium iodide dosage by age

We do not give our own dosing schedules. The table below shows the single doses from the WHO 2017 guideline, which match the CDC and FDA tables. WHO gives the amount as stable iodine, CDC and FDA as potassium iodide.

Age group	Potassium iodide (KI)	Stable iodine	130 mg KI tablet	65 mg KI tablet
Adults and adolescents over 12, including pregnant and breastfeeding women	130 mg	100 mg	1	2
Children 3 to 12 years	65 mg	50 mg	½	1
Children 1 month to 3 years	32 mg	25 mg	use liquid	½
Newborns, birth to 1 month	16 mg	12.5 mg	use liquid	¼

A few notes from the sources:

Tablet strength differs between countries. Common products are 130 mg or 65 mg KI tablets, and FDA also lists a liquid with 65 mg KI per mL. Some countries use potassium iodate (KIO₃) instead, with different milligram amounts. Always check the leaflet in your own pack.
Adolescents: CDC and FDA say a 12 to 18 year old who weighs over 70 kg (150 lb) should take the adult dose. CDC and FDA list a separate 12 to 18 group with a 65 mg dose for those below that weight. Follow the official announcement in your country.
The official announcement and the leaflet come first. National plans can differ from these international tables.

Who should not take potassium iodide

According to FDA, people should not take KI if they have:

a known sensitivity (allergy) to iodine,
dermatitis herpetiformis,
hypocomplementemic vasculitis,
a nodular thyroid together with heart disease.

FDA advises caution for people with multinodular goitre, Graves’ disease or autoimmune thyroiditis, especially if dosing goes on for more than a few days. A seafood or shellfish allergy does not necessarily mean you are sensitive to iodine. If you are not sure, ask a health professional.

Special groups

Newborns under 1 month: a single dose only. WHO recommends thyroid hormone monitoring and a paediatrician’s check within the first week. CDC notes that more than one dose can cause hypothyroidism and later developmental problems.
Pregnant women: the adult dose, usually once. FDA says KI can suppress thyroid function in the unborn baby, so pregnant women are a priority for sheltering and evacuation. WHO advises telling your doctor afterwards.
Breastfeeding women: the adult dose, once, and the baby gets its own age-appropriate dose. WHO says breastfeeding can continue. CDC suggests considering a temporary pause, if possible, until evacuation, because radioactive iodine can pass into breast milk. Follow local health advice.
Adults over 40: WHO says thyroid blocking is generally not indicated, except for emergency responders, because the cancer risk is low and the risk of side effects rises with age. CDC and FDA recommend it for this group only at very high predicted exposure, which in practice means only when officials say so.
No repeat doses for newborns, pregnant and breastfeeding women, or people over 60, according to the WHO guideline, unless a doctor advises otherwise case by case.

The sources differ on some details. Timing windows, advice on breastfeeding and the guidance for adults over 40 vary slightly between WHO, CDC and FDA, and national plans can differ again. We do not settle this for you. Follow the instructions of the authorities in your country, and if you have a thyroid or heart condition, talk to your doctor before an emergency.

Side effects

At the recommended dose, taken once, side effects are unlikely. WHO, CDC and FDA list:

stomach upset and diarrhoea,
skin rash,
swollen salivary glands,
“iodism”: metallic taste, burning mouth and throat, sore teeth and gums, cold-like symptoms,
allergic reactions. FDA says that swelling of the face, lips, tongue or throat, or trouble breathing, needs immediate medical attention.

The risk grows with age, with higher or repeated doses and in people with existing thyroid disorders.

Why salt, supplements and iodine tincture cannot replace tablets

Iodized salt: WHO says table salt does not contain enough iodine to block the thyroid, and eating large amounts of it is a health hazard in itself. CDC says the same about iodine-rich foods.
Iodine dietary supplements: CDC warns that they may not protect the thyroid and can harm you. Use only approved KI products.
Iodine tincture, skin antiseptics, throat sprays and Lugol’s solution: never drink these as a substitute. Japan’s National Institute of Radiological Sciences warned in 2011 that such products are not meant for internal use and contain too little iodine to protect.

How distribution works depends on your country

There is no single global system. WHO says tablets may be pre-distributed to households, schools, hospitals and pharmacies, and that authorities decide who takes them, where and when. Some examples:

United States: the NRC says states and local authorities decide whether to hand out KI before an accident or as soon as possible after. FDA has approved KI for sale without a prescription.
Poland: the government has stored tablets with local authorities, to be issued at designated points only after an official announcement.
Elsewhere: WHO recommends that countries plan stockpiles and distribution in advance, and some pre-distribute tablets to households near nuclear power plants. Check what your national authority does.

To find out what applies to you, check the website of your national civil protection or public health authority, or your local emergency management office. Look for how tablets are distributed, whether you live in a zone with pre-distribution and how alerts reach you. If you buy tablets yourself where that is allowed, WHO stresses that you still take them only when the authorities instruct.

If you are told to stay indoors, do not go out to collect tablets on your own. Follow the instructions you are given.

What you can do today

Find out from your national authority how potassium iodide is distributed where you live.
Make sure your phone receives emergency alerts, and keep a battery or crank radio with spare batteries.
If you or someone at home has a thyroid condition or takes regular medication, ask your doctor at the next visit what to do if an instruction is issued.
If you have tablets, keep them in the original pack with the leaflet, and note the strength (130 mg or 65 mg) on your family emergency plan.
Keep a water and food supply so you can stay indoors for several days if told to shelter.
Check your home first aid kit and your go-bag, and see our 72-hour bag checklist.
Follow verified updates in our news section.

FAQ

When should I take potassium iodide?

Only when public health or emergency authorities tell you to, during a nuclear or radiological emergency that involves radioactive iodine. WHO and FDA both say it should not be taken as a general precaution. Officials announce who should take it, when and how much.

Does potassium iodide protect against radiation?

Not in general. It protects only the thyroid, and only against radioactive iodine. It does not protect other organs, does not work against other radioactive substances and does not shield you from external radiation.

What is the potassium iodide dose for adults and children?

WHO, CDC and FDA give the same single doses: 130 mg KI for adults and adolescents over 12, 65 mg for children 3 to 12, 32 mg for children 1 month to 3 years and 16 mg for newborns. Always follow the official announcement and the leaflet in your pack.

Can I use iodized salt, iodine supplements or iodine tincture instead?

No. Iodized salt contains far too little iodine and eating large amounts is harmful. CDC warns that supplements may not protect and can cause harm. Iodine tincture, skin antiseptics and Lugol's solution are not substitutes and should never be drunk for this purpose.

Where do I get potassium iodide tablets?

It depends on your country. Some countries pre-distribute tablets to households near nuclear plants, others store them for issue after an alert, and some allow purchase at pharmacies. Check your national civil protection or public health authority.

Sources

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This guide is educational. In an emergency, follow official alerts and first responders.