Potassium iodide pills and prevention of thyroid cancer from Japanese nuclear power plant

Peter Wehrwein

Contributor, Harvard Health

Why did Japanese officials gear up to distribute potassium iodide in response to the explosions at the Fukushima Daiichi nuclear power plant?

According to the Union of Concerned Scientists, a nonprofit group in Cambridge, Mass., the radioactive materials that pose the greatest threat after a nuclear power accident are radioactive iodine (the iodine-131 isotope, in particular) and radioactive cesium (cesium-137), and news reports have said both have been detected outside the plant.

Radioactive iodine is a byproduct of the fission (splitting) of the uranium in the fuel rods that power a nuclear power plant. Once radioactive iodine is in the body, it concentrates mainly in the thyroid gland, which is located in the neck, just below the Adam’s apple.  (See our related post on the thyroid gland and cancer from iodine-131.) But as a helpful fact sheet from the Centers for Disease Control and Prevention (CDC) explains, the gland cannot tell the difference between radioactive iodine and the radioactively stable version of the mineral it needs, so it will absorb both.

Potassium iodide pills (sometimes abbreviated as KI: the K stands for potassium, the I for iodine) don’t prevent radioactive iodine from entering the body, but they do keep it from getting accumulating in the thyroid gland. By flooding the body with non-radioactive iodine, the pills keep the gland from absorbing the radioactive iodine. Here is how the CDC fact sheet summed it up:

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.

Children and infants are more vulnerable to developing thyroid cancer from radioactive iodine than adults, so it’s important that they get the pills in a radiation emergency. But the pills can be hard to swallow, especially for infants, and potassium iodide dissolved in water has a harsh, salty taste. The FDA tip: grind the pills up and mix them into low-fat chocolate milk, orange juice, or flat soda.

Large doses of iodine over a long period of time can be dangerous, so potassium iodide pills  should be reserved for true emergencies. (So far, no one is recommending that anyone in the United States take them, although people are apparently stocking up.)

Many varieties of table salt are  “iodized,’ which means iodine has been added. But iodized table salt doesn’t contain enough of the mineral to saturate the thyroid gland and keep it from absorbing radioactive iodine.


  1. Shawn

    I found a good article that goes in some depth on how radiation works that ties in why you should always keep potassium iodide pills handy if you live near a plant.

  2. Andynie

    this is a very helpful article. specially in japan who recently have nuclear radiation crisis.
    [URL removed by moderator]

  3. Anonymous

    One concern is how do we know that potassium iodide will have side effect to our health.

  4. Faisal

    Surgery is the first option. Doctors will try and remove all of the cancer cells that they can find around your neck and throat region. They also work on the lymph nodes, checking for cancerous cells. In most cases, the surgery is successful.
    [URL removed by moderator]

  5. Fernando

    this is a very helpful article. specially in japan who recently have nuclear radiation crisis.

  6. cancer gastrico

    Thank you for this very informative article. It’s nice to know that there is a solution for this radioactive iodine from the explosion of nuclear power plant in Japan. Thanks for sharing this article.

  7. fernando

    Cancer is very rampant nowadays.
    A lot of people are struggling hard to fight against the disease.
    It’s so nice to see a very informative write-ups as this one.
    [URL removed by moderator]

  8. Osha Gray Davidson

    I’ve read that Americans are stocking up on KI causing a shortage of the drug in Japan where it’s actually needed. Are these simply anecdotes or do you know if KI is actually in short supply in Japan?

    • P.J. Skerrett

      Osha — I haven’t seen any reports like that. I did see early on that while some Americans were wringing their hands and rushing to stockpile a drug they didn’t need, the response by Japanese people close to the affected area was calmer and more organized.

  9. Ulrich Decher

    Correction to my earlier comment. Panic is not the only adverse effect of distributing KI. There are also heatlth effects from overdosing.

    “At least seven people have reported reactions to the drug, often called by its chemical name, KI, including two who said they were suffering from serious symptoms including vomiting, racing heart and dizziness or vertigo.”


    The article also mentions possibly shutting down the function of the thyroid gland.

    Taking KI at this time is a really bad idea. Harvard should not be encouraging it.

    Ulrich Decher Phd Nuclear Engineering

  10. Ulrich Decher

    I am surprised that you would use the Union Of Concerned Scientits as the only authoritative reference on the technical subject of radiation health effects.

    The are other independent organizations on this subject which don’t have an anti-nuclear ax to grind. I recommend:

    Health Physics Society


    United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)

    Ulrich Decher Phd Nuclear Engineering

  11. Ulrich Decher

    There is no indication form what we know today about the nuclear events in Japan that K-I pills are necessary in the US or Japan for that matter.

    The only impact of the pills is to create unnecessary panic in populations which can have a definite negative health effect.

    Ulrich Decher PhD Nuclear Engineering

  12. Steph

    Traveling to Hawaii 3/18-3/28. Would it be wise to take teh KI pills with us in case of another big explosion? What is the dosing for prevention? Thanks.

  13. Carol Dunn

    Iodine is disappearing from stores shelves in the US.

    Could you provide some clarification about how important it is to only take iodine tablets if there is proof that you have exposed a large dosage of radiation, and not in advance on the chance that you might be exposed to low level radation?

  14. Rachel Anderson

    Dear Mr. Wehrwein,
    Should Americans be taking potassium iodide pills to prevent uptake of the radioactive iodine released into the atmosphere by the recent Japanese nuclear explosions?
    Thank you,
    Rachel Anderson

  15. Troy Jones

    March 14, 2011 – 50,000 potassium iodide tablets have been donated by Nukepills.com to Tokushukai Hospital in Toyko, Japan for immediate distribution to those affected by the nuclear reactor accident. This effort was facilitated by Brigham and Women’s Hospital, a teaching facility of Harvard Medical School, in Boston, MA.

  16. Sal A

    Should one stockpile pills or liquid? I’m readying some information that only liquid potassium iodide is approved for use by the FDA for the purposes of treating radiation in emergency situations. Does it matter whether or not a treatment being purchased is FDA approved or not?

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