The U.S. State Department has issued potassium iodide (KI) to personnel living in Japan but is not yet advising anyone to actually take the tablets, which protect against certain forms of radiation. What's the State Department waiting for? Can you overdose on KI?
It's highly unlikely. One of the risks people near Japan's damaged nuclear reactors face is that radioactive iodine will get into their blood and eventually the thyroid gland. To prevent this from happening, they can take potassium iodide, which saturates the thyroid with safe iodine and prevents it from absorbing the radioactive alternative. Because the goal of the treatment is to completely sate the thyroid, potassium iodide tablets contain 100 milligrams of iodine—nearly 700 * times the recommended daily amount. Despite the high dose, iodine poisoning isn't a big concern, since you'd need to ingest several grams of the substance in a very short period of time to get sick. As for potassium, there's nothing to worry about. A tablet of potassium iodide contains less than one-tenth the potassium you'd get from a banana (PDF).
While acute toxicity is unlikely, chronically elevated levels of iodine can be dangerous. They can cause goiter and inhibit thyroid functioning. It would likely take several weeks of ingesting the recommended dose of potassium iodide for those disorders to occur, though. The KI tablets are also dangerous for people with allergies or certain thyroid or skin conditions.
Doctors have been prescribing potassium iodide for more than a century, long before we figured out how to unleash the power of the atom. Pediatricians gave it to coughing children to break up mucus since at least the early 1900s. It wasn't until the 1950s that clinicians realized the compound could saturate the thyroid and prevent it from absorbing radioactive iodine in the event of a nuclear attack or accident.
Since then, public health advocates, the government, and the nuclear power industry have been arguing about how much potassium iodide should be on hand to protect the public in the event of a nuclear accident. The government stockpiles potassium iodide for residents within a 10-mile radius of a nuclear plant. After Sept. 11, 2001, which raised the specter of a terrorist-induced radiation emergency, Congress mandated that the radius be extended to 20 miles (PDF). In 2007, however, the Bush administration waived that portion of the law, arguing that evacuation would be more effective than stockpiling and distributing the tablets. But the decision has not put the issue to rest. Last year, 13 members of Congress from Florida urged Health and Human Services Chief Kathleen Sebelius to reconsider the plan, arguing that it would be nearly impossible to evacuate the peninsula (PDF) in a timely manner.
You'll have to wait to start your own stockpile of potassium iodide. Anbex, the company that distributes the medicine directly to the public, is currently out of tablets. When supplies become available, you can get a two-week supply for $10. The pills officially expire in seven years, but they're probably effective for much longer than that.
Correction, March 22, 2011: The article originally stated that a potassium iodide tablet contains seven times the recommended daily amount. In fact, it's 700 times. (Return to the corrected sentence.)
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