The standard adult dose of potassium iodide (KI) for radiation protection is 130 mg, taken once daily. Children, infants, and adolescents need lower doses based on age and weight. KI only protects the thyroid gland, and only against one specific type of radiation exposure: radioactive iodine, the kind released during a nuclear power plant accident.
How KI Protects the Thyroid
Your thyroid gland absorbs iodine from your bloodstream to produce hormones. It can’t distinguish between normal iodine and radioactive iodine. If radioactive iodine enters your body after a nuclear accident, your thyroid will absorb and store it, which can cause thyroid cancer over time.
Potassium iodide works by flooding your thyroid with stable, non-radioactive iodine before the radioactive form arrives. Once the gland is saturated, it stops absorbing iodine entirely. Any excess, radioactive or not, passes through your body and is excreted in urine within days. This process is called iodine thyroid blocking.
KI is not a general radiation antidote. It does not protect against external radiation from contaminated surfaces or ground deposits. It does not protect against other radioactive substances released in a nuclear event. It does not prevent radioactive iodine from entering your body. It only prevents your thyroid from accumulating it.
Dosage by Age and Weight
FDA-approved KI tablets come in two strengths: 65 mg and 130 mg. The dose is taken once per day for as long as authorities recommend, typically only a few days. Here’s the breakdown:
- Adults (19 and older): 130 mg (one 130 mg tablet or two 65 mg tablets)
- Adolescents 13 to 18, weighing 150 pounds or more: 130 mg
- Adolescents 13 to 18, weighing under 150 pounds: 65 mg
- Children 4 to 12 years: 65 mg
- Children 1 month to 3 years: 32 mg
- Newborns (birth to 1 month): 16 mg
The weight cutoff for adolescents matters. A teenager who weighs 150 pounds or more takes the full adult dose, while one under that threshold takes half.
Giving KI to Infants and Young Children
Infants and small children obviously can’t swallow tablets. The FDA provides instructions for making a liquid mixture at home. Crush one 65 mg tablet into a fine powder in a small bowl using the back of a metal spoon. Add four teaspoons of water and stir until the powder dissolves completely. Then mix the solution with four teaspoons of low-fat milk (white or chocolate), orange juice, flat soda, raspberry syrup, or infant formula to mask the taste.
This creates a mixture where each teaspoon contains roughly 8 mg of KI. From there, you measure the right amount for the child’s age:
- Children 4 to 12: 8 teaspoons of the mixture (the full amount, equal to 65 mg)
- Children 1 month to 3 years: 4 teaspoons (about 32 mg)
- Newborns: 2 teaspoons (about 16 mg)
KI oral solution is also available commercially in 1-ounce bottles with a dropper marked in milliliter increments. Each milliliter of solution contains 65 mg. For newborns, the dose is just 0.25 mL. For children 1 month through 3 years, 0.5 mL.
Timing Makes a Major Difference
KI works best when taken before or at the very beginning of exposure to radioactive iodine. The thyroid needs time to absorb enough stable iodine to become fully saturated. Taking it several hours after exposure still provides some protection, but the benefit drops off quickly. This is why public health authorities issue KI instructions as early as possible during a nuclear emergency rather than waiting for confirmed exposure levels.
Dosing is once per day. Repeated dosing beyond a few days increases the risk of thyroid complications, particularly in vulnerable groups like newborns and people with pre-existing thyroid conditions. Authorities will advise when it’s safe to stop.
Who Should Be Cautious
Most people tolerate a short course of KI without problems, but certain groups face higher risk. People with known iodine sensitivity should avoid it. The same goes for people with two rare conditions: dermatitis herpetiformis (a skin condition) and hypocomplementemic vasculitis, both linked to iodine hypersensitivity. Notably, a shellfish or seafood allergy does not automatically mean you’re allergic to iodine.
People with nodular thyroid disease and heart disease should not take KI. Those with Graves’ disease, multinodular goiter, or autoimmune thyroiditis (Hashimoto’s) can take it if needed, but with caution, especially if dosing extends beyond a few days. The concern is that a sudden surge of iodine can push an already-dysfunctional thyroid into overactivity or underactivity.
Newborns are the most sensitive age group. Their small thyroids saturate quickly, and excess iodine can temporarily suppress thyroid function during a critical window of brain development. That’s why the newborn dose is just 16 mg, roughly one-eighth of the adult dose.
Storage and Shelf Life
KI tablets are remarkably stable. The FDA has a formal shelf life extension program that allows government stockpiles to be tested and recertified well past their printed expiration dates. Potassium iodide is a simple salt, and when stored in a cool, dry place in its original packaging, it retains potency for years. If you keep a supply at home, store it away from heat and moisture and check the expiration date periodically, but don’t assume expired tablets are useless. They may still be effective.
What KI Won’t Do
People sometimes stockpile KI expecting broad-spectrum radiation protection. It provides none. In a nuclear emergency, radioactive iodine is just one of several dangerous isotopes released. KI has zero effect on the others. It also does nothing for radiation burns, radiation sickness from high-dose external exposure, or contamination on skin and clothing. Evacuation, sheltering in place, and avoiding contaminated food and water are all separate, critical steps. KI is one narrow tool that protects one organ against one isotope.