Pharmacies and drug stores are selling more potassium iodide as radiation exposure fears heighten amid Japan's nuclear emergency.
Potassium iodide prevents radioactive iodide from entering the thyroid gland.
But medical experts say the supplement doesn't protect other parts of the body, and isn't a definite treatment for radiation exposure.
Americans are also being warned not to take the pills prematurely because it could be harmful.
"I would not recommend that people buy potassium iodide and certainly not that they take potassium iodide," said Dr. Jonathan Fielding, director of public health for Los Angeles County. "It could cause allergic reactions, intestinal upset, [and] gland problems."
Public health officials said the nuclear scare in Japan doesn't pose a threat to the United States -- even for those living in Hawaii and on the West Coast.