The International Atomic Energy Agency said over the weekend that Japan had "distributed 230,000 units of stable iodine to evacuation centres" near the Fukushima Daiichi and Fukushima Daini nuclear power plants.
Damage to those plants from Friday's earthquake and tsunami has increased the risk that people in the area could be exposed to radiation.
If that happens, here's why taking iodine tablets might help.
In this fact sheet, the Centers for Disease Control and Prevention explain that the body needs iodine -- in a nonradioactive form -- to make thyroid hormones, which regulate metabolism. People usually get the stable iodine they need from food.
But absorbing radioactive iodine-131, which is present in the steam released from failing power plants like the ones in Japan, can cause cancer. Once breathed into the lungs or consumed by eating or drinking contaminated food or beverages, radioactive iodine travels through the body and quickly is absorbed by the thyroid gland, where it can damage DNA.