Fears of radiation sickness stemming from the unfolding nuclear crisis in Japan have prompted a surge in US sales of potassium iodide.
Potassium iodide (KI), a salt, floods the thyroid gland with non-radioactive iodine which lessens the amount of radiation the gland can absorb.
Pharmacies do not generally carry KI, but several manufacturers have reported being out of stock.
The US government has a stockpile on hand in case of emergencies.
Online retailer nukepills.com said via its Twitter feed that it had sold out of KI pills on Tuesday.
It received 3,800 orders in 18 hours on Sunday and has shipped 50,000 pills to Tokyo, with the help of Harvard Medical School.
Debby Fleming Wurdack, co-owner of Fleming Pharmaceuticals, which produces a KI solution on demand, told the BBC the firm would be out of stock by the end of the day.
"This is insanity," Ms Fleming Wurdack said.
The company received more than 350 calls with orders from clients ranging from housewives to state governments on Tuesday, she said.
It also received orders from companies intending to send KI to their employees in Japan. Read more>>
See also: Foods that contain high levels of natural iodine