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.
Ms Fleming Wurdack told the BBC that her company had ordered additional raw materials to manufacture more KI solution.
She expects the orders will continue for months.
At present, the US government recommends that states have sufficient KI on hand for residents in a 10-mile (16km) radius of nuclear plants.
A 2002 law, authored by Representative Ed Markey, expanded that requirement to 20 miles (32km), but the provision has never been implemented.
In a letter to the White House Office of Science and Technology Policy on Monday, Mr Markey called the failure to implement the law a "subversion of the intent of Congress".
A spokeswoman for the Department of Health and Human Services told Bloomberg News that the administration intended to review its rules on KI provision.
For maximum effectiveness, KI, a common salt similar to table salt, should be taken before exposure to radiation.
Its protective properties last about 24 hours.
KI tablets can prevent the body absorbing radioactive iodine, thus mitigating an increased threat of thyroid cancer, but do have side effects.
Children and pregnant and nursing women are priority KI recipients because they are more susceptible to radiation poisoning.
Japanese authorities say they have distributed 230,000 units of KI to evacuation centres close to nuclear facilities.