Until recently, potassium iodide, the preferred protection against radiation-induced thyroid cancer, had a modest following among nuclear reactor workers, survivalists and those preoccupied with the possible aftereffects of nuclear holocaust.

But that was before Sept. 11, before anthrax, before India-Pakistan and before dirty bombs. Today, potassium iodide is all the rage -- something lots of people want in their medicine cabinets next to their supplies of Cipro to protect them against anthrax.

At Anbex Inc., which has been making potassium iodide since the 1980s, sales have risen from a pre-Sept. 11 "few hundred" 14-pill packages a year to "tens of thousands of packages per month" in mid-June, Anbex President Alan Morris said.

"Every time [Secretary of Defense Donald H.] Rumsfeld or [Office of Homeland Security Director Tom] Ridge gets on TV, there's a sales spike," added Troy Jones, of Nukepills.com, Anbex's online distributor. "Ridge just says the word 'nuclear,' and our phones start to ring."

Potassium iodide, represented by the chemical symbol KI, is no magic bullet. It keeps the thyroid gland from absorbing radioactive iodine but does nothing to protect against any other radioactive isotope, let alone fend off the heat and blast effects of a nuclear explosion.

Yet with July 4 just around the corner, business shows no signs of slowing down. "I expect orders to go right up to July 3," Jones said. "If something happens, people are thinking New York, and they're thinking D.C."

In fact, both Nukepills.com and KI4U, another potassium iodide distributor, agree that the greater Washington region has become one of the country's hottest markets. "India-Pakistan was driving sales in May and earlier this month," said KI4U President Shane Connor. "Then came the dirty nuke, and now things are getting pretty crazy."

Also maddening for both companies, however, is the reluctance of consumers to admit that reality has transformed a remedy once dismissed as the caprice of conspiracy theorists into an asset for family first-aid kits.

"We've been selling to the federal government, to individuals in D.C., Maryland, Virginia, Baltimore and even the Eastern Shore, and to a lot of businesses around the Beltway," Jones said. "But they don't want anybody to know -- it's like they're buying adult accessories."

One purchaser with no qualms is Jaci Longan, who spends much of her day driving long distances to visit prospective clients for the New York-based risk-management consulting firm Marsh & McLennan Cos. Two weeks ago she decided she needed some potassium iodide tablets.

"Everything that went on with 9/11 and the dirty bomb scare freaked me out," Longan said in an interview from her home in Arnold, Md. "Who knows where I'm going to be, or which way the wind will be blowing?"

Longan, 41, said she first began to think about nuclear radiation when she lived in Southern Maryland near the Calvert Cliffs Nuclear Power Plant, but the dirty bomb scare prompted her to call Nukepills.com, where Jones "gave me an explanation of how it works, so I bought three packs."

She put "one in my purse, one in my car and one in my house," she said. "I have a brother, and sisters, and 10 nieces and nephews who I wish would buy some, too."

Potassium iodide is a relatively simple salt that takes advantage of the fact that every person's thyroid gland needs iodine and will take it from the first available source.

If that source is radioactive material from a nuclear accident or explosion, or from a dirty bomb or nuclear waste, the victim is at risk of thyroid cancer. Children are especially vulnerable.

But if a person's thyroid is already saturated with safe iodine from a pill, it will ignore the radioactive iodine, which will be harmlessly excreted. One 130-milligram tablet a day will do the job for an adult, less for a child. There are no significant side effects, and the pills can be bought without a prescription.

If you can find them.

The potassium iodide companies make the overwhelming majority of their sales online. One drugstore in the region is Familymeds Pharmacy in Prince Frederick, almost in the shadow of Calvert Cliffs.

"We're working with our county health department, since they started offering the pills to anyone living within 10 miles of the reactor," said Philip Price, Familymeds' pharmacist and pharmacy manager. "I've actually sold more than I expected -- maybe 40 to 50 packs since April 25." The pills cost $13.95 a pack.

The danger of iodine poisoning is no myth. Scientists blame it for at least 2,000 cases of thyroid cancer after the 1985 nuclear accident at Chernobyl, in the former Soviet Union, and radioactive iodine is viewed as a palpable danger in the event of an accident at most nuclear power plants.

But potassium iodide offers no protection against any other radioactive element. Dirty bombs would most likely spread isotopes of cesium, cobalt, strontium or americium, while the blast and heat effects of a nuclear explosion would eclipse the short-term dangers of the accompanying fallout.

For years, the tablets' limited usefulness made them controversial. Some scientists worried that potassium iodide would be regarded as a cure-all for nuclear fallout. The nuclear power industry opposed public stockpiling, fearing that reactors would be seen as unsafe.

The Nuclear Regulatory Commission in 1998 suggested that states "consider potassium iodide as a supplement to evacuation and sheltering," NRC spokeswoman Sue Gagner said. This was a mild hardening of the NRC's long-held position that only emergency workers and people unable to escape from an emergency area needed a supply of tablets.

In December 2001, the NRC bought 9 million tablets from Anbex and offered them to states so that anyone living within 10 miles of a nuclear power plant could have two free pills, buying enough time either to get out of town or wait for the radiation danger to pass.

At the same time, the federal Department of Health and Human Services bought 1.6 million pills for a stockpile to backstop the NRC's supply, triggering a brief debate about whether the pills should simply be distributed to everyone. The important thing, HHS said, was to be able to get pills in a timely fashion to where they were needed most.

Gagner said the NRC has had requests from 14 of the 33 states with a reactor less than 10 miles away and has shipped 4.5 million tablets. Maryland has ordered pills, but Virginia has not. The District does not qualify.

The NRC's lukewarm posture has not endeared the agency to potassium iodide's business community. Anbex's Morris said the company began selling potassium iodide after the 1979 near-nuclear accident at Three Mile Island, figuring that such a close call would provide a market for years to come.

"But the NRC said it was unnecessary and gave a false sense of security," Morris said. "So none of the state governments bought it, because 'the Feds say it's no good.' We sold a few hundred packages per year."

Morris noticed a first flicker of interest in the last quarter of 1999, when fears of possible terrorist attacks with the arrival of the new millennium resulted in $200,000 in new tablet orders, "but on January 2 the market went away," Morris said.

Then came Sept. 11. "Within a week, people started talking about terrorism," Morris said, "and if the threat's real, then obviously a nuclear power plant is an attractive target."

Sales "went from zero to 60 in a heartbeat," he continued, "and it continues to go up. January was a great month, February was better, and, halfway through June, we have already sold more than we sold in the entire month of May."

All this has left the tiny companies that sell potassium iodide somewhat breathless. KI4U's Connor said the "family business" he founded in 1999 now has eight full-time employees and is in the process of recertifying more than 100,000 radiation meters and dosimeters it bought at auction from the government so it can resell them to emergency workers.

Anbex's Morris answers his own phone in an office in Palm Harbor, Fla., and so does Jones, his distributor, in Mooresville, N.C. Asked what position he holds in Nukepills.com, Jones replied, "President, I guess."

Potassium iodide tablets can protect the thyroid gland from radioactive iodine.Jaci Longan, 41, bought potassium iodide online two weeks ago, worried by the dirty bomb scare. She is shown on her porch with her cat Samantha.