WASHINGTON — Mounting concern about swine flu and shortages of the vaccine recently approved to battle it are refocusing attention on homeopathic remedies, which are increasingly being used in this country and abroad as an alternative to prevent or treat various forms of flu: swine, bird and seasonal. U.S. public health officials say that children under 4 are among the groups most at risk from swine flu and have designated them a priority group for vaccination.

Meanwhile, the Food and Drug Administration has mounted an aggressive campaign against products making unproven or unapproved claims to fight swine flu.

While the gold standard for drugs and vaccines is proof of effectiveness in the form of randomized, double-blind placebo-controlled trials, there is no rigorous evidence that homeopathy works better than a placebo for any condition. That hasn't stopped a growing number of Americans from using it to battle a panoply of ailments, including arthritis, herpes and flu. A federally funded survey in 2007 found that in the previous year, nearly 5 million Americans used homeopathic remedies, made from substances including duck liver, heavy metals such as arsenic, herbs and poison ivy, and diluted in water until they are virtually undetectable.

A form of medicine invented by a German physician in the 1700s, homeopathy is predicated on the belief that "like cures like" — that a disease can be treated using a substance that produces similar symptoms in healthy people. It seeks to stimulate the body's ability to heal itself through the ingestion of highly diluted substances that might be toxic at higher doses. Even though homeopathic medicines use substances so diluted that virtually no molecule of the active ingredient remains, proponents believe that water contains the "memory" of the original substance.

Many scientists dismiss homeopathy, which defies the laws of chemistry and physics, as quackery. Robert Park, a prominent physicist and critic at the University of Maryland who has written extensively about pseudoscience, has called it "voodoo science."

Unlike vaccines or prescription or over-the-counter drugs, homeopathic medicines, which account for annual U.S. sales of more than $200 million, do not need to demonstrate safety or effectiveness, although they must be labeled with a list of ingredients and the conditions for which they are being used. A 1938 exemption allows drugs listed in the Homeopathic Pharmacopeia of the United States to be sold without the scrutiny that governs standard medicines.

"I think consumers should be aware that many homeopathic products are manufactured and distributed without FDA approval," said Elizabeth Miller, the Internet and health fraud team leader in the FDA's Center for Drug Evaluation.

Homeopathic drugs "are not necessarily safe," said David Schardt of the nonprofit Washington-based Center for Science in the Public Interest. Homeopathic drugmakers "don't have to play by the same rules — and I think consumers probably don't realize that."

Most homeopathic drugs don't require a prescription because they are used for self-limiting conditions that resolve without treatment. Many are sold over the Internet, in health food stores and in pharmacies, where they may be placed next to conventional over-the-counter drugs, making it hard for consumers to tell the difference.

That is the case with Zicam, one of the best-known homeopathic remedies, which is supposed to fight colds.

In June the FDA warned consumers not to use three forms of Zicam because they had been linked to permanent loss of smell believed to be caused by zinc, the product's active ingredient. In 2006 Matrixx Initiatives, the drug's manufacturer, agreed to pay $12 million to settle 340 lawsuits brought by consumers who claimed that Zicam Cold Remedy, a nasal gel and the company's flagship product, had damaged or destroyed their sense of smell. Matrixx denied that Zicam was responsible and said that the damage was caused by a virus.

Washington Homeopathic Products, founded in 1873, has grown dramatically since 1991 when it was bought by farmer Joseph Lillard and his wife, Linda Sprankle-Lillard, who moved the storefront business from Bethesda, Md., to Berkeley Springs, W. Va. Eighteen years ago the firm had five employees, 3,000 customers and annual revenues of $150,000. Today it employs 40 people, serves more than 70,000 customers including 423 pharmacies and 1,000 health food stores, and claims annual revenues of about $3 million.

In its 12,500-square-foot manufacturing plant, the firm makes 1,700 items for ailments including tooth pain, headaches, burns and arthritis. It also manufactures drugs for 35 other companies.

Lillard said that Influenzinum, which he expects to make and ship soon in its 2009-2010 formulation, is one of his more popular products. He says he can't point to scientific studies showing that it works, although he says he uses the product and hasn't gotten the flu. "People tell me it works," he said. "I'm not sure what 'science' means."

Although Influenzinum, which is made by several companies, has been manufactured for decades, the FDA, which is mounting an aggressive swine flu fraud campaign is limiting marketing claims that Lillard's company can make. Until recently, the firm's Web site said Influenzinum "is used to treat flu symptoms and possibly prevent flu."

After the Post asked FDA officials about those claims, Lillard received a warning letter from the agency, which officials declined to discuss.

The letter says that the Web site's claims about Influenzinum 08-09, the product made last year, are "false and misleading" and ordered them "and all other promotional materials" immediately removed from the site.

Lillard said that he is complying with the FDA's order and that this year's Influenzinum will say it is "for flu."