CAMBRIDGE, Mass. - The unsettling little secret of Zicam Cold Remedy finally spilled out this week. Though widely sold for years for colds, it was never tested by federal regulators for safety like other drugs. And that was perfectly legal - until scores of consumers lost their sense of smell.

One little word on Zicam's label explains all this: "homeopathic."

Zicam and hundreds of other homeopathic remedies - highly diluted drugs made from natural ingredients - are legally sold as treatments with explicit claims of medical benefit. Yet they don't require federal checks for safety, effectiveness or even the right ingredients.

They're somewhat similar to dietary supplements, which use many of the same natural ingredients and also aren't federally tested for safety or benefit.

Many scientists view homeopathic remedies as modern snake oil - ineffective but mostly harmless because the drugs in them are present in such tiny amounts.

But an Associated Press analysis of the Food and Drug Administration's side-effect reports found that more than 800 homeopathic ingredients were potentially implicated in health problems last year. Complaints ranged from vomiting to attempted suicide.

In the case of Zicam, the FDA says it tied the drug to reports from 130 consumers who said they lost their sense of smell.

The agency last week told Zicam maker Matrixx Initiatives to stop marketing three products that carry zinc gluconate: Zicam Cold Remedy Nasal Gel, Nasal Swabs and discontinued Swabs in Kids' Size. The agency said the drug must be tested for safety and benefit, like a conventional drug, before it is again marketed. And the FDA warned people not to use the three Zicam products.

"It never occurred to me they could be dangerous and there was no kind of oversight - like the FDA - that ensured there was safety," says former Zicam user David Richardson of Greensboro, N.C. He has complained to the FDA about losing his sense of smell and filed his case with a lawyer for a future lawsuit, joining hundreds of others who have claimed in recent years that they lost their sense of smell from Zicam cold products.

In its review of homeopathy, the AP also found that:

Active homeopathic ingredients are typically diluted down to 1 part per million or less, but some are present in much higher concentrations. The active ingredient in Zicam is 2 parts per 100.

The FDA has set strict limits for alcohol in medicine, especially for small children, but they don't apply to homeopathic remedies. The American Academy of Pediatrics has said no medicine should carry more than 5 percent alcohol. The FDA has acknowledged that some homeopathic syrups far surpass 10 percent alcohol.

The National Institutes of Health's alternative medicine center spent $3.8 million on homeopathic research from 2002 to 2007 but is now abandoning studies on homeopathic drugs. "The evidence is not there at this point," says the center's director, Josephine Briggs.

At least 20 ingredients used in conventional prescription drugs, like digitalis for heart trouble and morphine for pain, are also used in homeopathic remedies. Other homeopathic medicines are derived from cancerous or other diseased tissues. Many are formulated from powerful poisons like strychnine, arsenic or snake venom.

Homeopathy sprang from the inventive - some would say fanciful - mind of German physician and chemist Samuel Hahnemann in the late 1700s. (Hahnemann University Hospital in Philadelphia has no connection to homeopathy.) Experimenting on himself, he became convinced that if an ingredient causes a symptom in a healthy person, it will treat the disease that causes the same symptom. He also theorized that diluting ingredients to minuscule, even untraceable, concentrations paradoxically makes them more powerful.

To this day, homeopaths put forth mystical-sounding explanations involving "vital force" and "healing energy." And with arcane ingredients like "nux vomica" and "arsenicum album," many homeopathic medicines sound like something brewed in a druid's kettle.

In 1938, Congress passed a law granting homeopathic remedies the same legal status as regular pharmaceuticals. The law's principal author was Sen. Royal Copeland of New York, a trained homeopath. "He did it in such a sneaky way that nobody really noticed or paid attention," says medical author Natalie Robins.

And that law has remained in force ever since.

Almost reduced to obsolescence in the United States, homeopathic remedies have revived in recent decades with the burst of interest in vitamins, herbs, and other alternative treatments. Since 2002, the U.S. homeopathic remedy market has exploded by 89 percent, to an estimated $830 million last year, according to market research company Mintel. By 2007, homeopathic remedies were taken by almost four million Americans, federal data show.

Pharmacist Albert Lavender, retired deputy director of the FDA's unit overseeing drug labels, calls it "a big fraud" on the consumer.

"He might not get hurt most of the time," he says, "but his pocketbook is getting hurt all of the time."

Lavender says "it doesn't make sense" that the FDA requires homeopathic medicine to bear a label saying what it treats because, in his view, most of it treats nothing.

"Very often, the only active ingredient is alcohol, and patients don't know that, and they get a buzz on. The therapeutic effect is no greater or less than a martini," says Jerry Avorn, an expert in pharmaceutical safety at Harvard Medical School.

Richardson says he thought he was taking a government-approved drug when he took a whiff of homeopathic cold gel. He says he felt a burning sensation and hasn't smelled much since. A doctor who tested his sense of smell tentatively linked his condition to Zicam, Richardson's medical records show.

Some independent research also has blamed the active ingredient in Zicam, zinc gluconate, for such problems.

Zicam seller Matrixx Initiatives, of Scottsdale, Ariz., which grew out of a chewing gum company, paid $12 million in 2006 to settle lawsuits with about 340 Zicam patients. It has won a lawsuit in California, and several other federal cases were dismissed.

But complaints by dozens of patients remain before the courts. The Motley Rice law firm in Mount Pleasant, S.C., represents more than 300 with Zicam claims, says lead lawyer Lynn Seithel. She says the FDA warning last week "validates what our clients have been saying."

The company, which has sold more than a billion doses since the products came to market in 1999, says it settled in the past simply to reduce its legal exposure. The remedy has recently been sold with a redesigned spray nozzle, and the company argues that it is safe, citing academic studies that it funded. Matrixx says some people failed to follow package directions and stuck the nozzle too far up their noses.

Faced with the FDA warning, the company's acting president, William J. Hemelt, blamed much of the smelling problems on the colds that patients were treating. However, the company agreed to suspend shipments and reimburse customers who want refunds.

Questions have been raised about the touted safety record of other homeopathic remedies, too.

The FDA's own side-effect reports potentially implicate at least 843 homeopathic ingredients just in the year ending September 2008, the AP found. They were linked to side effects, including muscle and joint pain, in reports submitted by consumers, doctors, and others.

Though many homeopathic remedies consist mostly of sugar or alcohol, thousands of patients swear by their effectiveness.

Amanda Rafferty of Haverhill, Mass., took homeopathic sanguinaria canadensis, made from a toxic herb known as bloodroot, for her monthly migraine headaches. She says she went a full year without another migraine.

Her homeopath, Begabati Lennihan of Cambridge, Mass., treats headaches, colds, ear infections, digestive complaints, depression, and behavioral problems. Like other homeopaths, Lennihan considers not just symptoms but also temperaments, favorite foods, even dreams. However, if the problem shows up in an X-ray, she acknowledges, it "is going to be harder to fix with homeopathy."

Today's homeopaths are typically trained in part-time certificate programs. Lennihan took conventional nurse's training to bolster her homeopathic credentials. Only Connecticut, Arizona, and Nevada license homeopaths, and they insist on a medical degree as a prerequisite.

In the booming nonprescription market, many homeopathic remedies are sold for symptoms so vague and broad that it's virtually impossible to match treatment and ailment.

For example, one homeopathic ingredient, lithium carbonate, is used as a psychiatric drug by conventional medicine. In homeopathy, one advertisement pitches it for "rheumatic soreness in the heart region, paralytic stiffness all over, cerebral congestion, insomnia and epilepsy."