A popular homeopathic flu remedy boasts that it comes with no side effects, no drug interactions and won't make you drowsy. But the product also lacks something most people expect to find in their medicine: active ingredients.

Oscillococcinum (O-sill-o-cox-see-num), a tongue-twisting concoction used to treat flulike symptoms, is a staple in many European homes. Sales are steadily growing in the U.S., where it can be found everywhere from storefronts to major retailers.

Homeopathy critics, however, derisively call the product "oh-silly-no-see-um," a reference to the absence of active ingredients. It's products such as Oscillococcinum that have placed homeopathy in an awkward position: popular among holistic-minded consumers but scorned by scientists and most Western-trained doctors.

The British Medical Association vehemently objects to national funding for homeopathy treatment, considering any effect to be placebo. Around the world, activists have staged mass public "overdose" events outside pharmacies to demonstrate there's literally nothing inside the small white pills. One U.S. group, meanwhile, has offered $1 million to anyone who can prove homeopathy works and has challenged major drug retailers such as CVS, Rite-Aid and Walgreens to stop selling the products.

"Nobody, not even homeopaths have an idea how the remedies work," said Dr. Edzard Ernst, a longtime critic of homeopathy and professor of Complementary Medicine at Peninsula Medical School at the University of Exeter in the U.K.

Few things rile scientific skeptics more than homeopathy, a baffling form of alternative medicine in which patients are given highly diluted and vigorously shaken preparations to trigger the body's natural healing ability. Though it has been used for centuries and some studies have reported positive findings, the practice has no known scientific basis. Most analyses have concluded there's no evidence it works any better than a sugar pill.

Yet homeopathy hasn't just survived the years of scathing criticism; it's prospering. In the U.S., consumer sales of homeopathic treatments reached $870 million in 2009, growing 10 percent over the previous year, according to Nutrition Business Journal estimates.

For Oscillococcinum, sold in 60 countries, estimated annual retail sales in the U.S. are more than $20 million, according to the manufacturer, Boiron. It ranks 49th out of 318 cold and flu brand products that do more than $1 million in sales. Other popular homeopathic products include arnica gel for bruises and strains and diluted zinc remedies for colds.

"Some people feel these products shouldn't work due to the dilution level," said pharmacist Christophe Merville, director of education and pharmacy development for Boiron, the world's leading manufacturer of homeopathic medicines. But he said basic science studies have shown "that highly diluted solutions have biological properties that are different than water."

Ernst, who calls homeopathy the "worst example of faith-based medicine," said that even if the solution is structurally different, it doesn't matter. "After doing my washing up, the water in my sink is very different from pure water," he said. "Yet it would be silly to claim it had therapeutic effects."

Homeopathy is one of the most polarizing forms of complementary and alternative medicine in part because it's based on principles that defy the laws of chemistry and physics. One pillar is the assumption that "like cures like." Chopping a red onion, for example, can make your eyes tear and nose run. Seasonal rhinitis can trigger the same symptoms, so a homeopathic treatment derived from a red onion - Allium cepa - may be a possible remedy.

The second assumption proposes that diluting and violently shaking (or "succussing") the remedies makes them more effective, even if - and this is the part most scientists find hard to swallow - the final preparation no longer contains a single molecule of the original ingredient. The final product usually is a tiny ball of sugar the patient swallows, though homeopathic products also are sold as gels.

The mechanism behind the diluting and shaking remains a mystery. Some say homeopathic medicine may stimulate the body's natural defenses; others suggest homeopathic medicine retains a "memory" of the original substance in the water and the effect is due to nanoparticles.

Regardless, proponents say it shouldn't be discounted simply because it can't be explained. For years, no one knew how aspirin worked. And scientists still don't fully understand the mechanism behind a conventional drug such as Ritalin, argued Dr. Tim Fior, director of the Center for Integral Health in Lombard.

"Homeopathy challenges the belief in the molecular paradigm of medicines," said Fior, who on Wednesday will deliver an introductory lecture on homeopathy to medical students at the University of Illinois at Chicago. "Conventional pharmacology is based on - and profits immensely from - the idea that you can synthesize a molecule, patent it and produce it in bulk and then have a monopoly selling it. Homeopathic medicines are so dilute that they work more according to a biophysical or energetic paradigm."

People often use homeopathy to treat chronic pain, digestive issues, colds, influenza and allergies when they're not getting relief from conventional medicine. Homeopathic practitioners tend to spend more time with patients than regular doctors. The products also appeal to those looking for a "natural" or holistic product or who can't tolerate the side effects of conventional drugs.

Mona Grayson, 35, of Warrenville, Ill., turned to homeopathy for chronic digestive issues after her insurance expired and she could no longer cover the cost of her conventional treatment: $4,000 every eight weeks. Though she was tolerating her pricey medication, she had concerns about the long-term effects.

After an initial two-hour consultation with Fior, Grayson was given a remedy of phosphorus; she said she hasn't had problems since. "What matters to me is that I feel good," said Grayson, a raw food chef and happiness coach.

But does homeopathy provide anything beyond a placebo effect? Overall, many of the studies are small, of poor quality and funded by homeopathic manufacturers.

Dr. Iris Bell of the University of Arizona, one of the few homeopathy researchers to get federal funding, said the highest quality trials - double-blind, randomized, placebo-controlled studies - have had both negative and positive results. Her own work on fibromyalgia has shown individualized homeopathy did work better than the placebo.

Researchers also have shown that arthritis patients significantly benefited when they received homeopathy in conjunction with conventional treatment over six months. But the study, published in the journal Rheumatology, found the improvement was due to homeopathy's consultation process rather than its remedies.

"It has been a big problem bringing science to homeopathy," said Dr. Josephine Briggs, director of the National Center for Complementary and Alternative Medicine. With only a few exceptions, the center, a federal agency, hasn't funded any studies on homeopathy in the past five years. "On the other hand, the historical tradition has some real insights to treating humans in an individualized way," said Briggs, who said it might be appropriate to study the doctor-patient interaction.

At Merz Apothecary in Chicago, one of the largest homeopathic pharmacies in the country, president and co-owner Anthony Qaiyum summed up the thoughts of many homeopathic supporters. "Ultimately, who gives a damn whether it's scientifically proven if it works?" he said. "There are very valid questions about how it works, but whether it's my mind or the product, it's working and it's working without side effects."

Others see homeopathy as a safe way to complement treatment choices. "We don't always know why things work, but sometimes they do," said Freeport podiatrist Roland Tolliver, who uses it with his children and occasionally recommends arnica for patients with musculoskeletal issues.

"Regular medicine doesn't always work either," he said. "The most important thing is to leave all options open."

Critics say there's a risk in perpetuating the notion that homeopathy is equivalent to modern medicine, in part because people may forgo or delay conventional treatment. Moreover, it's unethical for pharmacists to prescribe placebos, said W. Steven Pray, a professor of pharmacy at Southwestern Oklahoma State University.

"You don't need placebos to generate placebo effects," Ernst has written. "Furthermore, if we allow the homoeopathic industry to sell placebos, we must do the same for Big Pharma. Imagine a world in which pharmaceutical companies could sell us placebos for all sorts of conditions just because some patients experience benefits through a placebo response."

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