If you're like roughly half of your fellow Americans, you probably popped a multi this morning. As the slightly acrid taste lingered on your tongue, you felt good knowing that something so quick and easy would help safeguard your health.

Multivitamins are the most popular dietary supplement on Earth. But here’s the sobering reality: They may be simply useless—or worse.

For the last several years, a Mount Fuji of evidence has piled up to show that multivitamins don’t do much of anything for the health of the average person. Though less conclusive, a growing body of evidence suggests that they may even shorten your life. Unless you are taking vitamins to address a specific deficiency, malnutrition or illness, gulping down a multivitamin in hopes of preventing disease or cheating the Grim Reaper may be one of the most prevalent medical myths of our time.

Yet Americans aren’t getting the message. In fact, as the economy remains stagnant, we are taking more vitamins than ever in the hopes that we can avoid a costly doctor’s visit. However misguided our thinking, there’s one sure bet on vitamins: With annual sales of more than $20 billion, there are pots of money to be made for an industry that operates in the shadows —money so big that hedge funds are tripping over themselves to get in on the action.

The victim is not just your wallet. It might be your health.

A look at the science

As recently as 2002, the Journal of the American Medical Association recommended that "all adults take one multivitamin daily." Your doctor probably told you to take one, confident that her advice reflected the scientific consensus. Your friends or family members may nudge you to jump on the vitamin train, and share their favorite brands and doses.

But for the last several years, many doctors have begun to reverse their previous recommendations. Two gigantic studies have caused what Prevention magazine called a “sea change” in clinical thinking.

The first study, published by the Agency for Healthcare Research and Quality in 2006, combed through 63 randomized, controlled trials (the highest standard of research) on multivitamins. Turns out they did absolutely nothing to prevent cancer or heart disease in most people, with the exception of those in developing countries, where nutritional deficiencies are common. In a second paper, published in 2009, scientists at the Fred Hutchinson Cancer Research Center followed 160,000 postmenopausal women for about 10 years, and found that multivitamins did zilch to prevent cancer, heart disease and all causes of death, regardless of what the women were eating (See PDF here).

There’s more: In a 2010 study, a team of French researchers, who published their work in the International Journal of Epidemiology, tracked over 8,000 volunteers who took either a multivitiamin or a dummy pill every day for six years. Results? The vitamin-takers showed no improvement in health or well-being over the placebo group.

Jaakko Mursu, a nutritional scientist, was among those in the medical community who had grown skeptical of multivitamins. So he led a team of researchers to study the issue. Even he was shocked by what he found, which was published as the Iowa Women's Health Study: Older women who took multivitamins were 6 percent more likely to die than others. This was true even though the women taking the vitamins tended to have healthier lifestyles than those who didn't (calcium was the only vitamin or mineral associated with decreased risk).

Useless is one thing. But harmful? That's really turning conventional wisdom upside-down.

So what’s the problem?

People take multivitamins under the theory that they act as a form of insurance against anything lacking in their diets. We hear all the time how we’re not getting enough fruits and vegetables, right?

Well, yes. But most of us in a rich country like the United States do, in fact, have a diet that is varied and plentiful enough that we don’t have specific vitamin deficiencies. We’re not keeling over from scurvy or rickets. In fact, those among us who take multivitamins and other vitamin supplements are actually the least likely to need them. Vitamin users tend to have higher levels of education, smoke less, exercise more, and choose healthier foods than non-users.

Even if you do have a specific deficiency, which a doctor would be able to determine through blood work and other diagnostic tools, it does not necessarily follow that a pill made in a laboratory is the way to get those nutrients. The vitamin industry is poorly regulated, and those little pills are not required to go through rigorous safety and efficacy tests. Current laws make it extremely difficult for the FDA to ban harmful ingredients once they are discovered—the battle over supplements containing ephedra, purported to reduce weight and increase energy, is a case in point. Despite a tsumani of evidence of the ingredient's harmful, even fatal, effects, a 2004 FDA ban was challenged by manufacturers, and eventually overturned in court, only to be restored in 2006 in the U.S. Court of Appeals.

Unlike medicines, vitamin packages, for the most part, are not required to alert consumers to harmful side-effects. And then there’s the problem of illegal sales of counterfeit vitamins, nutrients and supplements over the Web, not to mention the possibility of contamination. The term "natural," frequently used in marketing vitamins, is reassuring to consumers, but means basically nothing: hemlock is natural, but you'd probably not want to take it as a supplement. Just ask Socrates.

It’s also possible to overdose on vitamins. Between the food you eat during the course of a day (including items like cereal fortified with vitamins and minerals) and the supplements you ingest, you could be getting way over the recommended amount of a given item. Trader Joe’s Women’s Once Daily Multivitamin & Mineral supplement, for example, contains 200 percent of the recommended amount of Vitamin C, 286 percent of the recommended dosage of selenium, and over 400 percent of what you need in the way of Vitamin B12. More is better, right? Wrong.

According to Norman Hord, associate professor in the department of Food Science and Human Nutrition at Michigan State University, too much could hurt you. "Excesses of all nutrients, from water, to iron, to water-soluble B vitamins, can potentially cause toxicities," he says. “People who take vitamins and minerals in amounts above the established upper limits of the Dietary Reference Intakes (DRIs) may harm tissues where the vitamin is stored in their body…That’s why you shouldn’t take more than the recommended amount.”

High doses of folic acid have been associated with higher risk of precancerous colon polyps. Too much beta carotene has been linked to lung cancer. The chances of dying for people who take antioxidant supplements have been found to be 5 percent higher than non-users. As the New York Times pointed out in a 2009 article, “the selling point of antioxidant vitamins is that they mop up free radicals, the damaging molecular fragments linked to aging and disease.” The problem is that your body actually needs free radicals in order to fight off illnesses. Wipe them out and you may have just said good-bye to your body’s natural defense system.

Katherine Zeratsky, a registered dietician at the Mayo Clinic, explains that, "Most people, if they are eating a generally healthy diet—not a perfect diet, but a generally healthy diet—don't need a multivitamin…With things like breakfast cereals, nutrition bars, foritified grains and bread, because all those foods are fortified with other minerals, we are getting more than we realize and more than we need."

So how did nearly half of us end up shelling out hard-earned money for something that begins to look like the equivalent of 19th-century bloodletting? As is so often the case, bad things can happen when money, science and politics collide.

How America became vitamin-crazed

Paul A. Offit, the chief of the infectious diseases division of the Children’s Hospital of Philadelphia, is the ultimate medical nofunik. Rigorously dedicated to evidence-based medicine, he has raised the ire of many with his crusade to debunk the widespread belief that autism is caused by vaccines. Now he’s written a book challenging the fervent belief in vitamin supplements and some of the claims of alternative medicine, Do You Believe in Magic? The Sense and Nonsense of Alternative Medicine. In Offit’s view, there’s really no such thing as “alternative medicine.” If a treatment is proved to enhance health or prevent illness, it’s medicine, plain and simple. Unfortunately, the problem with claims made about many popular treatments, including vitamin supplementation, is that they haven’t passed the evidence test.

In recent op-eds in the New York Times and the Guardian, Offit explains how the vitamin supplement industry has gotten away with selling snake oil for the past several decades. America’s vitamin craze started with one Dr. Linus Pauling, whom Offit colorfully describes as “a man who was so spectacularly right that he won two Nobel prizes and so spectacularly wrong that he was arguably the world's greatest quack.”

Pauling, whose work in chemistry won him the Nobel Prize in 1954 and who later became a peace activist (winning the Nobel Peace Prize), became interested in longevity in his 60s, and his deep desire for a long life seems to have clouded his judgment. He developed a correspondence with an amateur chemist who claimed that high doses of Vitamin C could expand the human lifespan. Pauling developed tremendous enthusiasm for Vitamin C, touting it as the means of curing the common cold, which caused sales to skyrocket. Undaunted by numerous studies that contradicted his claims, he began to promote the use of antioxidant combinations as a cure for everything from mental illness to kidney failure.

By the mid-'70s, Americans were rushing to the pharmacy to follow Pauling’s advice. “Although studies had failed to support him,” writes Offit, “Pauling believed that vitamins and supplements had one property that made them cure-alls, a property that continues to be hawked on everything from ketchup to pomegranate juice, and that, for sales impact, rivals words such as natural and organic: antioxidant.”

In December 1972, the FDA, alarmed at the growing vitamin obsession, tried to regulate supplements containing more than 150 percent of the recommended daily allowance. The goal was to force manufacturers to prove that they were safe. Vitamin-makers not only destroyed the bill, they were able to set things up so the FDA would be pretty much off their backs for good. Industry executives recruited William Proxmire, a Democratic senator from Wisconsin, to put together a bill blocking the FDA from requiring safety tests and regulating megavitamins.

The industry leaders pleaded that consumer costs would go up if they had to be bothered with regulations. Among those who said that the Proxmire bill was bunk were the American Medical Association, the FDA, the National Nutrition Consortium (which represents jointly the American Institute of Nutrition, the American Society for Clinical Nutrition, the American Dietetic Association, and the Society of Food Technologists), Ralph Nader’s Consumers' Union, the Pharmaceutical Manufacturers’ Association, the American Association of Retired Persons, and the president of the National Academy of Sciences.

On Aug. 14, 1974, at the hearing, Marsha Cohen, a lawyer with the Consumers Union, tried to introduce common sense into the proceedings. She set down eight cantaloupes in front of her and pointed out that pills containing Vitamin C might contain the same amount as what was in those fruits, or even 20 times as much — clearly much more than Mother Nature ever intended a human to consume at one sitting. As Offit put it: “Ms. Cohen was pointing out the industry’s Achilles’ heel: ingesting large quantities of vitamins is unnatural, the opposite of what manufacturers were promoting.”

Nevertheless, Proxmire won the day, and myth became policy. In 1976, in what Peter Barton Hutt, chief counsel to the FDA, called “the most humiliating defeat” in the agency’s history, his bill became law. That’s why it has taken so long for consumers to learn that taking vitamins could actually harm them.

In 1994, Linus Pauling died of prostate cancer—a cancer that has been linked to high doses of vitamins.

Offit is happy to report serious studies that support the use of certain supplements. The problem is that you can count them on one hand: “Of the 51,000 new supplements on the market, four might be of benefit for otherwise healthy people: omega-3 fatty acids to prevent heart disease; calcium and vitamin D in postmenopausal women, to prevent bone thinning; and folic acid during pregnancy, to prevent birth defects.”

Four out of 51,000. You’d have better odds at a slot machine.

Humans are not very scientific

Part of the great puzzle of the vitamin myth is how the public continues to ignore large clinical trials and mounting evidence. Respected organizations like Harvard Men’s Health Watch may try to educate the public with press releases featuring titles like, “It’s time to reassess the value, safety of multivitamin use” which warns that “the average man [should] give up the multivitamin, at least until scientists solve the puzzle of folic acid and cancer," but the enduring myth of multivitamins shows that hope is stronger than evidence.

In preparing this article, I spoke to friends and acquaintances, several of whom hold doctoral degrees, though only one in science or medicine. Most were conscientious consumers of vitamins, and even when I presented them with the evidence from large clinical trials, they tended to show great reluctance to question what was in the bottle in the cabinet.

Excuses ranged from “I don’t take megadoses” (never mind that any dose that gives your body more than the recommended daily allowance is a megadose) to “My doctor is very up on these things” (then he’d have to be rethinking his pro-vitamin advice) to “the studies always change and I’m going to just stick to what I’m doing” (yes, scientific opinion changes, and it increasingly suggests that vitamins may be useless or harmful). None of the people I spoke to had specific deficiencies that had been diagnosed through blood work, etc. They were what some doctors call the “worried well”— people who aren’t sick but think of vitamins as a little “extra” that will boost their health. (Hey, it’s easier than hitting the gym.)

The only exceptions to the vitamin-believers in my circle were the journalists, who tended to be more aware of changes in the news, and the one acquaintance who is an actual medical doctor. She threw up her hands and said: “This vitamin thing drives me crazy. You wouldn’t believe how hard it is to get people to accept evidence-based medicine. Even many doctors resist it.”

Part of the answer is that even highly educated people are uncomfortable about changing their beliefs— and they may be more stubborn because they trust that they have greater acumen in deciding what to believe and not to believe. And yet without a background in science, nutrition or biology, it’s very difficult for the lay person to even begin to understand what is happening in their bodies when they take a vitamin. Frankly, if we really get down to it, most of us are not terribly literate in science. A 2009 Pew poll shows that over half of us don’t know what a stem cell is, and nearly as many are unaware that atoms are larger than electrons. We’re only passingly familiar with the nation’s top scientific agencies, and yet we actually feel confident making declarative statements about the benefits of antioxidants, phytonutrients and sublingual vitamin delivery.

Perhaps that’s because our language and our thinking is derived from advertising much more than we realize. The multi-billion dollar industry hawking vitamins greets us with endless admonitions and claims in the drug store, the grocery aisle, our in-boxes, and every time we flip on the TV. We become faux-conversant in matters that actually require serious study. I’m not a scientist, but as a layperson who has spent several weeks reading about the most current studies and research, I have come to three conclusions:

Never take any vitamin or supplement without discussing it with your doctor and having appropriate diagnostic work done. Tips from well-meaning friends or relatives do not count. If your doctor prescribes preventative vitamins, ask her about the changing state of the evidence and demand information on clinical trials. The onus is on her to demonstrate the efficacy of taking supplements if she is prescribing them. Try to get your body’s nutrients through foods, rather than pills.

But please, don’t take my word for it. Keep abreast of the latest developments through the National Institutes of Health, the Mayo Clinic, peer-reviewed journals and other serious research agencies and groups. A scientific perspective is always one of questioning. If vitamin mythology is the disease, trustworthy information is the cure.