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I am among the millions of Americans who take dietary supplements — mostly vitamins, minerals, antioxidants and probiotics. I take them for several reasons, but not, for the most part, to treat and prevent colds and flu.

There is little or no evidence that they work for either ailment. Studies have shown they are ineffective in preventing or treating the flu, and only a few are modestly helpful in preventing or treating colds.

“These illnesses are self-limiting,’’ says Robert T. Schooley, head of the infectious-diseases division at the University of California at San Diego. “If you do nothing, you get better. If you stand on your head, you get better. If you get into a hot-air balloon and go to the South Pole, you get better. If you spend $1,000 on ginkgo biloba, you get better. Nothing except a flu shot prevents the flu, and no dietary supplements can prevent or treat it. And there is scant evidence that supplements can prevent or treat colds.’’

In 2012, an estimated 59 million Americans purchased complementary medical approaches, including dietary supplements, mostly to stay well, according to a survey conducted by the National Center for Health Statistics. Americans spend almost $30.2 billion a year on complementary approaches, the survey says.

“I think many people are focused on self-care and the concept of taking ownership of their treatments,” says Craig Hopp, program director and acting chief of the basic and mechanistic research branch of the National Center for Complementary and Integrative Health (NCCIH). “I also think a certain segment of the population is skeptical about pharmaceuticals. But when it comes to colds and flu, there is limited evidence that any of these approaches are truly effective.’’

The Food and Drug Administration does not have the same regulatory power over these products as it does over drugs, which must prove safety and efficacy before they are licensed. The FDA can take action against potentially dangerous supplements, but only after they are in the marketplace.

Each year, Americans suffer more than a billion colds, and between 5 percent and 20 percent of the population gets the flu. Both illnesses are caused by viruses and have some symptoms in common, but the flu usually is more severe. Unlike the flu, colds generally don’t cause serious complications, and both illnesses usually resolve within two weeks.

Three prescription-only antivirals have been approved to treat the flu. If you take one of them within 48 hours of becoming ill, the drug can lessen symptoms and shorten the illness by one or two days.

For preventing or treating colds, studies have shown limited value in complementary approaches. Some, in fact, have potentially harmful side effects, according to information from the NCCIH, which is part of the National Institutes of Health:

• When started within 24 hours and taken for less than two weeks, oral zinc lozenges may reduce the length of a cold. But zinc nasal sprays can be dangerous, having been linked to severe and permanent loss of smell.

• Twenty-four studies have looked at echinacea, which is made from a plant, and concluded there is “weak” evidence of benefit. NCCIH also notes that products vary greatly, which can produce inconsistent effects.

•Buckwheat honey can ease a child’s nighttime cough but should never be given to children younger than 1 because of the risk of botulism.

• Geranium extract may help relieve sinusitis, bronchitis and the common cold, but the quality of evidence is low.

• Saline nasal irrigation, which involves rinsing the nose and sinuses with salt water, usually with a neti pot, can help with cold symptoms, but using tap water can be risky; the practice was implicated in two 2011 deaths caused by an amoeba.

• American ginseng may shorten colds, but it doesn’t decrease the number of colds that people get. Also, it may interact with the anticoagulant warfarin.

• Some people find willow bark useful for treating fever and aches. The active ingredient in willow bark is salicin, the original source for what ultimately became aspirin. So why not just take aspirin?

• An evaluation of the large amount of research done on vitamin C and colds (29 studies involving more than 11,000 people) concluded that the vitamin doesn’t prevent colds in the general population and shortens colds only slightly. But it does seem to reduce the number of colds in people exposed to short periods of extreme physical stress, such as marathon runners and skiers.

When I was younger, I never bothered to get a flu shot. Then, about 20 years ago, I came down with what I think was the flu. I had fever, aches, no appetite and crushing fatigue. I’ve gotten the annual shot ever since.

As for colds, I avoid being around children with runny noses, and — I admit it — I do take vitamin C, despite studies showing it helps only after severe physical exercise. Because I exercise strenuously — and because I have run 12 marathons — I figure it can’t hurt and might help.

I haven’t had many colds in recent years, possibly because I had so many when my kids were little. I assume I’m now immune to all of the viruses that struck me then.

When I do get a cold, I find that running (not marathons, just easy running) helps. It releases norepinephrine, a natural decongestant, which temporarily relieves stuffiness.

The other thing that unfailingly makes me feel better when I have a cold is copious amounts of my mother’s swear-by remedy: hot chicken soup.

Research actually supports its therapeutic benefits. A 1978 study, for example, found that eating hot chicken soup was more effective at increasing nasal air flow and clearing mucus than drinking hot or cold water.

In 2000, a University of Nebraska study concluded hot chicken soup eases upper-respiratory symptoms by inhibiting neutrophils, the white blood cells produced when the body fights infection. The researcher used an old family recipe and store-bought varieties, and found they all worked equally well.

“Chicken soup doesn’t treat the virus, it treats the symptoms,” Schooley says. “But if it makes you feel better, by all means go for it.”