Patients who receive a flu shot and then get sick soon after are often too quick to identify a cause and effect, says Dr. Lori Heim. (Ali Haupt/Associated Press)

It isn't necessary to drink eight glasses of water a day. Flu shots don't cause the flu. And the idea that singles have better sex lives than married couples? Not true either.

When Dr. Aaron E. Carroll and Dr. Rachel C. Vreeman, both professors at the Indiana University School of Medicine, began examining more than 60 popular medical beliefs, they discovered that most were myths. The pair has catalogued these false or highly questionable claims in their recently released book, Don't Swallow Your Gum! Myths, Half-Truths, and Outright Lies About Your Body and Health.

Debunking a health myth is one thing, says Carroll, but convincing people to believe scientific evidence can be a challenge when it conflicts with their personal experiences.

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Take for example, the many parents who swear that their children are hyperactive after consuming sugar — despite studies that show no relationship between sweets and behavior. "There are people who are convinced that this is a truth," he says.

To help people discern between fact- and myth-based medical claims, Carroll recommends improving your knowledge of cause and effect. Just because a symptom seems to indicate a certain outcome doesn't mean the two are related. Patients should also hesitate to link a symptom to a diagnosis before discussing it with a medical professional — and even then, it's important to ask key questions about why the two are associated.

Cause and Effect

Medical misinformation tends to spread quickly and can persist for years when it's repeated by family, the media, the Internet and even well-meaning physicians. Dr. Lori Heim, president-elect of the American Academy of Family Physicians, says most of it is based on unchallenged or old data or assumptions that since have been disproved as family lore.

Often patients are too quick to identify a cause and effect, says Heim. This frequently happens, for example, with patients who receive a flu shot and then get sick soon after. They tend to blame the vaccine since it contains dead strains of the flu virus, but since it will never become active again, it's likely an allergic reaction or a coincidence.

We tend to trust research bearing the imprimatur of science, but not all studies are created equally — and patients should take that into account when judging medical information.

Sorting out cause and effect can be even trickier when dealing with scientific studies. In 1998, an article published in the Lancet discussed eight autistic children whose symptoms seemed to occur after receiving the vaccine for measles, mumps and rubella. Since then, some parent advocates have insisted that there is a link between vaccines and autism despite several studies that have demonstrated that the two are not related. One 2002 study arrived at that conclusion using data for more than 530,000 children — a far more robust sample than eight.

We tend to trust research bearing the imprimatur of science, but not all studies are created equally — and patients should take that into account when judging medical information.

"Huge, well-designed studies all say one thing and poorly designed studies say another," explains Carroll. "There can be a subtle difference that gets lost."

The gold standard, he says, is a randomized, controlled clinical trial. These often involve thousands of human volunteers who are assigned to different groups randomly, some of whom receive a placebo treatment.

Scientific reviews, which examine all of the literature published on one subject and attempt to interpret the overall findings conclusively, can also be a trustworthy source of medical information.

Asking Questions

While it's important to improve your health literacy, Dr. Carroll says patients should also discuss medical information with a physician and start by asking why he or she believes something to be true.

"It's a good habit to get into," he says. "It's OK to ask your doctor and, if they don't know, they should look it up."

A good physician, says Dr. Marc Siegel, an internist at New York University's Langone Medical Center, will rely on research and best practices to treat a patient, but should also account for unique circumstances. Siegel, for instance, doesn't believe that sugar makes normally behaving children more energetic, but he does think it can have a precipitating effect on already-hyper kids. Such caveats are why it's important to discuss health issues with a physician.

In general, it's important to be wary of health information shared by non-medical authority figures. A few months ago, Oprah Winfrey was criticized for giving airtime to dubious medical advice, including information about the link between autism and vaccines.

We're prone to believe inaccurate medical claims, Carroll says, because "as human beings we try to make sense of what we see … and [we] rely on personal experience."

But instead, he says, we'd be much better served by a healthy dose of skepticism.