Myth: A woman can't get pregnant during her period.

While a woman is unlikely to conceive during menstruation, "nothing, when it comes to pregnancy, is impossible," said Aaron Carroll of Indiana University and co-author of Don't Swallow Your Gum: Myths, Half-truths and Outright Lies About Your Body and Health (St. Martin's Griffin, 2009).

Once inside a woman, sperm can wait for an egg for up to a week. Ovulation can occur soon after, or even during, the bleeding phase of a woman's menstrual cycle, giving patient sperm the chance to get lucky. The timing method of birth control doesn't work well, Carroll said, agreeing that couples who practice it are often called something they likely were trying to avoid: parents.

Relying on sunscreen to save you from skin cancer

Why is this healthy habit a bust? Because you’re probably applying it incorrectly. According to Francesca Fusco, M.D., spokesperson for the Skin Cancer Foundation, people tend to not use enough sunscreen or use it inconsistently or not early enough in life. They also often aren’t using a sunscreen that’s truly effective. Some of her tips for proper use include: Putting sunscreen on over any medication but under makeup; using the equivalent of a shot glass or two to cover your whole body—even under clothes—and then waiting at least 30 minutes before you go outside; and always using a sunscreen that contains the ingredient mexoryl. That last one is a biggie. Sunscreens without mexoryl—which is, to say, most of them—only protect against UVB wavelengths of light. But UVA waves are dangerous as well—possibly more so, considering that they can damage your skin without causing sunburn, leaving you unaware of your risk.

Buying “all-natural” health products

Certain natural health products and supplements might have some value, but the label “all-natural” doesn’t necessarily mean “safe.” Don’t feel too bad if you’ve leapt to that conclusion, though. It’s such a common mistake that the Canadian National Health Network began an education program aimed at making sure consumers were aware of the risks inherent in natural health products. According to the CHN, some natural health products might be toxic if you take too much, others can trigger unexpected allergic reactions, and still others react badly with medically prescribed drugs or with individual health issues, like pregnancy or heart disease. And, while the CHN reviews and labels natural health products for safety, most of the ones in the U.S. haven’t been tested or proven effective. They can be sold as long as they don’t claim to be able to treat or cure a specific disease. The best thing to do, before you start taking any supplement or look into any alternative cure, is to talk to your doctor. He or she will be able to help you make the best decisions for your body.

Drinking eight glasses of water a day

Woman drinking water//© Stockbyte/Getty Images
Drinking eight glasses of water a day

Admit it, this is one healthy habit that’s a royal pain. Luckily, it’s also completely unnecessary. For some people, eight glasses a day might actually be far too much, leading to sodium deficiencies and potentially life-threatening water intoxication, caused by kidneys not being able to keep up the intake of liquids. In 2002, a kidney specialist tried, in vain, to find any scientific evidence supporting the eight-glasses-a-day myth. His report, published in the American Journal of Physiology, concluded that this standard health advice was complete and utter bunk that, like many urban legends, stemmed from a tiny grain of truth. Apparently, the dietary guidelines provided by the Food and Nutrition Board of the National Research Council do say that humans need 1 milliliter of water for each calorie of food—adding up to about 10 cups a day. However, the same guidelines also say that we get most of this liquid from the water in solid food. There’s no need to drink more.

Taking antioxidant supplements

They’re supposed to reduce your risk of cancer and heart disease and even diminish the effects of aging, but, if you take antioxidants as a pill or some other drug-like form, chances are they aren’t doing anything at all. The basic idea behind the hype is that antioxidants, chemicals found in fruits and vegetables, can help reduce damage to various parts of your body by balancing unstable chemicals known as free radicals.
Without antioxidants, free radicals start trying to stabilize themselves—often by swiping molecules from your DNA, damaging it in the process. So far, so good. The free radical-fighting power of antioxidants has been demonstrated in the lab and people who eat plant-heavy diets are less likely to suffer from the diseases linked to free radicals. But, as Dr. Lisa Melton wrote in an article in the August 2006 issue of New Scientist magazine, many studies have shown that people who get their antioxidants from popular supplements receive none of the health benefits. In fact, Melton cited a few studies that even suggested antioxidant supplements were leading to worse internal damage, including a 1992 study by the National Cancer Institute that had to be cancelled after the patients taking beta carotene supplements actually began developing higher rates of lung cancer than those taking sugar pills.

Trusting your eyesight to carrots

If you think these vegetables will improve your vision, think again. While carrots do contain vitamin A, which is a major player in keeping your eyes working properly, you really only need a small amount of it—and no matter how much vitamin A you consume, it’s not going to magically eliminate your need for glasses. In fact, if you eat too much vitamin A, you can end up with a toxic—although not usually fatal—reaction. The idea that more carrots means better vision might actually be a relic of a World War II-era military disinformation campaign. According to the online World Carrot Museum, British intelligence began spreading the myth during the blitz as a plausible explanation for why their fighter pilots were suddenly able to spot Nazi planes at night. In reality, the British had simply developed a better radar system and didn’t want the enemy to find out about it.

Following a low-fat diet

Significantly cutting the fat in your diet is supposed to lead to weight loss, cancer prevention and a healthier heart. Turns out, those promises might just be empty intellectual calories. In 2006, the Women’s Health Initiative—a several-billion dollar, eight-year study of the effects of low-fat diets—finally came to an end. The results were shocking. Not only did the women who followed “fat-free diets” show no decrease in cancer or heart disease rates compared to their fat-eating counterparts, but they also weren’t any skinnier. And, the researchers said, the study probably applied to men as well. If you follow the medical literature, however, there’ve been plenty of studies, dating back to the early 1990s, which show low-fat diets aren’t as effective as they’re made out to be. In fact, there’s even some evidence that the behaviors they inspire might be harmful. A 2007 study in the journal Human Reproduction found that women who carefully avoided full-fat dairy products were more likely to experience a certain type of infertility.