When you ask a doctor a question, you expect an answer backed by the latest science. But the reality is, the field of medicine is always advancing, and “there's too much knowledge for any one physician to master,” says Renée Fox, PhD, professor emerita at the University of Pennsylvania, who studies the sociology of medical research, education and ethics.

As a result, certain popular beliefs that seem to make sense persist long after they've been challenged by studies. So we asked a range of experts, “What health myths drive you nuts?”

Here, they bust five vexing misconceptions.

RELATED: 15 Diseases Doctors Often Get Wrong

MYTH: “Rest is best for back pain.”

In years past, doctors tended to assume the worst—that back pain was a sign of a serious injury—and thought the way to heal was to stay off your feet. Today we know that most soreness stems from the muscles, ligaments and joints. Yet “many ER and primary care doctors still default to 'rest' out of uncertainty or a fear of liability,” said Dr. Robert Eastlack, codirector of the San Diego Spine Fellowship at the Scripps Clinic for Orthopedic Surgery.

The vast majority of back problems— from lumbar sprains to slipped disks—benefit from activity, according to 2007 guidelines from the American College of Physicians. Studies have shown that bed rest actually makes pain worse and last longer.

“The key is to keep moving,” said Eastlack— to stretch out tight muscle fibers and prevent your joints from stiffening.

He suggested starting with gentle daily exercise, like walking or swimming. Then gradually work your way up to more vigorous activities, like running, yoga or spin, as you become able to tolerate them. If the pain worsens or hasn't improved significantly in four to six weeks, get it checked out by your doctor.

RELATED: 15 Natural Back Pain Remedies

MYTH: “Hormone replacement therapy causes cancer.”

Docs can be skittish about prescribing estrogen to ease symptoms of menopause, such as hot flashes and night sweats. But they're probably clinging to outdated findings from a 2002 NIH Womens Health Initiative study that linked hormone replacement therapy (HRT) to a slightly elevated risk of breast cancer—even though more recent research has demonstrated that that risk is not associated with how HRT is formulated and prescribed today.

Several large studies have shown that taking estrogen is safe, said Dr. Mary Jane Minkin, a clinical professor of obstetrics, gynecology and reproductive sciences at Yale School of Medicine: “It really makes me crazy when doctors tell patients they'll suffer terrible consequences from hormone therapy.”

Typically it's needed for only two to three years to treat menopausal symptoms, she adds. There are women who can’t take estrogen (say, because of a history of breast cancer or blood clots). For them there's another option to ease hot flashes: Brisdelle, a very low-dose antidepressant.

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MYTH: “If medical tests come back negative, painful sex is probably in your head.”

When a patient says she has discomfort during intercourse, doctors typically consider a host of diagnoses—from dryness and infections to endometriosis, polyps and fibroids. But when they get to the end of their list, they're often stumped, says Libby Edwards, MD, a dermatologist in Charlotte, N.C., who specializes in genital skin conditions: “Our residency programs generally aren't teaching about other causes of painful sex.”

There's a little-known syndrome called vestibulodynia, which cant be tested for and is far more common than previously thought— affecting an estimated 12 percent of women at some point in their lives. The elusive disorder causes tenderness in the vaginal opening upon penetration, even by a tampon. Symptoms may come on slowly or suddenly; they can last for a few months or go on for years. Gynecology journals and textbooks include hardly anything about it, says Dr. Minkin. And the majority of women have to see at least three practitioners before they get a diagnosis, according to a Harvard Medical School study. But once you know you have it, there are a variety of ways to treat the pain, says Dr. Minkin, including medication and cognitive behavioral therapy.

RELATED: 10 Ways to Deal With Painful Sex

MYTH: “A history of breast cancer only matters if it's on your mother's side.”

Doctors who finished their training before the discovery of the BRCA genes in the mid- 1990s were taught to assess a woman's inherited risk by asking about her female first-degree relatives (mother and sisters). Faulty logic has helped that outdated approach persist, says Louise Morrell, MD, a medical oncologist specializing in breast cancer and genetics in Boca Raton, Fla. “We intuitively associate a female disease with the female side of the family,” she explains. Indeed, a recent study by researchers at Emory University School of Medicine found that women with a paternal history of the disease were less likely to get a referral for genetic counseling than women with a maternal history.

“For every single gene you have, you get one copy from your mom and one copy from your dad,” explains Dr. Morrell. That means the mutations associated with breast cancer are just as likely to come from Dad's DNA. “Knowing you are at a higher risk for the disease could save your life,” she adds. Make sure your provider takes a thorough family history. If you're still concerned, consider making an appointment with a genetic counselor (find one at nsgc.org). You might be a candidate for earlier mammograms or more frequent screenings, additional screening tests such as MRIs or other preventive strategies, like taking the drug tamoxifen.

RELATED: 12 Things That Probably Don't Increase Breast Cancer Risk

MYTH: “Younger women aren't at risk of stroke.”

The prominent signs of stroke in a woman under 45—dizziness or a headache—are not the same symptoms doctors look for in older patients (including weakness on one side and trouble talking), explains David Newman-Toker, MD, associate professor in the department of neurology at Johns Hopkins University School of Medicine. In a recent study, he found that stroke victims under 45 are nearly seven times as likely to be diagnosed incorrectly (with, for example, an inner ear infection or a migraine). And female stroke victims in general are 30 percent more likely to be mistakenly sent home from the hospital. “If you're a young woman, its a double whammy,” says Dr. Newman-Toker.

Strokes are on the rise in younger women, according to the American Heart Association, largely due to health factors like obesity. But another common cause is injury to blood vessels in the neck. It could happen from a major trauma like a car accident, or even from a more minor incident, such as tweaking your neck on a roller coaster. Hormonal birth control can also increase the risk of stroke in women who get migraines or who have an undiagnosed bloodclotting disorder. Pregnant women with gestational diabetes or pre-eclampsia are in danger as well. To make up for what physicians miss, know the signs, urges Dr. Newman- Toker. If you ever get a sudden onset of dizziness or a severe headache— and possibly hiccups or nausea—that sends you to the ER, ask the doc this question: “Why do you think it's not a stroke?” Says Dr. Newman-Toker, “If he can't answer in a way that sounds halfway intelligible, speak to another doctor.”

Docs dispel these common myths they keep hearing from patients.

“Wiping back to front leads to a UTI.”
The warnings you heard growing up? Flush 'em. “Clearly, if you take stool and put it on your urethra, you're going to get an infection,” says Lauren Streicher, MD, associate clinical professor of obstetrics and gynecology at Northwestern Universitys Feinberg School of Medicine. But that's not the same as wiping back to front. (Just wipe your behind separately!)

“Drinking more water can clear up my skin.”
If only. “One of the main causes of acne is an accumulation of dead skin cells and oil—which has nothing to do with how hydrated you are,” says Amy Ross, MD, a dermatologist in Palm Harbor, Fla.

“My fillings will give me mercury poisoning.”
The amount of mercury in silver fillings is nowhere near enough to cause problems. “As long as a filling is intact, I recommend leaving it alone,” says Matthew Nejad, DDS, a dentist in Beverly Hills. “Removing it could lead to complications.”

“You'll get a yeast infection from sitting in a wet bathing suit.”
“It's true that yeast like to grow in warm, moist places. But a vagina is a warm, moist place all the time—whether you have a bathing suit on or not,” says Dr. Streicher.

This article originally appeared on Health.com.