Some medical myths come straight from the doctor

Mom was right about lots of things: eating your vegetables, washing your hands and never running with scissors.

But some of her other medical advice — such as waiting an hour after eating before swimming — just doesn't hold water, according to the authors of two new books about health myths.

But mothers shouldn't get all the blame for dubious medical advice. Doctors dispense plenty of it, too, says Andrew Adesman, a pediatrician at New York's Schneider Children's Hospital and author of Babyfacts: The Truth About Your Child's Health From Newborn Through Preschool (Wiley, $15.95).

The problem is that many important medical questions have never been answered, at least not with rigorous clinical trials. When in doubt, doctors often fall back on what their own mentors taught them, without questioning the evidence on which their advice is based, say Indiana University School of Medicine pediatricians Aaron Carroll and Rachel Vreeman.

"It's amazing how often doctors will speak with a lot of authority when there's no evidence that they're right and, unfortunately, even when there's some evidence to show that they're wrong," says Carroll, co-author with Vreeman of Don't Swallow Your Gum! Myths, Half-Truths and Outright Lies About Your Body and Health (St. Martin's Griffin, $13.95). "A frighteningly large amount of what we do is just our best guess."

Carroll and Vreeman say patients should feel free to ask their doctors about the sources of their data. In their book, they go over some of the basic types of medical studies, noting which are the strongest.

So what can patients believe? Not these medical wives' tales:

Sugar makes kids hyper. Parents insist this one is true, even though 12 studies have shown no effect between children's behavior and the sugar in their foods.

You can prevent colds with vitamin C, echinacea or zinc. All colds eventually go away on their own after a few days, so people can be tempted to credit their recovery to something they did to find relief. But 16 studies show that echinacea is no better than a sugar pill. Thirty studies including a total of 11,000 people found vitamin C had no effect. Three of four well-done studies of zinc found no effect. But using zinc nasal gel can destroy the sense of smell for years.

Adding cereal to a baby's diet will help him sleep longer. Parents and pediatricians alike are perpetuating that myth, perhaps because sleep-deprived parents are willing to believe anything that might help them and their infants get more sleep. But studies dating to 1974 show that babies who are fed cereal get no more sleep than other babies.

Iron in baby formula causes constipation. Studies actually have found no difference in baby's bowel movements, whether infants received iron-fortified food or not. But babies who get iron-fortified formula are less likely to be anemic.

Teething causes a fever. Studies show that teething babies are no more likely to run a fever than others.

Going out in cold or wet weather makes you sick. Colds and flus are seasonal and tend to strike during the winter. But even when scientists put cold viruses directly into people's noses, people who were chilled were no more likely to become ill than those who were warm and comfortable.

You should wait an hour after eating before you go swimming. Scientists have labeled that advice "questionable" since 1961. Exercising after eating a big meal may make people feel uncomfortable, but it won't cause life-threatening cramps.

You can catch poison ivy from someone who has it. The oil in poison ivy, urushiol, is what causes the itchy rash — whether the oil comes directly from the plant or from clothing or gardening tools on which the oil lands. But the rash itself isn't contagious, as long as a person has since washed off the oil.

You need to stay awake if you have a concussion. Most people with concussions will not fall into comas if they go to sleep. If a doctor says the concussion needs no treatment, there's no reason not to sleep.

Never wake a sleepwalker. No sleepwalker has ever died as a result of being woken up.

You can determine the gender of your baby by timing when you have sex. Some people try to conceive a boy by having sex in the middle of a woman's menstrual cycle, hoping that the more mobile sperm carrying a Y chromosome are more likely to penetrate the cervical mucus, which is thicker at this time of the month. But an analysis of six studies found there were no more boys conceived at this time of the month than girls.

Birth control pills don't work as well if you're on antibiotics. A review by the American Academy of Family Physicians found common antibiotics have no effect on the pill.