Medical myths are all too common in popular culture, and some are referenced so frequently that we just assume they are correct.  We’ve heard them from the time we’re born right through till adulthood, and we probably don’t take too much time to do the research to see what evidence there is to support each of the claims.  Some are so common, that even doctors believe them (according to a British study in 2007).  Before we go further, let us define the word “myth”:

In popular use, a myth can be a collectively held belief that has no basis in fact according to the speaker. It can also be referenced as fiction, or a half-truth, and in the most general sense, it is an untrue story.

I have fallen victim to several of the half-truths below, and thus, this list was born.  Below, a collection of medical myths derived from websites LiveScience, ListVerse, and HowStuffWorks.  Some of the myths have been edited for brevity, and the copyright is owned by the respective authors.

MedicalMyth 1:  Cracking your knuckles will cause arthritis later in life…

What’s that cracking sound?  When you stretch your knuckles apart, you are moving the bones apart and forming a gas bubble.  The sound is the bubble popping.

The cracking sound in the knuckles is caused by the bones moving apart and forming a gas bubble – the sound is the bubble bursting. It is quite common to hear someone warning a knuckle-cracker that they will get arthritis, but the worst that can happen to a compulsive-cracker is that their finger joints may weaken over time. Arthritis is caused by a variety of things (such as crystal formations in the case of gout) – but knuckle cracking isn’t one of them.

MedicalMyth 2:  Chewing gum takes seven years to pass through your system…

I am sure we have all been told at least once in our life by a concerned adult, not to swallow gum as it will take seven years to leave our bodies. This is right up there with the whole “fruit seed growing a tree in your stomach” silliness, but while most adults realize the tree story is a myth, they don’t realize that the gum one is too. It is true that gum is not digestible in the human body, but it simply passes whole through your system. It doesn’t stick to your insides, it just continues along with any food you have eaten and pops out the other end.

MedicalMyth 3:  You should drink at least eight glasses of water a day…

The origins of this myth is most likely the fact that a 1945 government agency said that the human body needed around 8 glasses of fluid a day. This included the fluid from all of the foods we eat and drinks like tea and coffee. Somehow over time “fluid” turned to “water” and the modern water myth arose. This also lead to silly slogans like “if you are thirsty it is too late” – a concept that would seem to have been invented by water bottlers who have something to gain from excess water consumption in the population in general. So, in reality, if you are thirsty, drink some water. If you are not, don’t.

MedicalMyth 4:  You lose most of your body heat through your head…

A military study many years ago tested the loss of temperature in soldiers when exposed to very cold temperatures. They found rapid heat loss in the head – and so the idea that we lose heat through our heads was born. But what they didn’t tell you was that the soldiers were fully clothed except for their heads. This obviously skews the statistics considerably. The fact is, completely naked, you lose approximately 10% of your body heat through the head – the other 90% is lost via the other parts of your body.

MedicalMyth 5:  Eating turkey makes you sleepy because it contains tryptophan…

This is one of the most common myths on this list – and it pops up every year around Thanksgiving. But actually, chicken and ground beef contain almost identical quantities of tryptophan as turkey does. Other foods such as cheese and pork contain significantly more of the chemical than turkey. So why do people think turkey makes them sleepy? It is most likely due to turkey appearing at very large meals often eaten during the day rather than the evening. The heavy meal slows blood flow which can cause drowsiness, and the timing can have a huge psychological impact: in other words, you are imagining it.

Medical
Myth 6:  Eating at night makes you fat…

Secret snackers rejoice! This is a complete myth. It doesn’t matter what time of day you eat, as long as you eat only the total calories that you burn each day, you will not gain weight. If you eat fewer calories than you burn, you will lose weight, and if you eat more calories, you will gain. It is as simple as that. Having said that, the routine of three meals a day at the same time each day can have other benefits in life (routine is good and it helps humans work more effectively), but snacks at night are no worse than snacks in the morning or afternoon.

Medical Myth 7:   We use only 10 percent of our brains…

Physicians and comedians alike, including Jerry Seinfeld, love to cite this one. It’s sometimes erroneously credited to Albert Einstein. But MRI scans, PET scans and other imaging studies show no dormant areas of the brain, and even viewing individual neurons or cells reveals no inactive areas, the new paper points out. Metabolic studies of how brain cells process chemicals show no nonfunctioning areas. The myth probably originated with self-improvement hucksters in the early 1900s who wanted to convince people that they had yet not reached their full potential, Carroll figures. It also doesn’t jibe with the fact that our other organs run at full tilt.

Medical Myth 8:  Fingernails and hair grow after death…

Most physicians queried on this one initially thought it was true. Upon further reflection, they realized it’s impossible. Here’s what happens: “As the body’s skin is drying out, soft tissue, especially skin, is retracting,” Vreeman said. “The nails appear much more prominent as the skin dries out. The same is true, but less obvious, with hair. As the skin is shrinking back, the hair looks more prominent or sticks up a bit.”

MedicalMyth 9:  Shaved hair grows back faster, coarser and darker…

A 1928 clinical trial compared hair growth in shaved patches to growth in non-shaved patches. The hair which replaced the shaved hair was no darker or thicker, and did not grow in faster. More recent studies have confirmed that one. Here’s the deal: When hair first comes in after being shaved, it grows with a blunt edge on top, Carroll and Vreeman explain. Over time, the blunt edge gets worn so it may seem thicker than it actually is. Hair that’s just emerging can be darker too, because it hasn’t been bleached by the sun.

MedicalMyth 10:  Chocolate and Fried Foods Give You Acne…

Some speculate that this myth dates back to the baby-boom generation, who had worse acne than their parents and also more access to chocolate and fried foods. Wherever this idea came from, it’s wrong. Pimples form when oil glands under the skin produce too much of a waxy oil called sebum, which the body uses to keep skin lubricated. But when excess sebum and dead skin cells block pores, that area of the skin gets irritated, swollen, and turns red — the telltale signs of a pimple. It is unknown why sebaceous glands produce excess sebum, but hormones are the prime suspects, which explains why teenagers are affected more than others. Stress and heredity may also be factors, but chocolate bars and onion rings are off the hook.

MedicalMyth 11:  Coffee Will Sober You Up…

If you’ve had too much to drink, no amount of coffee, soda, water or anything else is going to sober you up. The only thing that will do the trick is time. The liver can metabolize only about one standard drink (12 ounces of beer, 6 ounces of wine, or 1.5 ounces of hard liquor) per hour, so if you’re drinking more than that every 60 minutes, you’ll have alcohol in your system for some time. The idea of coffee’s sobering effect may have started because caffeine acts as a stimulant, counteracting the sedative effect of alcohol to a small degree. However, it has no effect on the amount of alcohol in the blood. So if you’ve been drinking, spend your money on a cab rather than a cappuccino.

MedicalMyth 12:  Cold Weather Can Give You a Cold…

“Put your jacket on or you’ll catch a cold!” How times have you heard that? You may not want to tell her this, but dear old Mom was wrong. Viruses (more than 200 different kinds) cause colds, not cold weather. In order for you to catch a cold, the virus must travel from a sick person’s body to yours. This usually happens via airborne droplets you inhale when an infected person coughs or sneezes. You can also get a cold virus by shaking hands with an infected person or by using something where the virus has found a temporary home, such as a phone or door handle. Colds are more prevalent during the colder months because people tend to spend more time inside, making it much easier for viruses to jump from person to person.

MedicalMyth 13:  Wait 30 Minutes After Eating Before Swimming…

For a kid, nothing ruins the fun of a carefree summer day like a worried parent banning swimming right after the big cookout, fearing that the child will get cramps and drown. There is a slight chance of minor abdominal cramping, but for the vast majority of people, this isn’t dangerous. The body does divert blood flow from the muscles to the gastrointestinal system to spur digestion, but not in amounts that diminish muscle function. Listen to your body and swim when you’re comfortable — just like you probably don’t run a marathon right after Thanksgiving dinner, you don’t want to start swimming laps right after a seven-course picnic. It’s perfectly safe, though, to eat a light meal and then get wet. After all, athletes commonly eat right before competing.

MedicalMyth 14:  You Can Get the Flu from a Flu Shot…

Vaccinations are misunderstood because they’re created from the offending viruses themselves. But when you get a flu shot, you’re not being injected with a whole virus — you’re receiving an inactivated, or dead, virus. That means the part of the virus that can infect you and make you sick is turned off, but the part of the virus that stimulates your body to create antibodies is still on. The body’s antibodies will kill the flu virus should you come into contact with it later. Even pregnant women are advised to get flu vaccinations, so you know they’re safe. The only people who should avoid them are those who have severe allergies to eggs, because eggs are used to create the vaccines. No vaccine is 100-percent effective, so there is still a chance you can get the flu after receiving the shot, but that doesn’t mean the vaccination gave it to you.

MedicalMyth 15:  Reading in dim light ruins your eyesight…

The fearful idea that reading in dim light could ruin one’seyesight probably has its origins in the physiological experienceof eye strain. Suboptimal lighting can create a sensation ofhaving difficulty in focusing. It also decreases the rate ofblinking and leads to discomfort from drying, particularly inconditions of voluntary squinting.The important counterpointis that these effects do not persist.

The majority consensus in ophthalmology, as outlined in a collectionof educational material for patients, is that reading in dimlight does not damage your eyes.  Although it can cause eyestrain with multiple temporary negative effects, it is unlikelyto cause a permanent change on the function or structure ofthe eyes.

So why do people believe these myths, and others?

Michael Shermer spells it out brilliantly, when he discusses why people believe strange things.

References:
http://www.livescience.com/health/071220-medical-myths.html 
http://listverse.com/2009/02/15/top-10-common-medical-myths/ 
http://www.bmj.com/cgi/content/full/337/dec17_2/a2769
http://www.bmj.com/cgi/content/full/335/7633/1288
http://health.howstuffworks.com/9-medical-myths1.htm