Some claim drinking eight glasses of water a day leads to good health, while reading in dim light damages eyesight.
Others believe we only use 10% of our brains or that shaving legs causes hair to grow back thicker.
But a review of evidence by US researchers surrounding seven commonly-hold beliefs suggests they are actually "medical myths".
Some are utterly untrue, while others have no evidential proof, the British Medical Journal reports.
Researchers from the Indiana University School of Medicine in Indianapolis hunted medical literature for evidence on each claim.
They found no evidence supporting the need to drink eight glasses of water a day.
* Medical myths *
In fact, studies suggest that adequate fluid intake is often met by drinking juice, milk, and even caffeine-rich tea and coffee.
Data also suggests drinking excessive amounts of water can be dangerous.
The belief that we only use 10% of our brains appears to be completely untrue.
Studies of patients with brain damage suggest that damage to almost any area of the brain has specific and lasting effects on mental, vegetative and behavioural capabilities.
Absence of evidence does not necessarily mean absence of effect
Dr David Tovey
Editor of Clinical Evidence journal
Brain imaging studies also show that no area of the brain is completely silent or inactive.
And the belief that hair and fingernails continue to grow after death may be an optical illusion caused by retraction of the skin after death.
The actual growth of hair and nails requires a complex interplay of hormonal regulation not present after death.
Again, illusion may be to blame for the belief that shaving hair causes it to grow back faster, darker, and coarser, report author Rachel Vreeman told the BMJ.
The stubble resulting from shaving grows out without the finer taper seen at the ends of unshaven hair, giving the impression of thickness and coarseness.
Again, expert opinion is that reading in dim light does not damage your eyes. And there is little evidence to support the banning mobile phones from hospitals on the basis of electromagnetic interference.
Finally, eating turkey - and the tryptophan amino acid it contains - does not make people especially drowsy.
Indeed, turkey, chicken and minced beef contain similar amounts of tryptophan.
THE SEVEN MEDICAL BELIEFS
- Drink at least eight glasses of water a day
- We use only 10% of our brains
- Hair and fingernails continue to grow after death
- Reading in dim light ruins your eyesight
- Shaving causes hair to grow back faster or coarser
- Mobile phones are dangerous in hospitals
- Eating turkey makes people especially drowsy
The researchers explained: "Any large meal can induce sleepiness because blood flow and oxygenation to the brain decrease, and meals rich in protein or carbohydrate may cause drowsiness. Wine may also play a role."
Dr David Tovey, editor of Clinical Evidence journal, said: "The difficulty is it is often hard to disprove a theory.
"On the flip-side, absence of evidence does not necessarily mean absence of effect.
"Where reliable evidence becomes really important is in helping people make serious decisions about harms and risks.
"Many of these 'myths' are innocuous. However, we are still finding evidence that runs contrary to current practice and what we expect."
He gave the example of the relatively recent U-turn in advice over sleeping positions for babies to cut cot deaths.
Experts now recommend babies are positioned on their backs when sleeping to reduce the risk of sudden infant death.
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