Should we drink at least eight glasses of water a day? Does shaving hair cause it to grow back faster or coarser? Does reading in dim light ruin your eyesight?

These are just some of the common medical myths that are unproven or untrue, according to a study in this week’s Christmas issue of the British Medical Journal.

Researchers in the United States selected seven medical beliefs, espoused by both physicians and members of the general public, for critical review. They then searched for evidence to support or refute each of these claims.

The quality of evidence was taken into account and instances in which no evidence supported the claim were noted.

The results show that all of these medical beliefs range from unproven to untrue. For example, they found no evidence supporting the need to drink eight glasses of water a day. In fact, studies suggest that adequate fluid intake is often met by the consumption of juice, milk, and even caffeinated beverages. Clinical evidence also points to the dangers of drinking excessive amounts of water.

The belief that we only use ten percent of our brains is refuted by studies of patients with brain damage, which suggest that damage to almost any area of the brain has specific and lasting effects on mental, vegetative, and behavioural capabilities, say the authors. Brain imaging studies also show that no area of the brain is completely silent or inactive.

The belief that hair and fingernails continue to grow after death may be an optical illusion caused by retraction of the skin after death, they add. 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, they say. 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.

Finally, expert opinion is that reading in dim light does not damage your eyes, and there is little evidence to support beliefs such as banning mobile phones from hospitals on the basis of electromagnetic interference.

Despite their popularity, all of these medical beliefs range from unproven to untrue, say the authors. They suggest that physicians should constantly evaluate the validity of their knowledge.