a few years ago i drank massive amounts of water Erev Yom Kippur (maybe 8 liters)
i had a very severe headache the next morning. turns out it was 'hyponatremia" a dilution of the concentration of sodium (and other critical atoms) in the bloodstream.
worst headache i ever had
turns out hyonatremia can be FATAL, lo aleynu, even if caused (it has many causes) by drinking too much water.
i once read a case report in the medical literature of a man who drank water all day long, for a few days, because it helped his toothache. he died from hyponatremia
this is not an urban legend