The fact that water can cause fatal overdoses is not well known. Often called "water intoxication" or "hyponatremia" (low salt)1, water poisoning results when too much water is ingested in too short a time without replenishing electrolytes, particularly sodium and potassium.

The human body (as with other animals) maintains an equilibrium of electrolytes in the bloodstream as part of its normal operation. Sweating causes the body to lose salt, but the primary cause of hyponatremia is drinking enormous amounts of water, which dilutes the salt in the body to a dangerous level. As the electrolyte levels get dangerously out of balance, body systems begin to fail and the sufferer exhibits signs of crisis such as diarrhea, over-salivation, stupor, vomiting, muscle tremors, confusion, frequent urination and other general symptoms of illness, and their brain begins to swell. This swelling is called a 'brain edema' or 'cerebral edema' and can lead to brain damage, paralysis, and sometimes death.

Water poisoning deaths are a real (although small) problem with recreational MDMA use; for more information about this see the MDMA Health & Water Issues page. There are occasional deaths in the US and Canada from people accidentally drinking too much water, and some from intentional or forced overconsumption, such as the case of a young man required to drink two or more gallons of water as part of a college fraternity hazing ritual.

How susceptible one is to the dangers of hyponatremia seems to be partially dependent on body weight, how much food is in the system, and other factors not yet understood which make up individual variation in response. The body has a system for regulating electrolyte balance and it eliminates excess fluids through urination. Some people's urine regulation systems (partially controlled by Anti-Diuretic Hormone [ADH], also called vasopressin) do not respond as quickly in some circumstances.

Health professionals recommend taking electrolytes dissolved in liquid or eating salty snacks when drinking large amounts of water to make sure that a proper balance is being maintained. For instance, sometimes when people are trying to avoid an alcohol hangover, they may drink (or have their friends force them to drink) more water than they would normally drink. In such cases, salty crackers, chips, or some other kind of salt source can help the body absorb the water and eliminate it properly without risking further throwing the electrolytes out of balance.