One of the most famous sayings in toxicology is this: The dose makes the poison.
In other words, a milligram of the notorious poison arsenic is unlikely to kill you. Make that 200 milligrams and a cemetery plot awaits. Seems obvious, right? But, what if we're talking about something that is not a poison, a benign substance--say, a drink of cold water? Let's make it really pure water, free of arsenic and any other toxic substances.
If the size of the drink increases enough, for example, if instead of drinking a standard 8-ounce glass of water, you gulp down 160 ounces, then can we call water a poison? Wonderful, healthy water? Absolutely yes.
A year ago, a jury awarded more than $16 million to the family of a young woman who died of water poisoning after entering a Sacramento radio station contest called "Hold your wee for a Wii." The 28-year-old contestant had been trying to win the game console for her children. She died at home several hours after coming in second in the contest. The jury noted that even callers to the radio show had warned against the stunt.
What happens in a water overdose? The kidneys are overwhelmed by the tidal wave of incoming fluid and cannot cycle the excess away. The cells in the body begin to bloat with excess fluid. Electrolyte balances are disrupted, meaning that normal chemical metabolism falters. Tissues start to swell, most disastrously in the brain. The radio show contest even complained during the show of a blinding headache. A coma follows and, if the imbalances cannot be corrected, then comes death.
So the dose does make the poison rather dramatically in the case of water, changing it from a safe and healthy substance to a lethal one. With a classic poison like arsenic, the point is a little different. Put simply, a person is more likely to survive a low dose than a high one.
I recently read a piece by a physician urging athletes to be careful not to drink too much water when they're rehydrating. He titled his piece "The Balancing Act." But, it's really just another way of saying that the dose makes the poison.