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What's the denominator here? Like water is toxic at 90g/1kg, what's the other 910g? Because I definitely drink over a litre of water a day and I'm doing fine.
If you weigh 100kg, drinking 90*100g=9kg of water produces a 50% chance of fatality. The definition of LD50 requires the dose to be given "all at once", and quite frankly, you would not be able to drink 9 liters of water all at once. LD50 becomes a lot less meaningful for anything where you would need an extreme concentration of the substance--e.g. THC is difficult to acquire in concentrations compatible with fatal overdose--or where consuming it at such quantities is simply infeasible.
People often say "consumed rapidly" but that phrasing doesn't really solve this problem with LD50 as a measure. Basically LD50 is meaningful near the bottom of this chart, less so near the top of this chart.
I'll note that another problem with LD50 is that it doesn't take into account serious harms that can occur with lower dosages. Drinking any amount of gasoline is likely to lead to serious brain damage, for example.
Everyone knows the LD50 is a binary condition! Either you live or you die! That's why I always dose just under the LD50 to make sure I never suffer any consequences.
It's per kg of body weight. So if you weigh 80kg (176lbs) then rapidly drinking 7.2L of water has a 50:50 chance to kill you - I think.
I believe it's body weight, so if you weighed 200lb (~90kg) you'd have to drink 8100g of water to possibly die and you have to drink it fast and not pee it out. There was a woman several years back that did die from this, a radio station did a contest "hold your wee for a wii"
Water is toxic without you needing to rupture your bladder. I've experienced water toxicity before, it gives you a headache and makes you disoriented.
And like you said, in what period of time?
It's complicated. Short version, over a small amount of time.
In the case of water, how it kills you is by diluting your blood, basically. Your kidneys will be working extremely hard (and quickly) to empty out the excess water, so for the most part, you've got to drink much faster than your kidneys can work.
That said, it's not just speed - other stuff gets cleaned out with your urine, like certain vitamins and such. Drinking excess water over a long time, but significantly more than what's on the chart, will drain you of certain nutrients / electrolytes, and that'll screw you, too.
Drinking sports drinks in that quantity could actually sidestep that particular problem, drastically raising the amount of water you can take in.
One way or another, though, while it's possible to hurt or kill yourself from drinking too much water, you have to bring it to some serious extremes and your body should be vehemently complaining during this process.
If ever you think you're doing something extreme and might possibly be slightly risky in this regard, just drink some electrolyte heavy stuff instead for a while - Gatorade, Powerade, etc. Then your only risk is basically outrunning your kidneys and your stomach should really be making you throw up if you try that.
You kinda mentioned that some substances bioaccumulate, but some also "biomultiply" like bacteria, viruses and prions. This plays a role in how a lethal dose can be administered to be effective.
There are known examples, most famously the Hold your wee for a Wii contestant. Mother just trying to win a gaming system for her children.
I believe it's happened either in sports or athletic events where water was used instead of something like Gatorade.