^^ Thanks BT. Just to add...
LD50 figures are species dependant, and what usually amounts to a human figure, has itself been derived from the species scaling model using rats, monkeys etc (this is not a linear, weight for weight scale)
Of course drugs are never fed to humans to see how many survive.
"Hey Dr Doom, yesterdays dose of LSD only killed 49 /100 people. In order to establish the true LD50 for humans, we'll need to up that dose slightly on the next trial " 
A good example of the potential inaccuracy of interspecies scaling appeared in "Science" in 1962. This involved an experiment with an elephant which went terribly wrong. The dose, 297mg was considered small by the interspecies scaling model, but following administration it was quickly apparent that something wasn't right.
What this shows, is that different species may respond differently to a drug, as it certainly appeared the case with
BABAR who was given what was
thought to be a relatively small dose for his weight
I've heard it said that around 1.0-1.2mg is where the mental expansion stops. Some have described muscle and nerve pains. I knew a guy who used to take massive amounts of acid when developing his 10 acres of land, before lifting heavy rocks by hand. He'd do 10+ hours a day, doing ~8-10 trips at a time (strong trips too) sometimes 2 x a day. He'd break only long enough for tolerance to drop. Over 3 months he seemed to achieve superhuman progress, but looked far worse for wear.
One morning he rang in some distress. For the past few days he'd been experiencing what he described as lightning bolts in his forearms; which he could feel, see and hear!! This and the impossible mental loops, eventually became too much and he bought an excavator
