system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do scientists actually calculate the lethality of poisons?

For example, I read that 5g of cyanide is enough to kill 35 people. What does that even mean? If I could somehow split 5g of solid cyanide (is that a thing?) into 35 parts and put it in 35 peoples’ Dr. Pepper they would all die? And how could you possibly test that?
gpt
Most drugs have what's called an LD50 - it's the estimated dose required that will kill half of all people who consume it.

There is an LD100 - the amount needed to kill, on average, every single person. But this is used less often because it's often a much, much higher dose than you'd need to kill most people. For example, an LD50 might be 0.5g, while an LD100 might be 7.0g

This can be calculated using animal models - often mice or pigs (or both) due to their similar biology to humans, or through previous observations of people who have accidentally died to such a poison - as well as those who consumed the poison, but lived.

Generally speaking if 5g is enough to kill 10 people, say It means 0.5g is probably a little higher than the LD50 for that drug for an average person.

So you might not actually kill 10 people, it's more likely to be around 5, but it gives you a good scope of how toxic that compound actually is.

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#Extra Info

LD50 is usually based on body weight, eg g/kg. So when we talk about a certain amount of drug killing a certain number of people, we presume people of average weight and fitness.

But even then, you can get weird results using LD50 and not accounting for all the other factors.

A pellet of rat poison (let's say 10mg of  brodifacoum) in a healthy, well built, adult male's (100kg, great liver, good metabolism, no health conditions) meal is probably going to be no problem at all. Maybe he gets a few unexplained bruises for a couple of weeks, or his gums bleed when he brushes his teeth. If he's **very** unlucky, he may end up with a GI bleed, spend a night in hospital and receive a vitamin K injection.

That same pellet in the meal of an elderly man (60kg, reduced liver function, slow metabolism, pulmonary embolism last year) who is also taking blood thinners... Well that could easily be a death sentence.

Now the LD50 for brodifacoum is about 0.3mg/kg so on paper, both of our patients should survive (10mg would be the LD50 for a 30kg person). But that's where the other health factors and so on play a role.

It gets very complicated very quickly.