system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are lethal doses for humans determined?
gpt
Lethal dosage is calculated in terms of the LD50. This is the dose that statistically should kill half of the test subjects it's administered to based on body weight. 

Since we obviously can't test lethality of something on humans, we use laboratory animals like mice whose biology is well known and can be compared to ours.

Say you take a population of ten mice whose body weight is each 1kg. You find out after repeated trials that it takes a dose of 100mg to kill an average of five mice each time. This means the LD50 of the poison is 100mg/kg. 

Scaling that up, we can say it would statistically take a 10,000mg dose to kill half of a population of 100kg humans. That would be about ten grams of poison, which isn't terribly toxic. One of the most potent toxins on earth- botulinum- has an LD50 in the nanogram (1 billionth of a gram) range.