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How are LD50s in Humans Determined?
Q: I'd like more information on how to interpret Lethal Doses, or LD50's. I know that they are presented by the amount of a substance in mg, per the weight of the subject in kg; so mg/kg. However, I am wondering how you might determine the LD50 for human's...since rats and other such testing animals are much lighter.

Let's take caffeine for example. It's LD50 is "75mg/kg, or approximately 4500mgs". What would the lethal dose of caffeine be for human subjects? Thanks for the time as this will help me with both my math and criminology classes.

A: The numbers we give (75 mg/kg) are generally based on LD50's for mice or rats. These are traditionally measured in mg/kg. What this means (with your example) is that if you gave 100 rats (each weighing 1 kg) 75mg of caffeine, 50% of them would die. Using linear sca1ing, this would mean that the LD50 for a 2 kg rat would be 150mg. The LD50 for a 10 kg dog would be 750 mg, and the LD50 for a 60 kg person (130 lb) would be approximately 4500 mg (75mg x 60kg).

So, the 4500 mg is intended as an approximation of the LD50 of Caffeine in humans. At this point we could get into a long discussion of whether direct linear scaling is an appropriate method of approximating human LD50s. There is reason to believe that, due to differences in metabolism rates and various other factors, human LD50s may be somewhat lower than direct linear scaling would suggest. It has been suggested that multiplying direct scaling results by .7 might provide a more appropriate approximation. This would bring the 4500 mg down to 3150 mg as a potentially lethal dose for a 130 lb individual.

Again, it is important to know that this isn't necessarily a lethal dose, but rather is an approximation of the dose at which 50% of the test subjects would die.

 Asked By : chem major Answered By : fire Published Date : 3 / 23 / 2002 Last Edited Date : 3 / 23 / 2002 Question ID : 2954

Categories: [ Dosage ] [ Caffeine ]

Ask Erowid v1.7 - Jul, 2005