LD50

For example, the LD50 of LSD is 46 mg/kg intravenously in mice -- compare to an active dosage of .001 mg/kg (1 microgram/kg) in humans.

There is a wonderful book called Molecules at an Exhibition in which the author does interesting profiles of compounds that appear in everyday things or situations, as well as unusual compounds like sarin, an extremely potent neurotoxin. As I was reading it, I noticed that he would mention the LD50 of almost every one, in sort of a morbid kind of way. I mentioned it to my cousin, who is doing a PhD in biophysics, and he said that it's one of the first things that chemists tend to ask about a compound, as in: "Cool, what's the LD50?"

OBTW, a lethal dose of caffeine is about 10 grams. That's around 100-200 cups of coffee, depending on your body mass, and how strong you like your java.

LD₅₀ (oral, rat) - 5 mg/kg
LD₅₀ (dermal, rabbit) - 10 g/kg.

An LD50 test is performed by administering gradually increasing dosages to successive test populations over a set time frame, usually 14 days, until 50% of the subjects die, after which surviving animals are killed.

The LC50 test is another measure of acute toxicity, but of concentrations in air or water rather than an administered dosage, and is expressed in parts per million (ppm) or milligrams per cubic meter, and the test animal, and the duration of exposure:

LC₅₀ (rat) - 500 ppm/4 hr
LC₅₀ (rabbit) - 5 mg/m³/2 hr.

Nevertheless, use of the LD50 test is widespread not only as a measure of chemical toxicity, but to:

For example, the LD 50 value of DDT for rats, injected orally, is 87. So, a dosage of 87 miligrams of DDT for every kilogram the rat weighs has a 50% chance of killing the rat.

Some facts:

1. The LD 50 does not scale exactly, either from mice to humans or from 50% to 100%. People vary. People take six times the "lethal dose" (not the LD 50, but a higher dose that in a treating physician's opinion usually causes death) of something and manage somehow to survive because bodies differ.
2. Human-specific LD 50 data is not available because the FDA won't let drug companies take a pool of 1200 or so humans and feed them a drug until 600 are dead.
3. Because a single test may kill as many as 1000 animals, the United States and other members of the Organisation for Economic Cooperation and Development agreed in December 2000 to phase out the LD 50 test in favor of alternatives that greatly reduce (or even eliminate) deaths of the test animals.
4. Even adjusting for the test animal's weight, the LD50 for one species is often quite different from that for another. Thus any LD 50 value gives only a rough estimate of the risk to humans.
5. The way in which the chemical is administered also has a marked effect on LD 50 values. The chemical may be fed, injected, applied to the animal's skin, etc., and each method usually generates a different LD 50.

So, even though the LD 50 dosage for rats of a given substance is not a dead-sure (pun intended) way to get yourself killed, never be exposed to an LD 50 dose of a hazardous chemical. Even if you survive you're not going to be in good shape.

Oh yeah, L.D. 50 is also the snazzy title of Mudvaynes debut album. It ties in neatly with the cover art and the the general theme of the album, which involves such things as the evolution of man and drugs, among other things

A few LD₅₀ (Lethal dose for 50% of the population studied) values: