Answer: The LD50 stands for “Lethal Dose for 50% of subjects” or “Median Lethal dose”.
In pharmacological studies of drug compounds, it is important to know the dosage of drug that can kill the subject. Too much of any substance can have consequences, many of them (even water) can be lethal. It is therefore important to know what that concentration is where a substance loses its positive benefits and starts harming the subject.
The LD50 is generally measure in grams of substance per grams of body mass. Common units include milligrams per kilogram, but some substances may be so acutely toxic, that their LD50 is reported as nanograms per kilogram body mass. A substance with are low LD50 is more toxic, since it takes a lower concentration of substance to produce the same lethality.
An LD1 may also be a useful measure, which is the concentration at which 1% of subjects are killed. Sometimes LD50/30 is reported, which refers to the LD50 dosage if the subject is untreated over the next 30 days.
The LD50 is a measurement of acute toxicity rather than a measure of chronic toxicity. A subject exposed to a substance at a dosage well below the LD50 may experience adverse side effects after long term exposure. Staying at doses lower than the LD50 does not guarantee that there are no side effects.
The LD50 can also be measured for radiation exposure.