Lethal Dose, 50%. Refers to the dosage of a given drug required to kill 50% of a test population. The LD50 is usually measured in units of mass of drug per units of body mass; for example, mg/kg. This type of measurement is used to help ensure that the size of the specimen has no effect on the dosage measurement, as a large specimen can generally take a larger dose of a toxin without fatal effects. The measure is often known colloquially as the "semilethal dose;" while it may not always be lethal to a given subject, it will be lethal around half the time.
The LD50 for nicotine in rats is around 50 mg/kg, but it is only 3 mg/kg in mice. What's the LD50 for humans, you ask? Well, if you're willing to participate in the experiment to find out, go buy some cigarettes and knock yourself out (literally).
by progamer124 May 16, 2004
Sometimes used to describe the percentage of a woman's brain that is missing. The root of this comes from the abbreviation LD50 or Lethal Dose required to kill half of a population. In this case the LD stands for Lacking Discernment. Depending on the amount of brain missing, the number 50 can be substituted with any percentage greater than 50. Can be used in casual bar conversations so that no attention is drawn by the woman of topic.
Hey Ben, that girl is definitely LD50. She's so fucking stupid that I've caught her cheating on me 3 separate times and she still denies it!
by Bojangles0928 January 18, 2010