Dear Pandey Govind, I send you some articles on the topic

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LD50 is the dose which is lethal (causing death) to 50% of tested group of animals in a given population. LD50 means the 'median lethal dose'.

‘Median lethal dose’ (LD50) is the dose which is lethal to 50% of a test sample. It is an estimator of ‘lethality’ and the most common expression used to rate the potency of ‘toxicants’. Other terms used for prediction of illness or ‘lethality’ are- ‘no observed effect level’ (NOEL), ‘maximum nontoxic dose’ (MNTD) and ‘maximum tolerated dose’ or ‘minimum toxic dose’ (MTD). ‘Therapeutic index’ or ‘therapeutic ratio’ is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount which causes ‘death’ (in animal studies) or ‘toxicity’ (in human studies). In other words, the ‘therapeutic index’ is the ratio given by the ‘lethal dose or ‘toxic dose’ divided by the ‘therapeutic dose’. In animal studies, the ‘therapeutic index’ is the lethal dose of a drug for 50% of the population (LD50) divided by the minimum effective dose for 50% of the population (ED50). ‘Lethality’ is not determined in human clinical trials; instead, the dose which produces a toxicity in 50% of the population (TD50), is used to calculate the ‘therapeutic index’. While the ‘lethal dose’ is important to determine in animal studies, there are usually severe toxicities which occur at ‘sublethal dose’ in humans, and these toxicities often limit the maximum dose of a drug. A higher ‘therapeutic index’ is preferable to a lower one: a patient would have to take a much higher dose of such a drug to reach the lethal/toxic threshold than the dose taken to elicit the therapeutic effect.