What is the name for the measure of a drug's relative safety for use, computed by the ratio of the lethal dose for 50 percent of the population over the effective dose?

It is called THERAPEUTIC INDEX. Therapeutic index is the comparison of the amount of a particular drug that causes the desired therapeutic effects and the amount that causes toxicity. The index can be low or high. A drug with a low therapeutic index will require only a small increase in dose to produce toxic effects in users. 

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