What is the therapeutic index of drugs?

The 50% effective dose (ED50) refers to the dose that can cause 50% of the maximum reaction intensity in quantitative reaction and 50% of the subjects to have a positive reaction in qualitative reaction. By analogy, if the effect is convulsion or death, it is called 50% convulsion or 50% lethal dose (LD50). The smaller the ED50, the greater the LD50, indicating that the drug is safer. Generally, the ratio of LD50 to ED50 of drugs is called therapeutic index (TI) to indicate the safety of drugs. However, if the dose-effect curve of a drug is not parallel to its dose-toxicity curve, the TI value cannot fully reflect the safety of the drug, so some people use the values of LD5 and ED95 or the distance between LD 1 and ED99 to indicate the safety of the drug.

There are two indicators of drug safety: therapeutic index and safe range (relatively reliable). It is better to evaluate the safety of drugs by combining safety index (SI) and safety margin. Safety index = ld5/ed95；; Safety limit = (LD1-ed99)/ed99×100%. Clinically, the safety of drugs is sometimes expressed by the safety margin, which refers to the distance between the minimum effective dose and the toxic dose. In order to ensure the safety of clinical medication, the treatment index and safety range must be considered comprehensively.