What is the therapeutic index?

  • The therapeutic index is the ratio of the dose that produced toxicity to the dose that produces a clinically desired effect:

TI = \frac{TD_{50}}{ED_{50}}

  • It is a statement of the relative safety of a drug – the larger the TI, the safer the drug
  • In animal models, the LD50 may be used instead of the TD50 when calculating the therapeutic index