The therapeutic index of a drug defines the margins of safety or benefit/risk ratio. Therapeutic index (TI) can be defined as:
TI = LD50/ED50
- LD50 is 50%-lethal dose
- ED50 is 50% effective dose
In both graphs the dose response curves for therapeutic effectivity of a drug (green line) and toxicity (red line) are displayed. The higher the dose, the more effective a drug becomes, but also the more adverse effects and toxicity will become apparent.
Drug A can be dosed at maximal effective response without giving any toxicity. However, drug B, when dosed just above half maximum effective response already starts to give signs of toxicity. Dosed at maximal effective response, drug B causes almost half maximal toxicity. So, it can be concluded that drag A is much more desirable for the patient than drug B.
More information about therapeutic drug concentrations is to be found in the pharmacokinetic section.