How is the Therapeutic Index calculated?

• A. TD50/ED50
• B. ED50/TD50
• C. TD50/LD50
• D. LD50/TD50

Answer: (A)

The Therapeutic Index reflects the safety margin between what dose produces a therapeutic effect and what dose causes toxicity. It is calculated as the dose that causes toxicity in 50% of individuals (TD50) divided by the dose that produces the desired therapeutic effect in 50% of individuals (ED50). A larger ratio means a wider safety margin, so the drug can be used more safely relative to its efficacy. For example, if TD50 is 200 mg and ED50 is 50 mg, the TI is 4, indicating a reasonable safety buffer. If the TI is small, the margin between effective and toxic doses is narrow, requiring careful dosing and monitoring. The other forms (like ED50/TD50 or TD50/LD50) reverse the relationship or replace toxicity with lethality, which changes the meaning and isn’t the standard clinical definition. In many animal studies, TI is expressed as LD50/ED50, but in clinical contexts the TD50/ED50 definition is used.