Variables
Description
What is this formula?
The Therapeutic Index (TI) is a classical pharmacological measure of drug safety that compares the median lethal dose to the median effective dose.
When to use it
Use this formula when comparing the relative safety of drugs, toxins, pesticides, or other biologically active compounds.
Example
A drug has a median lethal dose of 500 mg/kg and a median effective dose of 25 mg/kg.
Data:
LD50 = 500 mg/kg
ED50 = 25 mg/kg
Formula:
TI = LD50 / ED50
Substitution:
TI = 500 / 25
Result:
TI = 20
Interpretation:
The lethal dose is 20 times greater than the effective dose, indicating a relatively wide therapeutic margin.
Applications
- Drug development
- Toxicological evaluation
- Pharmacology education
- Comparative drug safety analysis
- Preclinical studies
Note
The Therapeutic Index is a historical and widely used measure of drug safety, but it is a simplification. Modern clinical pharmacology often prefers measures based on therapeutic windows, exposure-response relationships, or population risk assessments. Different definitions may exist depending on whether LD50, TD50, or other endpoints are used.