Therapeutic Index

The therapeutic index compares how much drug is needed to produce a desired effect with how much is needed to cause harm. It works as a simple safety ratio: a larger value usually means there is a wider gap between effective and toxic doses, so the drug is easier to use safely. Because both numbers come from population data (the 50% effective dose and the 50% toxic dose), it gives a rough measure of how forgiving a medicine is.

Use this when comparing the safety of medications or assessing whether a dose has a narrow or wide safety margin. It is especially useful in pharmacology, toxicology, and drug development when deciding how carefully a medicine must be monitored.

• Reversing the ratio and dividing ED₅₀ by TD₅₀
• Using a single patient's dose instead of the 50% population dose values
• Assuming a high therapeutic index guarantees a drug is completely safe