The therapeutic index indicates the relative margin of safety of a drug. A dose of the drug that produces the stated effect in 50% of individuals within the population is called the ‘median dose’. Depending on the stated effect it can be designated as ‘median effective dose’ (ED50) and median lethal dose (LD50).
Therapeutic Index (TI) = ED50/LD50
- The TI indicates how close the effectiveness does to the lethal dose for 50% of the test population. Thus, it gives an idea about the margin of safety.
- As the ED50 approaches the LD50, the danger of drug toxicity increases significantly. Therefore, a drug with a larger therapeutic index is safer than one with a smaller therapeutic index. Hence, drugs with a lesser therapeutic index should be administered cautiously.