A drug with larger therapeutic index is safer than one with smaller therapeutic index.

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.