The protective index is similar to the [[therapeutic index]], but concerns toxicity ([[TD50|TD<sub>50</sub>]]) rather than lethality ([[{{LD50|LD<sub>50</sub>]]}}); thus, the protective index is a smaller ratio. Toxicity can take many forms, as drugs typically have multiple side effects of varying severity, so a specific criterion of toxicity must be specified for the protective index to be meaningful. Ideally a choice is made such that the harm caused by the toxicity just outweighs the benefit of the drug's effect. Thus, the protective index is a more accurate measure of the benefit-to-risk ratio than the therapeutic index, but is less objectively defined. Nevertheless, the therapeutic index can be viewed as an [[upper bound]] to the protective index for a given substance.
