The<unk>  (TI; also referred to as<unk>  ratio) is a quantitative measurement of the relative safety of a drug. It is a comparison of the amount of a<unk>  agent that causes the<unk>  effect to the amount that causes toxicity. The related terms<unk>  window or safety window refer to a range of doses which optimize between efficacy and toxicity, achieving the greatest<unk>  benefit without resulting in unacceptable side-effects or toxicity. Classically, in an established clinical indication setting of an approved drug, TI refers to the ratio of the dose of drug that causes adverse effects at an incidence/severity not compatible with the targeted indication (e.g. toxic dose in 50% of subjects, TD) to the dose that leads to the desired pharmacological effect (e.g. efficacious dose in 50% of subjects, ED). In contrast, in a drug development setting TI is calculated based on plasma exposure levels. In the early days of pharmaceutical toxicology, TI was frequently determined in animals as lethal dose of a drug for 50% of the population (LD50) divided by the minimum effective dose for 50% of the population (ED50). Today, more sophisticated toxicity endpoints are used. : in animal studies, or for humans, For many drugs, there are severe toxicities that occur at sublethal doses in humans, and these toxicities often limit the maximum dose of a drug. A higher<unk>  is preferable to a lower one: a patient would have to take a much higher dose of such a drug to reach the toxic threshold than the dose taken to elicit the<unk>  effect. Generally, a drug or other<unk>  agent with a narrow<unk>  range (i.e. having little difference between toxic and<unk>  doses) may have its dosage adjusted according to measurements of the actual blood levels achieved in the person taking it. This may be achieved through<unk>  drug monitoring (TDM) protocols. TDM is recommended for use in the treatment of psychiatric disorders with lithium due to its narrow<unk>  range. Term Meaning ED Effective Dose TD Toxic Dose LD Lethal Dose TI<unk>  TR<unk>  Ratio
Therapeutic index
Dose area product
Relative biological effectiveness
Form constant
Equivalent dose
Effective dose 
Threshold dose
Partial volume 