For many drugs toxicity limits the maximum dose of a drug. Therapeutic Index (TI) is a comparison of the amount of drug that causes a therapeutic effect to the amount that causes toxicity. Classically TI refers to the ratio of the dose of drug that causes adverse effects (e.g. toxic dose in 50% of subjects or TD50) to the dose that leads to the desired pharmacological effect (e.g. efficacious dose in 50% of subjects, ED50). A higher therapeutic index 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 therapeutic effect. The related term Therapeutic Window refers to a range of doses which optimize between efficacy and toxicity, achieving the greatest therapeutic benefit without resulting in unacceptable side-effects or toxicity. In general, it is the exposure of a given tissue to drug (i.e. drug concentration over time), rather than dose, that drives the pharmacological and toxicological effects.