The Therapeutic window (or pharmaceutical window) of a drug is the range of drug dosages which can treat disease effectively while staying within the safety range. In other words, it is the dosages of a medication between the amount that gives an effect (effective dose) and the amount that gives more adverse effects than desired effects. For instance, medication with a small pharmaceutical window such as Tegretol must be administered with care and control, e.g. by frequently measuring blood concentration of the drug, since it easily gives adverse effects such as agranulocytosis.
More specifically, it is the range between the ED50 and the starting point of TD50 curve. It is believed that this index can help to avoid most of the potential side effects. It is worth noting that the window can vary by situation. In some cases, the therapeutic window can even be completely "closed", meaning that the adverse effects exceed the desired effects at all doses capable of providing the desired effect. This is the (politicized) assertion made by governments that prohibit certain drugs: that at any dose, they do more harm than good.
This index is believed to be more reliable than either the therapeutic index or the protective index, since this index considers the biological variation among individuals to a larger extent. The downside, however, is that it (like the protective index) also introduces an element of subjectivity.
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