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Phase III trials will generally have included selected groups of patients based on strict inclusion and exclusion criteria that will form the basis of its licenced indications and the product can only be marketed on this basis. For this reason, further studies will often be performed to either widen the licence indications to broader groups of patients such as the elderly (Phase IIIb) or evaluate drug safety and efficacy in a real‐world setting, over the longer term or against comparators, often involving many thousands of patients (Phase IV). It is important to emphasise that even though a drug may obtain a licence, it does not necessarily mean that there will be widespread use in patients, as many countries also mandate a health economic evaluation to ascertain its cost‐effectiveness (see Chapter 3).