Pharmacopoeial standards for preparations of drugs have largely been developed on the assumption that tests of purity of the ingredients and of the amounts present would ensure uniformity of therapeutic effect. However, differences in formulation may affect the biological availability of the ingredients and the subsequent therapeutic response in many ways.1 2 For example two batches of a preparation which chemically can be shown to contain identical amounts of a drug may produce different blood levels and even dissimilar effects,3 especially where the drug is poorly soluble in water in the pH range of body fluids. On the other hand, with some drugs even a twofold difference in blood levels is clinically unimportant. It has been stated that “until ‘equivalence’ of products has been proven, ‘non-equivalence’ of two formulations of the same drug should be assumed”.4 But there are always likely to be minor variations between different formulations of the same drug, and it would probably be difficult to achieve ‘equivalence’ or to demonstrate that it had been attained.
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