The terms potency and efficacy are often used imprecisely and therefore, confusingly. It is pertinent to make a clear distinction between them, particularly in relation to claims made for usefulness in therapeutics.
Potency is the amount (weight) of drug in relation to its effect, e.g. if weight-for-weight drug A has a greater effect than drug B, then drug A is more potent than drug B, although the maximum therapeutic effect obtainable may be similar with both drugs. The diuretic effect of bumetanide 1 mg is equivalent to frusemide 50 mg, thus bumetanide is more potent than frusemide but both drugs achieve about the same maximum effect. The difference in weight of drug that has to be administered is of no clinical significance unless it is great.
Pharmacological efficacy refers to the strength of response induced by occupancy of a receptor by an agonist (intrinsic activity); it is a specialised pharmacological concept. But clinicians are concerned with therapeutic efficacy, as follows.
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