There are many circumstances in which a fixed drug dose is likely to be ineffective or toxic in a significant number of individuals, e.g. cytotoxic chemotherapy, aminoglycoside antibiotics. It is usual then to calculate the dose according to body weight. Adjustment according to body surface area is also used and may be more appropriate, for this correlates better with many physiological phenomena, e.g. metabolic rate. The relationship between body surface area and weight is curvilinear but a reasonable approximation is that a 70 kg human has a body surface area of 1.8 m2. A combination of body weight and height gives a more precise value for surface area (which can be obtained from standard nomograms) and there are several more sophisticated methods.24
The issue takes on special significance in the case of children if only the adult drug dose is known; adjustment is then commonly made on the basis of body weight, or body surface area, among other factors (see p. 126).
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