The effects of extremes of age on drug action.

Many examples of age-related differences in the response of individuals to a particular drug have been documented. In many cases these differences can be related to modified distribution, metabolism or excretion of the drug. There is evidence that in neonates and the elderly, the extent and rate of these processes are less than those in older children and young adults. However, in a given age group there is no consistent trend for all drugs. In some cases the increased plasma concentration of the drug would be regarded as an advantage as it prolongs the response, but it is certainly not true for drugs such as streptomycin, warfarin and the hypoglycemic agents. Two points which emerge from the results of many studies with neonates and the elderly are that interindividual variations in plasma half-lives, metabolite and drug excretion and protein binding are significantly larger than those in young adults and that in the case of the elderly, chronological age does not necessarily reflect biological age. In combination these factors emphasise the importance of individual drug monitoring of plasma levels and hence of knowing the pharmacokinetic constants of the drug in the individual patient. In clinical practice, the dose of a drug is most commonly calculated on the basis of body weight or surface area. This is generally perfectly acceptable for older children and adults, but is not always a reliable formula for neonates and the very old. Simple changes in such parameters as urine pH can override calculations based upon body size.(ABSTRACT TRUNCATED AT 250 WORDS)