Several factors are known to operate in determining a particular individual’s ability to metabolise drugs. As a result, patients vary greatly in their drug-metabolising capacity. The present practice of prescribing fixed doses to such a population with its very fast and very slow metabolisers must contribute to the incidence of iatrogenic disease. It is possible and desirable mthat in future, simple techniques be devised to assess an individual’s drug-metabolising capacity, thus permitting ‘tailor-made’