Measurement of drug use in a defined population. Evaluation of the defined daily dose (DDD) methodology.

To evaluate the accuracy of the DDD per 1000 inhabitants per unit time as an estimate of the fraction of drug users within a population, DDD figures have been compared with the proportion of apparent drug users, i.e. individuals in the population for who a prescriptions had been dispensed. An individual-based prescription monitoring project provided the necessary data for eight drugs representing continuous long-term medication, short-term medication and medication falling in between those two categories. For digoxin, the long-term drug, the number of DDD/1000 inhabitants/day was about 40% below the proportion of apparent drug users. The DDD figure for antibiotics ranged from 4% below to 28% above the apparent users, and for the remaining drugs it ranged from 17% below (bendroflumethiazide, 1982) to 80% below (naproxen, 1985). The DDD methodology is a valuable first step in overall drug use measurement, but for more precise estimates of drug use it must be supplemented by other techniques.