Defined daily dose

The DDD is the assumed average daily maintenance dose for a drug for its main indication in adults.

Expressed as DDDs per 1000 inhabitants per day, for chronically used drugs

It can be interpreted as the proportion of the population that may receive treatment with a particular medicine on any given day.

For use in hospital settings, the unit is expressed as DDDs per 100 bed-days (adjusted for occupancy rate); it suggests the proportion of inpatients that may receive a DDD.

For medicines that are used for short-term periods, such as antimicrobials, the unit is expressed as DDDs per inhabitant per year; this provides an estimate of the number of days for which each person is treated with a particular medication in a year.

The defined daily dose (DDD) methodology was developed in response to the need to convert and standardize readily available volume data from sales statistics or pharmacy inventory data into medically meaningful units, to make crude estimates of the number of persons exposed to a particular medicine or class of medicines.

The DDD methodology is useful for working with readily available gross drug statistics and is relatively easy and inexpensive to use. However, the DDD methodology should

be used and interpreted with caution. The DDD is not a recommended or a prescribed dose, but a technical unit of comparison; it is usually the result of literature review and available information on use in various countries.

Prescribed daily dose

The prescribed daily dose (PDD) is another unit, developed as a means to validate the DDDs. The PDD is the average daily dose prescribed, as obtained from a representative sample of prescriptions.

Not useful to estimate incidence and prevalence of drug use or to quantify or identify patients who receive doses lower or higher than those considered effective and safe.