The defined daily dose (DDD) is the assumed average maintenance dose per day for a drug used in adults, assigned only to medicines with ATC codes by the WHO collaborating center in Oslo. DDDs do not necessarily correspond to the recommended prescribed daily dose (PDD) and are primarily used for drug utilization research to assess consumption patterns and trends. The WHO endorses the ATC/DDDs methodology for global use, providing a standard for drug utilization studies to enhance rational drug use, especially in middle and low-income countries.

1. DEFINED DAILY DOSE
(DDD)

2. DEFINITION
DEFINED DAILY DOSE (DDD):
The assumed average maintenance dose per day for a
drug used for its main indication in adults.
DDDs area only assigned for medicines given an ATC codes.
The DDDs are allocated to drugs by the WHO Collaborating
Centre in Oslo, working in close association with the WHO
International Working Group on Drug Statistics Methodology.

3. Only one DDD is assigned per ATC code and route of
administration (e.g. oral formulation). The DDD is sometimes a
dose that is rarely or never prescribed because it is an average of
two or more commonly used doses.
DDDs are not established for all medicines with an ATC code.
Major drug groups without DDDs are topical products (most
products in ATC group D), sera (ATC group J06), vaccines
(ATC group J07), antineoplastic agents (ATC group L01),
general and local anesthetics (ATC group N01),
ophthalmologicals and otologicals (most products in ATC
group S), allergen extracts (ATC group V01) and contrast
media (ATC group V08).

4. The DDD is a unit of measurement and does not necessarily
correspond to the recommended or Prescribed Daily Dose
(PDD).
Therapeutic doses for individual patients and patient groups
will often differ from the DDD as they will be based on
individual characteristics such as age, weight, ethnic differences,
type and severity of disease, and pharmacokinetic
considerations.

5. Drug utilization data presented in DDDs give a rough estimate of
consumption and not an exact picture of actual use.
 DDDs provide a fixed unit of measurement independent of price,
currencies, package size and strength enabling the researcher to
assess trends in drug utilization and to perform comparisons between
population groups.

6. By applying DDD it is possible to:
Examine changes in drug utilization over time
Make international comparisons
Evaluate the effect of an intervention on drug use
Document the relative therapy intensity with various groups of
drugs
Follow the changes in the use of a class of drugs
Evaluate regulatory effects and effects of interventions on
prescribing patterns.

7. DDD for Children
•DDDs are normally assigned based on use in adults.
•For medical products approved for use in children, the dose
recommendations will differ based on age and body weight.
• Many medical products used in children are not approved by
regulatory agencies for such use, and the usual documentation
used by the WHO Collaborating Centre regarding dose regimens
is not available.
•Paediatric DDDs are challenging to assign and problems related
to Drug Utilization Research (DUR) in children cannot be solved
by such means.

8. DDD Changes
DDDs sometimes need to be reviewed because dosages may
change over time, e.g. due to the introduction of new main
indications or new research making it necessary to change the
DDD.

9. Prescribed Daily Dose (PDD)
It is important to underline that the DDD is a technical unit (fixed
unit of measurement) and does not necessarily correspond to the
recommended or prescribed daily dose (PDD).
•The PDD can be determined from studies of prescriptions,
medical or pharmacy records, and it is important to relate the PDD
to the diagnosis on which the drug is used.
The prescribed daily dose (PDD) is defined as the average dose
prescribed according to a representative sample of
prescriptions.

10.  The PDD will give the average daily amount of a drug that is
actually prescribed. When there is a substantial discrepancy
between the PDD and the DDD, it is important to take this into
consideration when evaluating and interpreting drug utilization
figures.
For drugs where the recommended dosage differs for
different indications (e.g. antipsychotics) it is important that
diagnosis is linked to the prescribed daily dose given.

11. Pharmacoepidemiological information (e.g. sex, age and
mono/combined therapy) is also important in order to interpret a
PDD.
The PDD can vary according to both the illness treated and
national policies and practices.
For example, the PDDs of anti-infectives may vary according to
the severity of the infection.
There are also international differences between PDDs, which
can be up to four or five fold higher/lower. PDDs in Asian
populations are often lower than in Caucasian populations.

12. Responsible Organizations of
ATC/DDD methodology
• Who is who?
• -The World Health Organization:
• TheWHO endorsed the ATC/DDD methodology for
global use in 1996as an internationalstandard for drug
utilizationstudiesinan effort to ensure universalaccess to
essentialdrugs and to stimulate rational use of drugs
particularly in middle/low income countries.

13. - The WHO Collaborating Centre for Drug Statistics
Methodology :
The central body responsible for the development and maintenance
of the ATC/DDD system.
This Centre is located at the Norwegian Institute of Public Health
(NIPH) and the main activities of the Centre are drawn up in an
agreement between the WHO Headquarters and the Government of
Norway.

14. These activities are:
oTo classify drugs according to the ATC system.
oTo establish DDDs for drugs which have been assigned an ATC
code.
oTo review and revise as necessary the ATC classification system
and DDDs.
oTo stimulate and influence the practical use of the ATC system by
cooperating with researchers in the drug utilization field.
oTo organize training courses in the ATC/DDD methodology and
to lecture such courses and seminars organized by others.
oTo provide technical support to countries in setting up their
national medicines classification systems and build capacity in the
use of medicines consumption information.

15. WHO International Working Group for Drug Statistics
Methodology :
This body includes 12 expert members selected by WHO
Headquarters, who represent a wide range of geographical and
professional backgrounds.
All six WHO regions are represented in the group.
This body gives expert advice to the WHO Collaborating
Centre.

16. The main terms of reference of the Working Group are:
To continue the scientific development of the ATC/DDD system.
To discuss and approve all new ATC codes, DDD assignments
and alterations to existing ATC codes and DDDs.
To develop further the use of the ATC/DDD system as an
international standard for drug utilization studies.
To revise as necessary the guidelines for assignment and change
of ATC codes and DDDs.
To revise as necessary the procedures for applications for
assignment of and changes to ATC codes and DDDs to ensure they
are consistent and transparent.

17. To assess the sources and availability of statistics on drug use
internationally, and to encourage the systematic collection of
comprehensive drug use statistics in all countries and regions using
the ATC/DDD system as the international standard.
To develop methods, manuals and guidelines for the practical
application and appropriate use of the ATC/DDD system in drug
utilization studies in a variety of settings, particularly those
applicable to developing countries.
To work with groups involved in rational drug use initiatives to
integrate methods for measurement of drug use in assessing needs
and outcomes of interventions with the aim of improving drug use.

18. Guidelines
•The ATC/DDD guidelines and the Index were both published for
the first time in the current format in 1990.
•The system has been in use for statistics since 1975.
•Both publications are updated annually and available electronically
and in hard copies.
•English and Spanish versions of the publications are available.

19. Guidelines for ATC Classification and DDD assignment
•A comprehensive guide regarding ATC and DDD can be found here.
• The Guidelines includes information on the ATC Classification
system and general principles of DDDs assignment.
•Information necessary for allocating ATC codes and DDDs at a
national or local level can be found in the Guidelines.