Guidelines for ATC classification and DDD assignment - WHOCC

D. Principles for reviewing and changing DDD
As the dosages used may change over time, it will always be necessary to make
some alterations. The International Working Group for Drug Statistics
Methodology may review a DDD whenever the Group finds it appropriate.
Changes of DDDs should be kept to a minimum and avoided as far as possible.
Too many alterations will always be disadvantageous for long-term studies on drug
utilization. Before alterations are made, difficulties arising for the users are
weighed against the benefits achieved by the alteration.
- The same principles used to assign new DDDs also apply when DDDs are
reviewed.
- Changes are generally not made unless they are at least in the order of 50 %.
This rule is not used for the three year revision of DDDs, where smaller
alterations are allowed. Further, minor alterations are allowed for important
drugs, which are frequently used (e.g. the DDD for cimetidine was changed from
1.0 g to 0.8 g).
DDD review after three years
All newly assigned DDDs are reviewed during the third year after inclusion in the
ATC Index with DDDs. The DDDs are reviewed at the first semi-annual meeting of
the International Working Group for Drug Statistics Methodology. The following
are considered:
- Recommended dosages as listed in drug catalogues in different countries and/or
published in peer reviewed scientific journals or major international textbooks.
- Data on prescribed daily doses (PDDs) from a range of countries. Figures
showing the prescribed daily dose (PDD) are important when reviewing an
assigned DDD. Usually more data concerning PDDs are available after a three
years period than at the time of marketing.
- Established main indication and therapy profile of the preparation (i.e. has the
main indication changed?)
- Existing DDDs in the ATC group.
- Written objections to the DDD which have been received.
When reviewing combination products, changes in the DDDs for the different
active ingredients are an important consideration.
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