Guidelines for ATC classification and DDD assignment - WHOCC

For all combination products where the DDD assigned deviates from the principles
given above, a list of DDDs are available from the Centre (published on the website
www.whocc.no).
3. Other factors
a) Fixed dose groups
For some groups of products, it has been considered most appropriate to estimate
the average use for products within a group instead of establishing accurate doses
for every product, e.g. cough mixtures in ATC group R05 and multivitamins in
ATC group A11. For the multivitamins the composition of various products may
differ, but the average recommended dose is usually the same. Such DDDs are
called "fixed dose".
In some ATC groups, it has been decided to use fixed DDDs for all combination
products given in e.g. number of tablets regardless of strength. These rules are
clearly stated in the chapters in the respective ATC levels in this publication (e.g.
ATC group A02AD, A02BD and A02BX).
For eye drops used in glaucoma therapy, a fixed dose regardless of strength has
been established in the different subgroups. This is based on the assumption that,
per dosage given, only one drop is applied in each eye, regardless of strength.
When fixed doses are assigned, these will be further described in the guidelines for
the different ATC groups.
b) Depot formulations
Depot formulations (e.g. sustained release formulations) are usually assigned the
same DDDs as the ordinary dosage forms. Exceptions to this main rule are
described in the guidelines for the different ATC groups.
c) Intermittent dosing
In certain therapeutic groups, e.g. hormones, many of the products are administered
intermittently. In such cases, the dose administered is divided by the number of
days in the treatment period to obtain the average daily dose. This means that
medicament free periods in between courses are included in the treatment period.
This also applies to such drugs as depot antipsychotics and contraceptive pills,
which are given intermittently.
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