Postal address:
WHO Collaborating Centre for Drug Statistics Methodology
Norwegian Institute of Public Health
P.O.Box 4404 Nydalen
0403 Oslo

Visiting/delivery address:
Marcus Thranes gate 6
0473 Oslo

Tel:  +47 21 07 81 60
Fax: +47 21 07 81 46

Use of ATC/DDD

As noted above, the main purpose of the ATC/DDD system is as a tool for presenting drug utilization statistics with the aim of improving drug use.  This is the purpose for which the system was developed and it is with this purpose in mind that all decisions about ATC/DDD classification are made.  Consequently, using the system for other purposes can be inappropriate.  The system has been used since the early 1970s in drug utilization studies where it has been demonstrated to be suitable for national and international comparisons of drug utilization, for the evaluation of long term trends in drug use, for assessing the impact of certain events on drug use and for providing denominator data in investigations of drug safety .

ATC codes are includes in some international drug ctalogues (e.g. Martindale, European Drug Index, Index Nominum) and in several national drug catalogues.

A publication entitled
“Introduction to Drug Utilization Research” is available here.





Drug utilization
The ATC/DDD system can be used for collection of drug utilization statistics in a variety of settings and from a variety of sources. 

Examples are: 

  • Sales data such as wholesale data at a national, regional or local level.
  • Dispensing data either comprehensive or sampled.  Computerised pharmacies can easily collect data on drugs dispensed.  Alternatively, sample data can be collected manually.  Reimbursement systems, which operate in a number of countries at the national level provide comprehensive dispensing data down to the individual prescription level, as all prescriptions are submitted and recorded for reimbursement.  This is generally called “claims” data.  Similar data are often available through health insurance or health maintenance organisations.

These databases can sometimes allow collection of demographic information on the patients, and information on dose, duration of treatment and co-prescribing.  Less commonly, linkage to hospital and medical databases can provide information on indications, and outcomes such as hospitalisation, use of specific medical services, and adverse drug reactions. 

  • Patient encounter based data.  This is usually collected by specially designed sampling studies such as those carried out by market research organisations.  However, increasing use of information technology at the medical practice level will make such data available more widely in the near future.  These methods have the advantage of potentially providing accurate information on Prescribed Daily Doses, patient demographics, duration of therapy, co-prescribing, indications, morbidity and co-morbidity, and sometimes outcomes. 
  • Patient survey data.  Collection of data at the patient level can provide information about actual drug consumption and takes into account compliance in filling prescriptions and taking medications as prescribed.  It can also provide qualitative information about perceptions, beliefs, and attitudes to the use of medicines. 
  • Health Facility data.  Data on medication use at all the above levels is often available in health care settings such as hospitals and health centres at regional, district, or village level.

Use of the ATC/DDD system allows standardisation of drug groupings and a stable drug utilization metric to enable comparisons of drug use between countries, regions, and other health care settings, and to examine trends in drug use over time and in different settings.

Drug consumption figures should preferably be presented as numbers of DDDs/1000 inhabitants/day or, when in-hospital drug use is considered, as DDDs per 100 bed days.  Sales or prescription data presented in DDD/1000 inhabitants/day may provide a rough estimate of the proportion of the population within a defined area treated daily with certain drugs.  For example, the figure 10 DDDs/1000 inhabitants/day indicates that 1% of the population on average gets a certain treatment daily.

For antiinfectives (or other drugs normally used in short periods) it is often considered most appropriate to present the figures as numbers of DDDs per inhabitant per year, which will give an estimate of the number of days for which each inhabitant is, on average, treated annually. 

For example, 5 DDDs/inhabitant/year indicates that the consumption is equivalent to the treatment of every inhabitant with a 5 days course during a certain year. 

Alternatively, if the standard treatment period is known, the total number of DDDs can be calculated as the number of treatment courses, and the number of treatment courses can then be related to the total population.

For some drug groups where DDDs have not been established, alternative ways of presenting data are recommended.  For example, consumption of dermatological preparations can be presented in grams of ointment, cream etc, and antineoplastic agents ATC group L01 can be presented in grams of active ingredient.

When there is a known discrepancy between the prescribed daily dose (PDD) and the DDD, it is important to take this into account when interpreting drug consumption figures.  Caution should also be taken in situations where the recommended dosage differs from one indication to another (e.g. antipsychotics), in severe versus mild disease (e.g. antibiotics) and where PDDs may differ from one population to another (e.g., according to sex, age, ethnicity or geographic location).

Finally, it should be taken into considerations that some prescribed medications are not dispensed, and the patient does not always take all the medications, which are dispensed.  Specially designed studies are required to measure actual drug intake at the patient level.

Since alterations of ATC and DDDs do occur it is important to be aware of which version of the ATC index is used in drug consumption studies especially when comparing the data over time and when making international comparisons.

Improving drug use (to the top)
Collecting and publishing drug utilization statistics are critical elements in the process of improving the prescription and dispensing of medicines.  For drug utilization statistics to have the best possible impact on drug use, the statistics need to be used in a focused and active manner.

Examples of ways in which drug utilization statistics based on ATC and DDDs have been and can be used to improve drug use include the following:

National publications, which provide clinicians, pharmacists and others with a profile of drug consumption in the country (with or without comparisons between countries or between areas within the country).

  • Publications providing feedback within health services to individual health facilities, groups of health care providers, or individual health providers. 
  • Use of drug utilization statistics by national health systems, universities, drug information centres, and others to identify possible over use, underuse or misuse of individual drugs or therapeutic groups.  Depending on the situation this information can then be used to initiate specific studies or specific educational interventions.  Educational interventions may include articles in drug bulletins, articles in scientific journals, letters to clinicians, etc. 

Drug Safety Assessment (to the top)
Estimates of frequency trends in spontaneously reported cases of suspected adverse reactions for certain population groups may be linked to trends in drug consumption, using the ATC/DDD system.  Use of the DDD/1000 inhabitants/day as a drug utilization metric as the denominator, where frequency of adverse reactions is the numerator, allows trends in the frequency of adverse reaction reports to be examined against trends in drug utilization.  For comparisons between drugs, validation by PDDs would be necessary.

The WHO Collaborating Centre for International Drug Monitoring (Uppsala Monitoring Centre), Sweden, receives spontaneous reports of suspected adverse reactions from national centres (96 countries are included in the programme, October 2009).  Information on all medicinal products appearing in these reports is stored in a drug register, linked to the reports database.  All single and multiple ingredient preparations are given an ATC code at the substance level, which allows flexible searches comprising different drug categories or groups of drugs.  The ATC system is also used for the grouping of drugs in output documents.

Drug costs, pricing and reimbursement and cost-containment (to the top)
Basing detailed reimbursement, therapeutic group reference pricing and other specific pricing decisions on the ATC and DDD assignments is a misuse of the system.  This is because the ATC and DDD assignments are designed solely to maintain a stable system of drug consumption measurement, which can be used to follow and compare trends in the utilization of drugs within and across therapeutic groups.  None the less, drug utilization data have a central role in the quality of care cycle and ATC and DDD methodologies can be helpful in following and comparing trends in cost, but need to be used with caution.

The DDD is a technical drug use metric.  DDDs do not necessarily reflect therapeutically equivalent doses of different drugs and therefore cannot be assumed to represent daily doses that produce similar treatment outcomes for all products within an ATC category.  Such estimates of therapeutic equivalence are very difficult to establish, particularly to the precision usually required for pricing decisions.  DDDs, if used with caution can be used to compare, for example, the costs of two formulations of the same drug.  However, it is usually not valid to use this metric to compare costs of different drugs or drug groups.  The relationships between therapeutically equivalent doses, the actual prescribed daily dose (PDD) and DDD usually differ between drugs and, for the same drug, between countries.  Moreover, even though PDDs commonly change over time altering a DDD complicates drug utilization research, hence there is a reluctance to alter a DDD. Alterations are not made unless there is evidence that changes in PDD are large, or there is some particular reason such as a change in the main indication.  For these reasons, DDDs are not suitable for comparing drugs for specific, detailed pricing, reimbursement and cost-containment decisions.

Similarly, basing reimbursement and pricing comparisons on inclusion of drugs in ATC groups is not recommended.  The main indications for drugs (on which ATC assignments are based) often differ widely between countries and, like the PDD, can change over time.  However, the ATC classifications can be useful when costs need to be aggregated into drug groups or therapeutic areas to determine, for example, to what extent increased costs can be attributed to increased use of a therapeutic group over time.

Pharmaceutical marketing purposes (to the top)
It is important to emphasise that the ATC classification does not necessarily reflect the recommended therapeutic use in all respects.  Therefore, the ATC system should not be used as a tool for marketing purposes concerning efficacy, mechanism of action or therapeutic profile in relation to other drugs.

It should be emphasised that assignment to different ATC groups does not mean a difference in therapeutic effectiveness and assignment to the same ATC group does not indicate therapeutic equivalence.

Concerning use of price comparisons for marketing purposes, see Drug cost, pricing and reimbursement and cost-containment.


Last updated: 2011-02-15