ATC Code Definition

Look your active substance up in the WHO Collaborating Centre's ATC/DDD Index, then confirm the code against the code the authority shows for an authorised product with the same substance(s), route and use. If you're looking at an EU centrally authorised product, the EMA register gives the code directly. If the substance isn't in the index yet, anyone can apply to the WHOCC for a new code.

Step 1: Search the ATC/DDD Index (WHOCC, ATC/DDD Index)

• Search by name, not brand. The search needs at least three letters, and trademarks aren't searchable.
• Search by code. This works at any level. The result shows the substance plus every level above it, so you can check you're in the right therapeutic group.
• Combinations need extra care. A name search may not pick up every active ingredient in a combination product.
• Use the current edition. The index is for 2026, last updated 20 January 2026. It links to the WHOCC Guidelines for ATC Classification and DDD Assignment, which explain why each substance is placed where it is.

Step 2: Apply the classification rules (WHOCC, Structure and principles)

• Main use decides. Products are classified "according to the main therapeutic use of the main active ingredient."
• Route and strength matter. There is "only one ATC code for each route of administration." A substance "can be given more than one ATC code if it is available in two or more strengths or routes of administration with clearly different therapeutic uses." So pick the code for your route and indication, not just the first hit.
• Combinations get their own codes. Products with two or more active ingredients "are regarded as combinations and given different ATC codes from plain products." For example, plain metformin is A10BA02. In the EMA register, vildagliptin/metformin products carry A10BD08 ^[1][2], while sitagliptin/metformin carries A10BD07 ^[3].
• Search under the INN. The index prefers international nonproprietary names (INN). Where there's no INN, it usually uses the USAN or BAN.

Step 3: Check it against a register

• EU centrally authorised products: each EMA record has an "ATC code (human)" field next to the pharmacotherapeutic group ^[2].
• Veterinary products: use ATCvet codes. EMA's veterinary register holds them separately.
• National products: registers such as Swissmedic, the Dutch CBG-MEB, Romania's ANMDMR and Hungary's OGYÉI can also be searched by ATC code.

Step 4: If no suitable code exists, apply for one (WHOCC, Application for ATC codes)

• Who can apply: "any user of the system (e.g. health authorities, pharmaceutical manufacturers, researchers and other users)."
• What to send: the chemical structure and pharmacology, main and other indications, marketing authorisation status, and a "proposed ATC classification with justification based on the evidence submitted."
• Terms: applying is free and must be done in English.
• When to apply: a new chemical entity "is normally not included in the ATC system before an application for marketing authorisations is ready for submission in at least one country." Some cases need approval in at least one country first.
• Timeline: you get a preliminary code within 6-8 weeks, unless the case goes to the Working Group. The WHO International Working Group for Drug Statistics Methodology formally approves new codes at its March and October meetings. The code is published as provisional for public comment, and becomes final if there are no justified objections.

Approach: I used the EMA centrally authorised medicines register for worked examples; the WHO ATC/DDD Index itself isn't in the datasets I can search. The lookup rules and application procedure come from the WHOCC's official pages.

Norway's two national drug statistics, both run by the Norwegian Institute of Public Health (FHI), are built on the ATC hierarchy. Every figure is reported against an ATC code at some level, from the national total ("TOTALT", all ATC codes) down through the 1st-level anatomical group to the 5th-level substance. Use is measured in defined daily doses (DDD), and the Prescription Register also counts individual users. Because each level is its own row, you can follow the same drug from the main group down to the substance.

1. Wholesale-based drug consumption statistics (Grossistbasert legemiddelforbruksstatistikk)

• Scope: FHI collects sales "from wholesaler to pharmacy, health institutions, grocery trade, kiosk and petrol stations." It covers "all sales of medicines, both over-the-counter and prescription medicines, as well as medicines for animals." Figures are "distributed by ATC codes" and measured as "DDD/1000 inhabitants/day and number of DDD." Public figures go back to 1999 (FHI, about the wholesale statistics).
• The metformin chain in 2025:

ATC level	Code and FHI label	DDD	DDD/1000 inhabitants/day
Total	TOTALT, all ATC codes	4,020,222,053 [4]	1,968.8 [5]
1st	A, digestive organs and metabolism	698,830,296 [6]	342.2 [7]
2nd	A10, drugs used in diabetes	215,875,637 [8]	105.7 [9]
3rd	A10B, blood glucose lowering drugs excl. insulins	172,617,525 [10]	84.5 [11]
4th	A10BA, biguanides	39,678,997 [12]	19.4 [13]
5th	A10BA02, metformin	39,678,997 [14]	19.4 [15]

The 4th and 5th levels show the same value, so in 2025 metformin made up all of the biguanide sales.

• Trends over time: each code has a time series. For example, the A10 diabetes group went from 177,311,854 DDD in 2023 ^[16] to 215,875,637 in 2025.

2. Norwegian Prescription Register (Legemiddelregisteret / Reseptregisteret)

• Scope: it covers medicines "dispensed by prescription from pharmacies to humans and animals" since 2004 (FHI, Legemiddelregisteret).
• ATC table: the "Per ATC-kode. 2004-2025" table breaks each ATC code down by sex and age group. Each row gives users, users per 1,000 population and DDD.
• 2025 figures, both sexes, all ages:
  • Group A: 1,498,407 users (267.84 per 1,000) ^[17]
  • A10: 479,173 users (85.65 per 1,000), 212,086,653 DDD ^[18]
  • A10BA02 metformin: 176,944 users (31.63 per 1,000), 38,882,579 DDD ^[19]
• Themed group tables: alongside the ATC table, FHI publishes groups that don't follow one ATC level, such as "ADHD-midler" ^[20], cardiovascular drugs ^[21] and contraceptives ^[22]. The weight-loss table still lists individual substances by ATC code, for example A08AA10 sibutramine ^[23].

How the two sources fit together

• Different coverage, same codes: the wholesale statistics count everything sold into the supply chain, including over-the-counter and institutional sales. The Prescription Register counts only prescription dispensing from pharmacies, but adds who the patients are. Both use the same ATC codes, so you can compare them directly.
• Comparing them: for 2025 metformin, wholesale shows 39.7 million DDD against 38.9 million dispensed on prescription.
• Where to start: the Norwegian Medical Products Agency (DMP) page "Statistikk for omsetning av legemiddel" sends readers to FHI for both sources ^[24][25].

Approach: I searched FHI's wholesale-based drug consumption statistics, the Norwegian Prescription Register statistics and DMP's statistics page. For how each source works, I used FHI's own pages, which don't say which ATC/DDD index version they use.