When you buy an API for a regulated market, the authority reviewing your finished product needs to know how that API was made. A Drug Master File is what lets them find out without the API maker handing you their process.
That arrangement solves a problem that would otherwise stop the generic industry working. The manufacturer spent years developing a route. The buyer needs it assessed. Neither wants the other holding the recipe.
Most buyers meet the term when a supplier says “we have a US DMF” and expect that to settle the matter. It does not. A DMF number on its own tells you very little.
Quick answer: A Drug Master File is a confidential submission to a regulatory authority describing how an active ingredient, packaging material, or excipient is manufactured, controlled, and tested. The owner keeps the process secret from the buyer, while the authority reviews it in support of the buyer’s drug application under a Letter of Authorization.
Table of Contents
- What a Drug Master File Is and Why It Exists
- US FDA Drug Master File Types
- Open Part vs Restricted Part
- The Letter of Access and Why You Need One
- What a Drug Master File Contains
- Review Status, Completeness Assessment, and Fees
- ASMF: The European Equivalent
- CEP: The EDQM Alternative Route
- US DMF vs EU ASMF vs CEP Compared
- Amendments, Annual Reports, and Process Changes
- What an Importer Should Actually Ask For
- Expert Tips
- Common Mistakes
- Frequently Asked Questions
What a Drug Master File Is and Why It Exists
A Drug Master File is a submission made voluntarily to a regulatory authority by a manufacturer of a drug substance, packaging component, or other material used in making a medicine.
It sits on file with no standing of its own. Nothing happens until someone references it in an application for a drug product. The owner is usually the API manufacturer; the applicant referencing it is usually a different company.
Consider what an API route is worth. Starting materials, catalysts, solvents, reaction conditions — the gap between a profitable route and an unprofitable one lives in those details.
A regulator needs to know what impurities that route generates and whether the process runs consistently. Without a master file system, the maker would hand all of it to every customer who files an application, and some of those are competitors.
The system resolves that. Chemistry goes to the agency; the buyer gets a summary, a specification, and permission for the agency to read the rest.
US FDA Drug Master File Types
The FDA organises Drug Master Files by what the file describes. The type tells you immediately what the supplier is offering.
Type II — drug substance and intermediates
The one importers deal with most. It covers the API itself, or an intermediate or starting material used to make it. When a supplier says “our paracetamol has a US DMF”, they mean a Type II filing.
Type III — packaging material
Container closure components: bottles, closures, films, foils, elastomeric stoppers, and the resins behind them. For finished dosage forms, this file supports the extractables and leachables questions in your dossier.
Type IV — excipient, colorant, flavour, or essence
Non-active ingredients where the maker wants composition or process kept confidential. Many excipients never need one, since compendial material with a sound specification is enough. Novel excipients and proprietary blends are where Type IV matters.
Type V — FDA-accepted reference information
A catch-all for information that fits nowhere else. The FDA asks to be consulted first, so it is not used casually.
Type I — discontinued
Type I once covered manufacturing sites, facilities, personnel, and procedures. The FDA discontinued it, and facility information now sits in registration and the other file types. Older documents still reference it.
Open Part vs Restricted Part
Every master file splits into two halves, whatever the region calls them.
The Open Part — the Applicant’s Part in Europe — holds what the buyer may see: specifications, analytical methods, a general process description, stability summaries, container closure, and the impurity profile at the level needed to justify limits.
The Restricted or Closed Part holds the confidential detail: full route, reaction conditions, in-process controls, equipment, validation data, and the origin of each impurity.
You receive the Open Part and never the Closed Part. A thin Open Part leaves your regulatory team unable to answer basic questions, and every deficiency turns into a slow relay through the supplier.
The Letter of Access and Why You Need One
The file is confidential, so the agency will not open it for your application unless the owner tells them to.
That authorisation is the Letter of Authorization in US practice and the Letter of Access in Europe. It names the file, the substance, the authorised applicant, and the application it supports. The holder issues it to you and files a copy with the agency.
Three things go wrong with these letters more often than anything else in API sourcing:
- It names the wrong legal entity — your trading arm instead of the applicant.
- It is generic and never identifies the application it supports.
- The holder issues it but never files their copy, so the reference fails at review.
Ask for a draft before you commit. It costs nothing and reveals whether the supplier’s regulatory function is real.
What a Drug Master File Contains
Modern files follow the Common Technical Document structure, so content lands in a familiar order in any region.
General information — nomenclature, structure, and physicochemical properties.
Manufacture — the manufacturer and site, the synthetic route, controls of materials and critical steps, process validation, and development history.
Characterisation — proof of structure, polymorphic form where relevant, and a full discussion of impurities: process-related, degradation products, residual solvents, and elemental impurities.
Control of the drug substance — the specification, analytical procedures, their validation, and batch data showing the process delivers against that specification.
Reference standards, container closure, and stability — including the protocol, data supporting the retest period, and the commitment to continue studies. Our guide to stability studies in pharmaceuticals covers those datasets.
Facility information sits alongside this through registration and inspection history, which is why a DMF and a GMP status are related but separate. See GMP, WHO-GMP, EU GMP, and US FDA explained.
Review Status, Completeness Assessment, and Fees
Here is where most buyer misunderstandings live. A DMF number does not mean the FDA has reviewed the file, and it certainly does not mean the FDA has approved it.
A number is issued when a file is received and acknowledged. Review happens only when the file is referenced in an application, or through the completeness assessment applied under the generic drug user fee framework.
The completeness assessment checks whether a Type II API file contains the elements required for review — not whether the science is acceptable. Files that pass appear on a published list available for reference.
That list helps importers: appearing on it means the file met the structural bar and its fee was paid. Technical review, when it comes, generates deficiencies that go to the holder rather than to you, yet still hold your application.
How the fees work
Under the US generic drug user fee programme, a Type II API DMF referenced in a generic application carries a one-time fee payable by the holder. Amounts reset each fiscal year, so any figure quoted here would go stale.
The structure matters more: paid once per file rather than per application, and the file cannot be referenced until the fee is paid and the assessment passed. European fees differ — the EDQM charges for CEP applications and changes, while ASMF costs sit inside procedure fees.
ASMF: The European Equivalent
Europe uses the Active Substance Master File procedure. The concept matches the US DMF, but details differ enough to trip up buyers who assume equivalence.
An ASMF has a formal two-part structure: the Applicant’s Part, which the buyer receives in full, and the Restricted Part, for authorities only. That split is defined in guidance rather than left to the owner’s judgement.
An ASMF is not held on a permanent central register the way a US DMF is. It goes to the authority handling a specific procedure, alongside the application it supports, and the same file may go to several authorities.
Assessment produces an ASMF assessment report. Later applicants sometimes benefit from work already done, but not automatically. See also how to import pharmaceuticals into Europe.
CEP: The EDQM Alternative Route
The Certificate of Suitability to the monographs of the European Pharmacopoeia — CEP, sometimes still written COS — works on different logic.
The manufacturer submits a dossier to the EDQM, which assesses whether the Ph. Eur. monograph, applied with any extra tests listed on the certificate, adequately controls the impurities that specific route produces.
If it does, the EDQM issues a certificate naming the substance, manufacturer, site, and any additional requirements. You get a copy and submit it in place of the drug substance chemistry section — a document you actually hold, from a body that already did the assessment.
CEPs are accepted across Ph. Eur. member states and by many other authorities recognising the scheme. The US FDA does not accept them, since it runs its own system.
One limit matters: a CEP only works where a Ph. Eur. monograph exists. New molecules without one must use the ASMF procedure.
US DMF vs EU ASMF vs CEP Compared
| Aspect | US DMF | EU ASMF | CEP / COS |
|---|---|---|---|
| Issuing / receiving body | US FDA | National authority or EMA, within the relevant procedure | EDQM (Council of Europe) |
| Structure | Open Part and Restricted (Closed) Part | Applicant’s Part and Restricted Part, format defined in guidance | Single dossier held by EDQM; a certificate is issued |
| Who assesses it | FDA reviewers, triggered by an application or completeness assessment | Assessors of the authority handling the marketing authorisation | EDQM assessors, independently of any product application |
| What the buyer receives | Open Part plus a Letter of Authorization | Applicant’s Part plus a Letter of Access | A copy of the certificate, plus the declaration of the specific route |
| Standing on its own | None — needs a referencing application | None — submitted with a procedure | Standalone approval of the substance route |
| Where it is accepted | United States | EU and EEA member states through the relevant procedures | Ph. Eur. member states and many authorities recognising the scheme; not the US FDA |
| Depends on a monograph | No | No | Yes — a Ph. Eur. monograph must exist |
| Fee model | One-time DMF fee under the generic user fee programme, paid by the holder | Costs sit inside procedure fees | EDQM application and variation fees, paid by the holder |
A supplier serving both markets usually maintains a US DMF and either a CEP or an ASMF. Ask which, for the exact site you will buy from. Our overview of pharmaceutical regulatory authorities around the world covers who else recognises them.
Amendments, Annual Reports, and Process Changes
A master file is not a one-time document, and neglect shows up in your application rather than the supplier’s.
Amendments are filed whenever content changes — a new specification, a revised analytical method, added stability data, a new site, or a route change.
Annual reports confirm the file’s status, list the year’s changes, and update the authorised parties. A file silent for years is a warning sign, and the FDA can close files that fall out of maintenance.
What happens when the API maker changes their process
This is the scenario that catches importers. The supplier improves a step, swaps a solvent, changes starting material vendor, or shifts production to a second site. The change goes in as an amendment, and the holder should notify every authorised party — though in practice notification is inconsistent, especially through distributors.
Consequences depend on the change. A minor analytical update may need nothing. A route change that alters the impurity profile can mean comparative data, a variation to your marketing authorisation, or fresh stability work.
Build the obligation into your supply agreement. A clause requiring written notice of any amendment, with a defined notice period before affected material ships, costs nothing to ask for.
What an Importer Should Actually Ask For
The supplier says they have a Drug Master File. Here is what turns that claim into something you can rely on.
- The DMF number and filing date, plus the market it covers — US, EU, or both.
- The exact site name and address on the file. Suppliers with several plants sometimes file only one.
- Whether the file appears on the FDA’s published list of Type II API DMFs available for reference.
- Whether the user fee has been paid for US filings.
- The Open Part or Applicant’s Part in full, not a two-page summary.
- A draft Letter of Authorization or Letter of Access naming your legal entity.
- For Europe, the CEP copy with annexes and the route declaration, or the ASMF details.
- Date of the last amendment and last annual report.
- The supplier’s notification commitment for future changes.
A supplier with real regulatory capability produces most of this within a week. See also what buyers should check before choosing an API supplier and documents required for pharmaceutical export for the wider evaluation.
Expert Tips
- Check the file against the site, not the company. Large API makers run several plants, and the DMF may cover one while your order is scheduled at another. Put the site address in the purchase agreement.
- Read the Open Part before signing anything. Its quality predicts how the relationship will run. A good one answers your regulatory team’s questions without an email to the supplier.
- Ask when the file was last amended. A DMF untouched for years usually means the product is not actively supported. Files can also be closed for lack of maintenance, and nobody tells you.
- Get the Letter of Authorization drafted early. Wrong entity names and missing application references are the most common avoidable delay.
- Compare the DMF specification with the COA you receive. Where the COA reports fewer tests, ask why before the first commercial batch. Our Certificate of Analysis guide shows what a complete COA looks like.
- Ask about impurities in specific terms. Named impurities, their origin, their limits, and whether any are genotoxic under the relevant guidance.
Common Mistakes
- Treating a DMF number as approval. A number means received, not reviewed. Timelines built on that assumption lose months when the first deficiency lands.
- Accepting a summary instead of the Open Part. Your regulatory team cannot draft the drug substance sections, so every question routes back through the supplier.
- Not verifying the Letter of Authorization reaches the agency. When the holder’s copy is never filed, the reference fails and the application stalls.
- Assuming a CEP works everywhere. It does not cover the US market, and companies planning a US filing on a CEP alone find the gap late.
- Buying through a trader without confirming access rights. A trader who cannot obtain a Letter of Authorization leaves you with material you cannot file.
- Confusing the DMF with GMP status. The file describes the process; inspection history shows whether the site runs it properly. A strong file at a weak site is still a problem.
Frequently Asked Questions
What is a Drug Master File in simple terms?
A Drug Master File is a confidential document a manufacturer files with a regulatory authority describing how a material is made, controlled, and tested. It usually covers an active ingredient, but can also cover packaging or excipients. The authority holds it and reviews it only when a drug application refers to it. The system exists so the manufacturer’s process stays confidential while still being assessed properly.
Is a Drug Master File approved by the FDA?
No. The FDA does not approve Drug Master Files. It assigns a number on receipt and reviews content only in the context of an application that references the file, or through the completeness assessment applied to Type II API files under the generic drug user fee programme. Files that pass that assessment appear on a published list as available for reference, which is a useful signal but still not approval of the chemistry.
What are the DMF types used by the US FDA?
Four types are currently in use. Type II covers drug substances, intermediates, and materials used in their preparation, and is the one API buyers deal with. Type III covers packaging materials. Type IV covers excipients, colorants, flavours, and similar materials. Type V covers other information the FDA has agreed to accept, and needs prior consultation. Type I, which covered manufacturing sites and facilities, has been discontinued.
Do I need a Letter of Access to reference a DMF?
Yes. The file is confidential, so the authority will not open it for your application without written authorisation from the owner. That document is the Letter of Authorization in US practice and the Letter of Access in European practice. It must name the file, the substance, your legal entity, and the application it supports, and the holder must also file their copy with the agency. Mismatched letters are a leading cause of delay.
What is the difference between a DMF and an ASMF?
Both keep the manufacturer’s process confidential from the buyer while letting authorities assess it. A US DMF sits on a permanent register with the FDA and is referenced by number. An ASMF goes to the authority handling a specific European procedure, alongside the application it supports. The ASMF uses a formally defined split into an Applicant’s Part and a Restricted Part, and assessment produces a report tied to that procedure.
Is a CEP better than an ASMF?
Neither is better in general terms. A CEP is easier for the buyer, because the EDQM has already assessed the route and you simply submit the certificate. It only works where a European Pharmacopoeia monograph exists, so new molecules cannot use it. An ASMF works for any substance but requires assessment within each procedure. Many suppliers hold a CEP for established molecules and use the ASMF route for the rest.
What happens if the API manufacturer changes their process?
The change is filed as an amendment, and the holder should notify every authorised party. Depending on the change, you may need a variation to your marketing authorisation, comparative data, or repeat stability work on the finished product. Route changes that alter the impurity profile carry the greatest impact. Contractual notification obligations are the only reliable protection, since practice varies widely when buying through distributors.
Can I get a Drug Master File if I buy through a trader?
Only if the trader can obtain a Letter of Authorization from the manufacturer who owns the file. Many can, and act as a legitimate channel. Some cannot, and that becomes clear only when the letter is requested. Confirm access rights in writing before ordering commercial quantities, and check that the site named on the file matches the site your material will come from.
Final Thoughts
A Drug Master File is a permissions system as much as a technical document. The chemistry sits with the authority, the summary sits with you, and a letter connects the two.
Buyers who understand that structure ask better questions early, and spend less time explaining to their regulatory team why an application is stuck.
Discuss Your API Documentation Requirements
AksharAvira Pharma supplies APIs, generics, and finished dosage forms to buyers in regulated and semi-regulated markets. If you need clarity on the regulatory documentation a specific market expects for a molecule, speak with our pharmaceutical experts.