Medical Subject Headings (MeSH)

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MeSH Tree with top 16 categories or facets
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Last Update

  • Updated.jpg This entry is out of date, and will not be updated, August 2018


See also Data science portal | Families of Subheadings | National Library of Medicine (U.S.) | PubMed - MEDLINE | Social tagging | Unified Medical Language System® (UMLS)

See also MeSH Webinar: "2016 MeSH Highlights" on January 20, 2016

  • NLM staff provides tour of the 2016 Medical Subject Headings (MeSH). The 30-minute presentation features a MeSH tree clean-up project; new clinical study publication type; changes to trees for diet, food and nutrition; restructuring in pharmacology and toxicology; new terms in psychology and health care. Indexing MeSH, and PubMed searching experts answered user questions. Questions and answers not addressed during the Webinar will be posted to this page as soon as possible. For more information about 2016 MeSH, see What's New for 2016 MeSH and the Introduction to MeSH - 2016.

MeSH Database

  • MeSH On Demand
    • identifies MeSH Terms in your text using NLM's Medical Text Indexer (MTI) tool; a list of MeSH Terms relevant to your text will be shown

Year end processing (YEP)

  • NLM performs maintenance to MEDLINE every year called year-end processing (YEP). YEP maintenance aims to change retrospectively Medical Subject Headings (MeSH®) on existing MEDLINE citations to conform to the new year's version of MeSH, and where possible make other global changes such as with Supplementary Concept Substance Names.
  • For more information, see MEDLINE®/PubMed® Year-End Processing (YEP)

Polyhierarchic structure

  • MeSH terms are arranged in a poly-hierarchic tree structure.
  • At the top of the MeSH Tree are major categories such as Anatomy or Organisms, and Diseases.
  • Increasing levels of specificity can be send as you descend the tree structure and the bottom parts of the hierarchy such as Metatarsal Bones and Conduct Disorder.
  • With 27,000 descriptors and more than 172,000 headings called Supplementary Concept Records, the MeSH vocabulary is one of the largest of its kind in the world. It includes thousands of cross-references to assist in locating appropriate terms; Vitamin C, for example, leads searchers to Ascorbic Acid.
  • The explode and focus functions help to increase recall and precision of biomedical literature searching.

Explode and focus

  • MEDLINE searchers may use MeSH terms at high levels of generality (mentioned as broadest categories above) or specificity (narrower terms at the bottom of the tree) depending on the goals of the search or needs of the user. Explode will retrieve articles indexed with the major heading and all specific, narrower terms that fall beneath it. Focus locates articles where the term is a major descriptor (not a minor one). Explode and focus can be used together to find articles where the major heading and terms underneath are the focus of the articles retrieved. Entry terms to MeSH descriptors are included in the MeSH record and consist of information such as variations in the form, word order, spelling and printed entry term. (There are historic reasons for this which go back to ELHILL days of searching MEDLINE over phone lines). Trade names of drugs and equipment are usually not printed but exist as non-print entry terms. When entry terms are used, PubMed substitutes it with the MeSH descriptor automatically.

MeSH for indexing

  • The MeSH thesaurus is used by NLM librarians to index articles from ~5,600 international biomedical journals (see Peer review) in the MEDLINE database. Each citation in MEDLINE is given up to ten to fifteen MeSH terms to describe its content. Incidentally, MeSH are also used to provide subject headings for bibliographic records (books, audiovisual materials and digital items) in library catalogues. NLM staff revise and update the MeSH thesaurus annually. Subject librarians are responsible for areas in which they have knowledge and expertise. NLM incorporates new subject headings or terms from the biomedical literature itself and scans emerging areas and new terms regularly. Health professionals in various scientific disciplines are also consulted about any proposed changes to the MeSH vocabulary and there is close coordination with other health organizations who produce specialized vocabularies.


  • In Medline, there are 83 floating, allowable subheadings or qualifiers. These 83 topical qualifiers are used to bring out specific aspects or facets of biomedical topics during the indexing and information retrieval process. Some common subheadings include drug therapy, diagnosis, etiology, surgery, etc. To search for specific subheadings in Ovid, enter your MeSH term followed by a forward slash and the subheading abbreviation - health care reform/ec for economics of health care reform. Post-coordinate searching is also possible and widely-used by health librarians in systematic review searching. (Certain pre-arranged "Families of Subheadings" can also be searched together and literally exploded.)

Using MeSH in health-related searching

  • When MeSH terms are not known (or used), health librarians can use textwording and/or keywording. As relevant citations are eventually located, terms associated with these citations can be reviewed and used to rerun a new more structured MeSH-based search. Health librarians constantly restrategize and rerun searches using natural language (See Gault et al) because there may be no relevant MeSH created yet. Keyword searching is a useful way to teach novice searchers or when very current articles are needed. Why? MeSH terms may not yet be created, relevant papers may not yet be indexed or the indexing may not be applied consistently to retrieve everything on a topic.

Using MeSH in health libraries

  • To improve subject use and access in health library OPACs
  • To develop taxonomies for indexing in databases of various kinds
  • To create print and digital pamphlet files
  • To assign key words to journal article submissions by authors
  • To look up definitions in a medical dictionary
  • To assign metadata to digital publications
  • To catalogue items for later search engine retrieval
  • To use in reference interviews to identify concepts and synonyms

Canadian context


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