Medical Subject Headings (MeSH)

Key facts

Produced by

National Library of Medicine (NLM)

Main uses

Indexing MEDLINE and PubMed citations; cataloging NLM collections

Structure

Descriptors arranged in hierarchical trees under 16 broad categories

Identifiers

Descriptors start with D, supplementary concepts with C, qualifiers with Q

Cost

Free to download and use

What is MeSH?

Medical Subject Headings (MeSH) is a thesaurus of standardized subject terms. Instead of relying on the words authors happen to use, each MEDLINE article is tagged with MeSH headings that describe its topics, so a search on one heading finds articles that use different wording for the same concept.

The National Library of Medicine (NLM) builds and maintains MeSH and publishes it free of charge. Beyond PubMed, MeSH terms are used to index conditions and interventions in ClinicalTrials.gov, and MeSH is one of the source vocabularies in the Unified Medical Language System (UMLS).

How MeSH is structured

MeSH has several record types:

A descriptor can appear in more than one tree, and PubMed searches on a MeSH heading include the narrower headings beneath it by default.

• Descriptors (main headings): the core subject terms, such as a disease, drug class or procedure, each with synonyms called entry terms.
• Tree numbers: positions in hierarchies under 16 broad categories, such as Anatomy, Diseases, Chemicals and Drugs, and Health Care.
• Qualifiers (subheadings): narrow a descriptor to an aspect, such as drug therapy, adverse effects or epidemiology.
• Supplementary Concept Records: specific chemicals, drugs, rare diseases and other concepts, each mapped to one or more descriptors.

Why MeSH matters

MeSH turns free-text literature and trial records into countable topics:

• Literature surveillance and systematic reviews use MeSH to build complete, reproducible PubMed searches on a disease or drug.
• Identifying key opinion leaders often starts with counting authors of articles indexed under a specific MeSH heading.
• In ClinicalTrials.gov data, MeSH terms group trials that describe the same condition in different words, which makes pipeline and competitor counts more consistent.
• Because MeSH is part of UMLS, its terms can be linked to clinical vocabularies such as SNOMED CT.

Limitations of MeSH indexing

MeSH describes what an article or trial is about, not clinical facts about individual patients, so it is not used to code diagnoses or claims. Indexing is applied to MEDLINE citations, and recent or non-MEDLINE records in PubMed may have no MeSH terms, so MeSH-only searches can miss new publications.