openapi: 3.1.0
info:
  title: NCBI Entrez E-utilities History Links API
  description: The Entrez Programming Utilities (E-utilities) provide a stable interface to the NCBI Entrez query and database system. Nine server-side programs allow searching, fetching, posting, linking, and summarizing records across PubMed and 38 NCBI databases including PubMed Central, Gene, Nuccore, and Protein. Supports searching 35M+ biomedical citations, retrieving abstracts, full-text links, MeSH terms, and related article metadata.
  version: v1.0.0
  termsOfService: https://www.ncbi.nlm.nih.gov/home/about/policies.shtml
  contact:
    name: NCBI Help Desk
    url: https://support.nlm.nih.gov/
    email: info@ncbi.nlm.nih.gov
  license:
    name: NCBI Data Use Policies
    url: https://www.ncbi.nlm.nih.gov/home/about/policies.shtml
servers:
- url: https://eutils.ncbi.nlm.nih.gov/entrez/eutils
  description: NCBI E-utilities production server
tags:
- name: Links
  description: Operations for finding linked records between databases
paths:
  /elink.fcgi:
    get:
      summary: ELink - Retrieve linked records
      description: Returns UIDs linked to an input set of UIDs in either the same database or a different Entrez database. Also returns LinkOut URLs for non-Entrez external resources, and checks for the existence of links in either direction.
      operationId: ELink
      tags:
      - Links
      parameters:
      - name: dbfrom
        in: query
        description: Database containing the input UIDs.
        required: true
        schema:
          type: string
          example: pubmed
      - name: db
        in: query
        description: Database to search for linked records. Omit when cmd is llinks or llinkslib.
        required: false
        schema:
          type: string
          example: pmc
      - name: id
        in: query
        description: Comma-delimited list of UIDs.
        required: false
        schema:
          type: string
      - name: cmd
        in: query
        description: ELink command mode.
        required: true
        schema:
          type: string
          enum:
          - neighbor
          - neighbor_score
          - neighbor_history
          - acheck
          - ncheck
          - lcheck
          - llinks
          - llinkslib
          - prlinks
          default: neighbor
      - name: linkname
        in: query
        description: Name of a specific link to retrieve (e.g., pubmed_pubmed, pubmed_pmc).
        required: false
        schema:
          type: string
      - name: term
        in: query
        description: Entrez query to filter linked UIDs.
        required: false
        schema:
          type: string
      - name: retmode
        in: query
        description: Response format (xml or json).
        required: false
        schema:
          type: string
          enum:
          - xml
          - json
          default: xml
      - name: holding
        in: query
        description: Name of a LinkOut provider to restrict LinkOut links.
        required: false
        schema:
          type: string
      - name: datetype
        in: query
        description: Type of date to limit the link results.
        required: false
        schema:
          type: string
          enum:
          - mdat
          - pdat
          - edat
          - crdt
          - mhda
      - name: reldate
        in: query
        description: Limit results to items published in the last n days.
        required: false
        schema:
          type: integer
      - name: mindate
        in: query
        description: Minimum date for date range filter.
        required: false
        schema:
          type: string
      - name: maxdate
        in: query
        description: Maximum date for date range filter.
        required: false
        schema:
          type: string
      - name: WebEnv
        in: query
        description: Web environment string from a previous call.
        required: false
        schema:
          type: string
      - name: query_key
        in: query
        description: Integer query key from a previous call.
        required: false
        schema:
          type: integer
      - name: tool
        in: query
        description: Name of application making the call.
        required: false
        schema:
          type: string
      - name: email
        in: query
        description: Contact email address.
        required: false
        schema:
          type: string
          format: email
      - name: api_key
        in: query
        description: API key to enable higher request rate.
        required: false
        schema:
          type: string
      responses:
        '200':
          description: Linked records or LinkOut URLs
          content:
            application/xml:
              schema:
                type: object
components:
  securitySchemes:
    ApiKeyAuth:
      type: apiKey
      in: query
      name: api_key
      description: 'Optional NCBI API key. Without a key: 3 requests/second. With a key: 10 requests/second. Register at https://www.ncbi.nlm.nih.gov/account/'