name: NIH API Rate Limits
description: NIH and NCBI APIs enforce rate limits to ensure fair access and system stability. Rate limits vary by API service and whether an API key is registered. Exceeding limits may result in temporary IP blocking.
url: https://www.ncbi.nlm.nih.gov/home/develop/api/
created: '2026-06-13'
modified: '2026-06-13'
rateLimits:
  - api: NCBI E-utilities (Entrez)
    baseURL: https://eutils.ncbi.nlm.nih.gov/entrez/eutils/
    documentationURL: https://www.ncbi.nlm.nih.gov/books/NBK25497/
    tiers:
      - name: Without API Key
        requestsPerSecond: 3
        requestsPerMinute: 180
        notes: Default limit. API key registration required as of December 1, 2018 for production use.
      - name: With Registered API Key
        requestsPerSecond: 10
        requestsPerMinute: 600
        notes: Obtain a free API key at https://support.nlm.nih.gov/kbArticle/?pn=KA-05317
      - name: Enhanced API Key
        requestsPerSecond: Custom
        notes: Contact info@ncbi.nlm.nih.gov with justification for limits above 10 rps
    enforcement:
      - Requests exceeding limits will receive HTTP 429 responses
      - Persistent violations may result in IP address blocking
      - Large batch jobs should be scheduled on weekends or 9 PM - 5 AM EST weekdays

  - api: PubChem PUG REST
    baseURL: https://pubchem.ncbi.nlm.nih.gov/rest/pug/
    documentationURL: https://pubchem.ncbi.nlm.nih.gov/docs/programmatic-access
    tiers:
      - name: Standard (No Key Required)
        requestsPerSecond: 5
        notes: Universal limit for all users and applications. No API key is available to increase this limit.
    enforcement:
      - Maximum 5 requests per second per user/application/organization
      - No API key whitelisting available for higher limits
      - Contact PubChem for bulk access optimization guidance
      - Large downloads should use FTP bulk data rather than the REST API

  - api: NIH RePORTER API
    baseURL: https://api.reporter.nih.gov/v2/
    documentationURL: https://api.reporter.nih.gov/
    tiers:
      - name: Standard (No Key Required)
        requestsPerSecond: 1
        notes: Recommended rate; no authentication required
    enforcement:
      - Post no more than 1 URL request per second
      - Large batch jobs should run on weekends or weekdays 9 PM - 5 AM EST
      - IP addresses violating usage policy may be blocked
      - Maximum 500 records per request
      - Maximum offset of 14,999 (projects) or 9,999 (publications)

  - api: ClinicalTrials.gov REST API
    baseURL: https://clinicaltrials.gov/api/v2/
    documentationURL: https://clinicaltrials.gov/data-api/api
    tiers:
      - name: Standard (No Key Required)
        notes: No specific rate limit published; reasonable use expected
    enforcement:
      - No API key required
      - Fair use policy applies
      - Bulk download recommended for large-scale data retrieval

  - api: NCBI Datasets v2 REST API
    baseURL: https://api.ncbi.nlm.nih.gov/datasets/v2/
    documentationURL: https://www.ncbi.nlm.nih.gov/datasets/docs/v2/api/rest-api/
    tiers:
      - name: Standard
        notes: Follows NCBI general API key policies; API key recommended for production use
    enforcement:
      - NCBI API key recommended for sustained programmatic access
      - Same key used across NCBI services

  - api: NCBI BLAST URL API
    baseURL: https://blast.ncbi.nlm.nih.gov/blast/Blast.cgi
    documentationURL: https://ncbi.github.io/blast-cloud/dev/api.html
    tiers:
      - name: Standard
        notes: Asynchronous submission model; poll for results rather than sustained high-frequency requests
    enforcement:
      - Jobs are queued; check status before polling for results
      - Avoid excessive polling; recommended interval is every 5-60 seconds
      - NCBI discourages running more than 5 simultaneous BLAST searches