name: PubMed / NCBI E-utilities Rate Limits
description: Rate limits for the NCBI Entrez E-utilities APIs and PMC APIs. Limits are enforced per IP address (unauthenticated) or per API key (authenticated). NCBI may block IPs that violate usage policies.
provider: PubMed / NCBI
url: https://www.ncbi.nlm.nih.gov/books/NBK25497/
rateLimits:

  - name: Unauthenticated Rate Limit
    description: Maximum requests per second from a single IP address without an API key
    plan: Anonymous
    value: 3
    unit: requests/second
    scope: per IP address
    enforcement: IP-level blocking for sustained violations
    notes: >
      NCBI enforces a hard limit of 3 E-utility requests per second from any
      single IP address without an API key. Exceeding this limit may result in
      temporary or permanent IP blocking.

  - name: Authenticated Rate Limit
    description: Maximum requests per second per API key for registered users
    plan: Registered API Key
    value: 10
    unit: requests/second
    scope: per API key
    enforcement: Per-key throttling; sustained violations may revoke key
    notes: >
      Users with a free NCBI API key can make up to 10 requests per second.
      Higher limits are available upon request for qualifying research institutions
      or applications. Contact NCBI support for elevated access.

  - name: Bulk Download Time Window
    description: Recommended off-peak hours for large-scale data retrieval
    plan: All Plans
    type: recommendation
    window:
      - weekends: all day
      - weekdays: 9:00 PM to 5:00 AM Eastern Time
    notes: >
      NCBI strongly recommends scheduling large batch jobs and bulk downloads
      outside of peak hours (weekdays 9 AM to 9 PM ET) to reduce load on
      production systems. Failure to comply may result in access restrictions.

  - name: Required Parameters for Automated Access
    description: Automated scripts and applications must include tool and email parameters
    plan: All Plans
    type: policy
    parameters:
      - name: tool
        description: Name of the application or script making requests
        required: true (for automated access)
      - name: email
        description: Contact email for the developer or organization
        required: true (for automated access)
    notes: >
      Including tool and email parameters allows NCBI to contact developers
      before blocking access in case of policy violations. Omitting these
      parameters from automated scripts increases the risk of IP blocking
      without prior notice.

enforcement:
  - type: IP blocking
    description: NCBI may block IPs that exceed rate limits or violate usage policies
    contact: info@ncbi.nlm.nih.gov
  - type: API key revocation
    description: API keys may be revoked for sustained policy violations
    contact: https://support.nlm.nih.gov/