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Having an API key will allow you to access the E-utilities at a faster rate, effective on May 1, 2018. After the stated date, if you do not have a key NCBI will limit your rate of access to 3 E-utility requests per second. Otherwise, you will receive an error message. Having a key will increase the limit to 10 requests/second. The stated limits apply to any combination of requests to EInfo, ESearch, ESummary, EFetch, ELink, EPost, ESpell, EGquery, and ECitMatch from a single computer (IP address).
Note that you won't need a key if you require less than 3 requests/second even after May 1, 2018. If you you expect your usage to exceed 10 requests/second, please contact us and we can negotiate a higher rate limit.

An API key is a unique string (value) that you include in your HTTP requests that identifies you to NCBI servers. You can generate a key through these steps:

  • Sign in to your NCBI account
    • (If you do not have an account: use Register for an NCBI account link at the bottom of the sign in box on the NCBI account page and follow the steps as provided.)
    • Use your existing NCBI account that you might have created for another purpose
  • Once signed in, access your account's settings by clicking on your user name that displays in the top right corner of any NCBI page.
  • Scroll down the page to the section titled API Key Management.
  • Click on the Create an API Key button that will generate a key (unique string of characters) displayed in the API Key Management box.
  • To use the key, copy the string and add it as the api_key parameter to any E-utilities request, for example:

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/einfo.fcgi?db=pubmed&api_key=ABCD123

For additional insight into NCBI's rationale for introducing API keys see the blog on API Keys. For detailed help and usage policy see our documentation.