europepmc.citations

stable Advanced

Europe PMC articles citing a PMID.

GET /v1/europepmc/citations

What you get#

A list of Europepmc results in Onekey's unified shape, in data (an array). An empty array is a normal, successful answer and is charged like any other; a source that reports "nothing found" as an error returns a free 404 not_found instead.

What it costs#

Advanced · 5 credits for a fresh fetch, charged only when it succeeds.
Free when served from cache (0.2 credit for a cached response over 100 KB).
Free if the request is invalid (never charged) or the source fails or reports not found (refunded).

Parameters#

Name	Type	Required	Default	What it is	Constraints
`pmid`	string	yes	(required)	PubMed id (PMID)	max 9 chars; must match `[0-9]{1,9}`; trimmed

Request#

curl "https://api.onekey.dev/v1/europepmc/citations?pmid=33087721" \
  -H "x-api-key: YOUR_API_KEY"

Try it

Add your key to run this call right here. No key? Get one free. The key stays in this tab.

Recorded response#

A real Europepmc response, recorded and returned in Onekey's shape. Fields the platform does not provide are null.

json

{
  "success": true,
  "data": [
    {
      "content": {
        "id": "42647127",
        "title": "Engineering antigenic breadth against SARS-CoV-2 by pairing divergent RBDs within a single mRNA immunogen.",
        "year": 2026
      },
      "author": {
        "displayName": "Montgomerie I, McKenzie RE, Palmer OR, Mason NC, Kuang J, Pankhurst TE, Draper SL, von Daake S, Eccles DA, Bird TW, Martin AL, Green I, White L, Robinson Z, Marshall A, Minnell JJ, Small SJ, Hermans IF, Painter GF, Ussher JE, Quiñones-Mateu ME, Patrick WM, Comoletti D, Connor LM."
      },
      "ext": {
        "source": "MED"
      }
    },
    {
      "content": {
        "id": "PPR1283099",
        "title": "A human monoclonal antibody from hybrid immunity exhibits exceptional neutralization breadth against antigenically divergent contemporary Omicron JN.1 lineage variants",
        "year": 2026
      },
      "author": {
        "displayName": "Rathour D, Shrivas S, Kumar N, Prasad C, Singh J, Singh G, Surendranath A, Rathi S, Verma S, Chauhan S, Singh B, Sutar J, Chattopadhyay S, Batra G, Sonar S, Eckerle I, Subbissi L, Mohan A, Asthana S, Deshpande S, Bhattacharya J."
      },
      "ext": {
        "source": "PPR"
      }
    }
  ],
  "metadata": {
    "platform": "europepmc",
    "endpoint": "citations",
    "cached": false,
    "stale": false,
    "age_s": 0,
    "fetched_at": "2026-01-01T12:00:00+00:00",
    "credits_charged": 5,
    "request_id": "7f3a9c2e5b1d4c8fa2e6d0b9c4f18e37"
  },
  "error": null
}

Field mapping#

How this endpoint's source fields become Onekey's unified schema. Fields the source lacks come back as null.

Source path	Onekey field	Transform
`id`	`content.id`	-
`title`	`content.title`	-
`authorString`	`author.displayName`	-
`pubYear`	`content.year`	`toNumber`
`source`	`ext.source`	-

Caching and freshness#

Fresh for about 1 hour. Repeat calls in that window are served from cache and are free.
Between 1 hour and 1 day the cached copy is still served instantly and marked stale: true while Onekey refreshes it in the background.
If the source is down when a refresh is needed, the last copy can be served for up to 3 days more (marked stale).
A "not found" is remembered for 5 minutes and is free.
The cache is shared across all customers, which is what keeps repeat calls cheap.

What can go wrong#

Status	Code	What to do
401	`unauthorized`	Check the `x-api-key` header.
402	`insufficient_credits`	Add credits; your balance must cover the price.
404	`not_found`	Unknown endpoint path.
422	`invalid_params`	Fix the parameters using the table above. Free.
429	`rate_limited`	Wait `Retry-After` seconds, then retry.
502	`upstream_error`	The source failed. Refunded; retry with backoff.

See Errors for the full list and retry guidance.

Status: stable (verified against the live source).