ClinicalTrials.gov (v2 API)

Base URL

https://clinicaltrials.gov/api/v2/

Auth

No API key required. Fully public.

Key Endpoints

Search studies

GET /studies

Key parameters:

• query.cond — condition/disease (e.g. breast cancer)
• query.intr — intervention/treatment (e.g. pembrolizumab)
• query.term — general search terms
• query.spons — sponsor
• query.id — NCT ID
• filter.overallStatus — pipe-delimited: RECRUITING|COMPLETED|ACTIVE_NOT_RECRUITING|...
• filter.phase — PHASE1|PHASE2|PHASE3|PHASE4|NA
• filter.geo — distance(lat,lon,dist) e.g. distance(38.89,-77.03,50mi)
• fields — comma-separated field list to reduce payload
• sort — e.g. LastUpdatePostDate:desc
• pageSize — results per page (default 10, max 1000)
• pageToken — cursor for next page (from nextPageToken in response)
• countTotal=true — include total count

Example — recruiting Phase 3 breast cancer trials:

/studies?query.cond=breast+cancer&filter.overallStatus=RECRUITING&filter.phase=PHASE3&pageSize=5&countTotal=true

Response structure:

{
  "totalCount": 1234,
  "studies": [
    {
      "protocolSection": {
        "identificationModule": {"nctId": "NCT05123456", "briefTitle": "..."},
        "statusModule": {"overallStatus": "RECRUITING"},
        "designModule": {"phases": ["PHASE3"], "enrollmentInfo": {"count": 500}},
        "conditionsModule": {"conditions": ["Breast Cancer"]},
        "eligibilityModule": {"minimumAge": "18 Years", "sex": "ALL"}
      }
    }
  ],
  "nextPageToken": "CAYQAg"
}

Single study by NCT ID

GET /studies/{nctId}

Example: /studies/NCT05123456

Study count

GET /stats/size?query.cond={condition}&filter.overallStatus=RECRUITING

Field metadata

GET /studies/metadata

Pagination

Uses cursor-based pagination via pageToken (NOT numeric offsets). Include countTotal=true on first request to get total.

Rate Limits

No API key. Be reasonable — a few requests per second. Bulk: https://clinicaltrials.gov/AllAPIJSON.zip