ClinicalTrials.gov v2 API Guide

Overview

ClinicalTrials.gov is the world's largest clinical trial registry, maintained by the U.S. National Library of Medicine (NLM) at NIH. It contains over 576,000 study records from 220+ countries covering interventional trials, observational studies, and expanded access programs. The v2 API provides structured JSON access with field-level filtering, cursor-based pagination, and statistics endpoints.

Key v2 improvements over the legacy API: JSON-native responses, sparse field selection via the

fields

nextPageToken

protocolSection

derivedSection

Authentication

No authentication required. All endpoints are publicly accessible without API keys or registration. Users should comply with NCBI usage policies and maintain reasonable request rates.

Core Endpoints

Search Studies

• URL:

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

• Parameters:

RECRUITING

COMPLETED

ACTIVE_NOT_RECRUITING

EARLY_PHASE1

PHASE1

PHASE2

PHASE3

PHASE4

NA

distance(lat,lng,dist)

LastUpdatePostDate:desc

json

csv

• Example:

curl "https://clinicaltrials.gov/api/v2/studies?query.cond=diabetes&query.intr=metformin&pageSize=1&fields=NCTId,BriefTitle,OverallStatus"

• Response:

{
  "studies": [{
    "protocolSection": {
      "identificationModule": {
        "nctId": "NCT06649773",
        "briefTitle": "The Experiment of Noiiglutide Injection in Type 2 Diabetes Patients"
      },
      "statusModule": { "overallStatus": "ACTIVE_NOT_RECRUITING" }
    }
  }],
  "nextPageToken": "ZVNj7o2Elu8o3lpo..."
}

Full responses include

protocolSection

identificationModule

statusModule

descriptionModule

conditionsModule

designModule

armsInterventionsModule

eligibilityModule

outcomesModule

contactsLocationsModule

Get Single Study

• URL:

  GET https://clinicaltrials.gov/api/v2/studies/{nctId}

curl "https://clinicaltrials.gov/api/v2/studies/NCT04280705?fields=NCTId,BriefTitle,OverallStatus,Phase,LeadSponsorName,EnrollmentCount,Condition,InterventionName"

{
  "protocolSection": {
    "identificationModule": {
      "nctId": "NCT04280705",
      "briefTitle": "Adaptive COVID-19 Treatment Trial (ACTT)"
    },
    "statusModule": {
      "overallStatus": "COMPLETED",
      "startDateStruct": { "date": "2020-02-21" },
      "completionDateStruct": { "date": "2020-05-21" }
    },
    "sponsorCollaboratorsModule": {
      "leadSponsor": { "name": "National Institute of Allergy and Infectious Diseases (NIAID)" }
    },
    "conditionsModule": { "conditions": ["COVID-19"] },
    "designModule": { "phases": ["PHASE3"], "enrollmentInfo": { "count": 1062 } },
    "armsInterventionsModule": {
      "interventions": [{ "name": "Placebo" }, { "name": "Remdesivir" }]
    }
  }
}

Database Statistics

• URL:

  GET https://clinicaltrials.gov/api/v2/stats/size

curl "https://clinicaltrials.gov/api/v2/stats/size"

{
  "totalStudies": 576554,
  "averageSizeBytes": 17186,
  "largestStudies": [
    { "id": "NCT02723955", "sizeBytes": 3596689 },
    { "id": "NCT03688620", "sizeBytes": 2865033 }
  ]
}

Field Value Statistics

• URL:

  GET https://clinicaltrials.gov/api/v2/stats/fieldValues/{fieldName}

curl "https://clinicaltrials.gov/api/v2/stats/fieldValues/Phase"

{
  "type": "ENUM",
  "piece": "Phase",
  "field": "protocolSection.designModule.phases",
  "missingStudiesCount": 136632,
  "topValues": [
    { "value": "NA", "studiesCount": 222829 },
    { "value": "PHASE2", "studiesCount": 87478 },
    { "value": "PHASE1", "studiesCount": 63716 },
    { "value": "PHASE3", "studiesCount": 48700 },
    { "value": "PHASE4", "studiesCount": 34911 },
    { "value": "EARLY_PHASE1", "studiesCount": 6179 }
  ]
}

Rate Limits

No formal rate limits are published for the v2 API. Follow NCBI usage guidelines: stay under 3 requests/second without an API key, up to 10/second with one. For bulk data access, use the AACT relational database (https://aact.ctti-clinicaltrials.org/) or downloadable flat files rather than paginating through the full API.

Academic Use Cases

• Systematic reviews: Use

  query.cond

  +

  query.intr

  +

  filter.overallStatus=COMPLETED

  to build PRISMA-compliant trial inventories. Paginate with

  nextPageToken

  to collect all records, then extract outcomes and enrollment for quantitative synthesis.
• Landscape mapping: Combine search with

  stats/fieldValues

  to map phase distributions, sponsor concentration, and geographic spread for a therapeutic area -- useful for identifying evidence gaps in grant proposals.
• Recruitment tracking: Filter by

  RECRUITING

  status and

  filter.geo

  to find active enrollment opportunities. Automate periodic queries for new trials in your domain.

Code Examples

Paginated Collection for Systematic Review

import requests, time

def collect_trials(condition, intervention, status="COMPLETED"):
    base = "https://clinicaltrials.gov/api/v2/studies"
    studies, token = [], None
    while True:
        params = {
            "query.cond": condition, "query.intr": intervention,
            "filter.overallStatus": status, "pageSize": 100,
            "fields": "NCTId,BriefTitle,Phase,EnrollmentCount,CompletionDate",
        }
        if token:
            params["pageToken"] = token
        data = requests.get(base, params=params).json()
        studies.extend(data.get("studies", []))
        token = data.get("nextPageToken")
        if not token:
            break
        time.sleep(0.34)
    return studies

trials = collect_trials("type 2 diabetes", "metformin")
print(f"Collected {len(trials)} completed metformin T2D trials")

Sponsor and Phase Analysis

import requests
from collections import Counter

params = {"query.cond": "Alzheimer's Disease", "pageSize": 100,
          "fields": "NCTId,Phase,LeadSponsorName"}
data = requests.get("https://clinicaltrials.gov/api/v2/studies", params=params).json()

phases, sponsors = Counter(), Counter()
for s in data["studies"]:
    p = s["protocolSection"]
    for ph in p.get("designModule", {}).get("phases", []):
        phases[ph] += 1
    sponsors[p.get("sponsorCollaboratorsModule", {})
              .get("leadSponsor", {}).get("name", "Unknown")] += 1

for ph, n in phases.most_common():
    print(f"{ph}: {n}")

References

• v2 API reference: https://clinicaltrials.gov/data-api/api
• About the API: https://clinicaltrials.gov/data-api/about-api
• AACT database (bulk access): https://aact.ctti-clinicaltrials.org/
• WHO ICTRP: https://trialsearch.who.int/
• NCBI usage policies: https://www.ncbi.nlm.nih.gov/home/about/policies/