ClinicalTrials.gov (NLM, launched February 2000 per FDAMA 1997): 500,000+ registered studies as of 2024. FDAAA 801 (2007): mandatory registration within 21 days of first enrollment for applicable clinical trials (ACTs -- Phase 2+ interventional trials of FDA-regulated drugs/biologics/devices); results reporting within 12 months of primary completion date; penalties up to $10,000/day; NIH grant withholding; but 2015 NEJM study found only 13% reporting on time. Study phases: Phase 0 (microdosing), Phase 1 (safety, 20-80 participants), Phase 2 (efficacy signal, 100-300), Phase 3 (pivotal RCTs, FDA approval basis), Phase 4 (post-marketing); observational (cohort/case-control/cross-sectional) studies phased differently. Key fields: NCT number, official title, brief summary, sponsor type (industry ~50%, NIH/federal ~20%, academic ~30%), study status, phase, allocation, intervention model, masking, primary completion date, enrollment, primary outcome measures, eligibility criteria (inclusion/exclusion), age range, gender, MeSH condition terms, intervention type (drug/device/behavioral/procedure). Disease area composition: oncology ~35%, diabetes/cardiology/psychiatry/ID follow. COVID-19 surge: ~11,000 COVID trials 2020-2021. Publication bias (file drawer problem): AllTrials campaign, Ben Goldacre, COMPARE project, RIAT. ClinicalTrials.gov API v2 at clinicaltrials.gov/api/v2/studies: no API key, pagination by pageSize/pageToken, protocolSection/resultsSection/statusModule/conditionsModule modules. Aggregate stats: ~40% completed, ~25% recruiting, ~15% terminated. Python API query: recruiting Phase 3 oncology trials by enrollment (top 10) + phase distribution for all cancer trials.
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