How to Read a ClinicalTrials.gov Listing

The short version

ClinicalTrials.gov is the US federal registry of clinical studies. By law, most interventional clinical trials studying drugs, biologics, or devices in the United States must be registered before enrollment begins, and results must be posted within 12 months of study completion. For anyone following peptide therapeutic research, ClinicalTrials.gov is the most reliable single source for the current status of ongoing and completed trials: what is being tested, in whom, against what comparator, and with what primary endpoint.

What ClinicalTrials.gov is

ClinicalTrials.gov was established by the FDA Modernization Act of 1997 and expanded significantly by the FDA Amendments Act of 2007 (FDAAA 801), which mandated registration for most phase II-IV trials of FDA-regulated products and required results reporting for applicable trials.^[1] The NIH issued final implementing regulations (42 CFR Part 11) in 2016, extending results reporting requirements and adding specific data element definitions.^[2]

The registry is maintained by the National Library of Medicine (NLM) and is searchable without registration. It is also one of the primary registries recognized by the World Health Organization International Clinical Trials Registry Platform (ICTRP), which aggregates records from 17 registries worldwide.^[3]

Most major international journals require trial registration in an ICMJE-approved registry as a condition of publication, meaning that any trial that eventually publishes in a top-tier journal will have a verifiable registry record that predates its results.^[7]

Anatomy of a listing

Each trial record on ClinicalTrials.gov is organized into several sections. Here is what to look for in each:

NCT identifier

A unique ClinicalTrials.gov identifier in the format NCT followed by eight digits (e.g., NCT03987919). This is the canonical citation identifier for the trial record; journals, regulatory documents, and this hub all use NCT IDs to link directly to trial records. If you see an NCT number in a paper, entering it at clinicaltrials.gov will take you directly to the record.

Sponsor and collaborators

The sponsor is the entity responsible for initiating and managing the trial. For pharmaceutical trials, the sponsor is almost always the drug's manufacturer. Collaborators may include academic medical centres, contract research organizations (CROs), or government agencies co-funding the trial. Knowing the sponsor is essential context for evaluating potential conflicts of interest.

Study type, design, and phase

"Interventional" studies test a defined intervention. "Observational" studies follow participants without assigning interventions. For interventional trials, the design section specifies allocation (randomized vs. non-randomized), intervention model (parallel assignment, crossover, sequential), masking (open label, single blind, double blind, quadruple), and the primary purpose (treatment, prevention, diagnostic, etc.). The phase field indicates Phase 1 through Phase 4 or "Not applicable."

Primary outcome measure

The registered primary outcome includes the measure name, a description, and the time frame at which it will be assessed. Cross-referencing this with the primary endpoint in the eventual publication is a key check for outcome switching. If the registered primary outcome differs from the one labeled "primary" in the paper, that discrepancy requires explanation.^[4]

Secondary and other outcome measures

All prespecified secondary outcomes are listed, though the hierarchical testing order may not be explicit in the registry record. The statistical analysis plan (SAP), which governs multiplicity control, is typically filed with the FDA or EMA rather than posted publicly. Some sponsors post the SAP as a linked document or publish it in a journal.

Eligibility criteria

The inclusion and exclusion criteria define the population in which the trial's results apply. Common exclusions in peptide metabolic trials include: severe renal or hepatic impairment, personal or family history of medullary thyroid carcinoma, prior pancreatitis, active cardiovascular event in the preceding 90 days, and pregnancy. These exclusions mean that the trial population differs from many real-world patients; eligibility criteria are the primary determinant of external validity (generalizability).

Enrollment and locations

The total planned enrollment is shown, along with the list of participating sites by country. Large pivotal trials may run at 200+ sites across 20+ countries. Multi-country trial populations provide broader demographic representation but also introduce geographic heterogeneity in baseline characteristics, standard of care, and background therapies.

Finding a peptide trial

The simplest search is by intervention name. On the ClinicalTrials.gov search page, enter the nonproprietary (generic) name of the peptide in the "Condition or disease" or "Other terms" field, or use the "Interventions" filter. For example, searching for "tirzepatide" will return all registered studies in which tirzepatide is an intervention.

Useful filters to apply after a basic search:

• Status. Restrict to "Recruiting" to find trials currently enrolling, or "Completed" to find trials with available results.
• Phase. Filter to Phase 3 to see the pivotal trials that form the basis for regulatory decisions.
• Study type: Interventional. Excludes observational studies.
• Funder type. "Industry" will return sponsor-funded trials; "NIH" or "U.S. Federal" will return publicly funded trials.

The SURPASS-2 trial of tirzepatide (NCT03987919) is a useful example to practice reading: it is a completed Phase 3 randomized double-blind double-dummy trial with results posted, a full eligibility criteria section, and a published journal article for cross-reference.^[6]

Study status categories

The status field tells you where the trial is in its lifecycle:

• Not yet recruiting. The trial is registered and approved but enrollment has not begun. This status sometimes persists for months while sites are being activated.
• Recruiting. Enrollment is open. Depending on the trial, you can contact the listed investigators or trial sites to inquire about eligibility.
• Enrolling by invitation. Enrollment is restricted to pre-identified participants, typically in follow-on or extension studies.
• Active, not recruiting. All participants have been enrolled and are in follow-up. Efficacy and safety data are being collected; the trial has not yet completed.
• Completed. The trial has finished enrollment and follow-up, and the primary completion date has passed. Results may or may not yet be posted.
• Suspended. Enrollment or treatment has been temporarily halted, typically for a safety review. Suspension may precede termination or may be lifted after the issue is resolved.
• Terminated. The trial was stopped before planned completion. The record should include a brief reason (e.g., safety signal, business decision, futility). Terminated trials with negative signals are important to identify and track, as their results may not be published in journals.
• Withdrawn. The trial was registered but was stopped before any participants were enrolled.

Finding results posted on ClinicalTrials.gov

Under FDAAA 801, sponsors of applicable clinical trials are required to post summary results on ClinicalTrials.gov within 12 months of the primary completion date (the date of final data collection for the primary outcome measure). Results that are submitted to the registry but pending FDA review are given a 30-day review window; the posted results appear under the "Results" tab of the trial record after that window.

The posted results section includes:

• Participant flow (how many were enrolled, completed, and discontinued at each stage).
• Baseline characteristics table (demographics, disease duration, baseline endpoint values).
• Outcome measures table (point estimates and variability measures for each prespecified outcome).
• Adverse events section (serious adverse events by system organ class, and adverse events occurring in a defined frequency threshold).

These posted results are often available before the peer-reviewed publication appears in a journal. They are a useful preliminary source but lack the full statistical context, subgroup analyses, and narrative interpretation that a journal paper provides. They are also not peer-reviewed; the sponsor is responsible for their accuracy.

From registry record to peer-reviewed publication

The registry record and the journal publication are complementary, not redundant. The registry record provides the prespecified protocol; the journal paper provides the statistical analysis and interpretation. Comparing the two is one of the most useful critical appraisal exercises a reader can do:

1. Check the registered primary outcome against the published primary endpoint.Outcome switching (a secondary outcome promoted to primary, or a different time point used) is not always disclosed in the paper.
2. Compare the registered enrollment target against the actual enrollment.A trial that enrolled substantially fewer participants than planned may be underpowered; a trial that enrolled more may have had its power recalculated on interim data.
3. Note if there are registered secondary outcomes that are not reported in the paper.Absent secondary outcomes with unknown results are a form of selective reporting.
4. Check the primary completion date against the results posting date.Delays beyond 12 months in results posting are a compliance failure and may indicate that unfavourable results are being withheld pending publication strategy.

Researchers have found that a significant proportion of registered trials are not published at all, and that published trials frequently report different primary outcomes from those registered. Developing the habit of cross-referencing NCT records with publications gives you a more accurate picture of the evidence base than reading published papers in isolation.^[4]