Peptide basics
The trial behind most GLP-1 weight claims ran for 68 weeks
On this page
The main semaglutide weight trial, STEP 1, ran for 68 weeks — about sixteen months.
Its registry record runs from June 2018 to primary completion in March 2020.
SELECT, the cardiovascular outcomes trial, ran from October 2018 to June 2023: a little under five years.
Duration decides the question. A 68-week trial can measure weight change; it cannot tell you what happens over a decade.
This page sets out how long the major GLP-1 trials ran and what that duration allows a result to support. It contains no dose, schedule or route and makes no recommendation.
The dates, from the registry
STEP 1 registered a start date of 4 June 2018 and a primary completion date of 30 March 2020. The treatment period it reports is 68 weeks. SURMOUNT-1 ran from 4 December 2019 to 1 April 2022, with a 72-week treatment period.13
The liraglutide SCALE obesity trial is older: 1 June 2011 to 18 March 2013. SELECT is much longer than any of them, running from 24 October 2018 to primary completion on 21 June 2023.24
Why weight trials are short and outcome trials are long
A weight trial measures a continuous quantity that moves within months. Once the change has plateaued, extending the trial adds cost without adding much information about the primary question.1
A cardiovascular outcomes trial measures events — heart attacks, strokes, deaths — which occur at low rates. It has to run for years and enrol many thousands of people simply to accumulate enough of them to compare. SELECT enrolled 17,604 people and ran nearly five years for exactly that reason.2
What a 68-week result cannot tell you
- What happens after the trial ends, whether treatment continues or stops.
- Whether an effect seen at 68 weeks persists at five or ten years.
- Harms that take longer than the trial to appear.
- Whether the population would have drifted back toward baseline given more time.1
How to check duration yourself
The registry record carries a start date and a primary completion date on the front page, and the treatment period is usually stated in the study design section. Both are visible before any results are published, which makes them a reliable way to size up a claim quickly.1
Common questions
Is primary completion the same as the trial finishing?
No. Primary completion is when the last participant was assessed for the main outcome. A trial can continue afterwards for secondary outcomes or extension periods.
Why does the calendar span exceed the treatment period?
Because participants are enrolled over many months. The calendar span covers recruitment plus treatment plus follow-up; the treatment period is what each individual experienced.
Are there longer weight-loss trials?
Extension studies exist and follow participants beyond the main period. They are generally smaller and less strictly controlled, and should be read as a different kind of evidence.
Sources
- Primary studySTEP 1 (NCT03548935) — start and primary completion datesClinicalTrials.gov, U.S. National Library of Medicine, 2021clinicaltrials.gov/study/NCT03548935 ↗↩ Back to text
- Primary studySELECT (NCT03574597) — start and primary completion datesClinicalTrials.gov, U.S. National Library of Medicine, 2023clinicaltrials.gov/study/NCT03574597 ↗↩ Back to text
- Primary studySURMOUNT-1 (NCT04184622) — start and primary completion datesClinicalTrials.gov, U.S. National Library of Medicine, 2023clinicaltrials.gov/study/NCT04184622 ↗↩ Back to text
- Primary studySCALE Obesity and Prediabetes (NCT01272219) — start and completion datesClinicalTrials.gov, U.S. National Library of Medicine, 2015clinicaltrials.gov/study/NCT01272219 ↗↩ Back to text