RESEARCH DIGEST / GLP-1 RECEPTOR AGONIST
Semaglutide cut mean body weight by 14.9% over 68 weeks in the STEP 1 trial. Here is what the studies measured.
A plain-spoken digest of the weight-management and metabolic literature — the trial figures stated calmly with their source, and the open questions kept in plain sight.
The short version
Semaglutide is a once-weekly injectable medicine (also made as a daily pill) that the FDA has approved to treat type 2 diabetes and to manage long-term weight. It is a copy of a natural gut hormone called GLP-1 (a signal your gut sends after you eat that tells the body to release insulin and tells the brain you are full). Because it is built to last about a week in the body, one dose keeps working all week.
In the largest weight trial, people lost on average about 15% of their body weight over roughly 16 months [1]. In people with heart disease, it lowered the risk of heart attacks and strokes [3]. The most common downside is an upset stomach — nausea, especially when the dose is first raised. What people report, including the downsides, is on the effects page. This site is a reading digest of the published studies. It is not a clinic and does not give medical advice or recommend a dose for anyone.
What the largest weight-management trials measured
In the STEP 1 randomized trial, once-weekly subcutaneous (injected under the skin) semaglutide 2.4 mg produced a mean body-weight change of -14.9% from baseline to week 68, versus -2.4% with placebo, in 1,961 adults with overweight or obesity but without diabetes — a treatment difference of roughly 12.4 percentage points [1]. The result held over the long term: in STEP 5, two years of the same once-weekly 2.4 mg dose produced sustained, clinically meaningful weight loss versus placebo [8].
The effect depends on continued treatment. In the STEP 4 randomized-withdrawal trial, people who kept taking semaglutide after a 20-week run-in lost a further 7.9% of body weight, while those switched to placebo regained 6.9% over the same window [9]. That pattern — loss while on treatment, regain after stopping — is the central fact of obesity pharmacotherapy and is covered in full on the semaglutide weight loss page.
The weight effect is driven by reduced food intake, not by burning more energy. In rodent studies, semaglutide reached the brainstem and the hypothalamic arcuate nucleus (a small brain region that sets hunger and fullness), reduced food intake and shifted food preference, with no fall in energy expenditure [4].
Beyond weight: the cardiovascular, kidney and liver evidence
Semaglutide's record extends past the scale. In the SELECT trial of 17,604 adults with established cardiovascular disease and overweight or obesity but no diabetes, once-weekly 2.4 mg reduced major adverse cardiovascular events versus placebo (hazard ratio 0.80; 95% CI 0.72-0.90; P<0.001) — a 20% relative reduction in cardiovascular death, heart attack or stroke [3]. The earlier SUSTAIN-6 trial in type 2 diabetes at high cardiovascular risk found a similar benefit (HR 0.74; 95% CI 0.58-0.95) [2].
In the FLOW trial of 3,533 people with type 2 diabetes and chronic kidney disease, once-weekly 1.0 mg reduced major kidney-disease events — kidney failure, a large drop in kidney function, or kidney or cardiovascular death — by 24% versus placebo (HR 0.76; 95% CI 0.66-0.88) [6]. A 2025 phase-3 trial (ESSENCE) in biopsy-confirmed fatty-liver disease (MASH) reported resolution of liver inflammation in 62.9% of treated patients versus 34.3% on placebo [12]. The mechanism and these trials are read in full on the Semaglutide research page.
The honest open questions
A favorable overall picture is not a clean one. A dedicated safety review concluded semaglutide has an overall favorable risk-benefit profile, with mostly mild-to-moderate transient gastrointestinal effects (nausea in roughly one-third of patients), an increased risk of gallstone disease, and pancreatic and thyroid-cancer signals for which definitive conclusions cannot yet be drawn given their low incidence [5]. The drug carries a boxed warning for thyroid C-cell tumors derived from rodent studies, though a 2024 assessment found no clear human thyroid-cancer signal [15].
Two practical realities round out the picture. Weight tends to return after stopping — in the STEP 1 extension, participants regained a mean of about 11.6 percentage points within a year of discontinuation [17]. And a meaningful share of the weight lost is lean (muscle) mass, not only fat [16]. These are not reasons to dismiss the evidence; they are the context a reader deserves, and they sit alongside the benefits on the Semaglutide effects page.