Semaglutide research: what the landmark trials measured.

The short version

The semaglutide research record is unusually deep for a single molecule: tens of thousands of trial participants across diabetes (SUSTAIN, PIONEER), weight management (STEP), heart outcomes (SELECT), kidney outcomes (FLOW) and now fatty liver (ESSENCE).

Start with the glucose story. In type 2 diabetes, semaglutide reliably lowers HbA1c (the three-month average blood-sugar marker) and trims weight. Then the outcome trials: it cut heart attacks and strokes in people with diabetes and in people with heart disease, and slowed kidney decline in diabetes with kidney disease. In a head-to-head against tirzepatide it lost on weight but still produced large losses.

Below, each finding is a single block with the trial, the size, the result and a citation. Where a study found a downside — like the eye-disease signal in SUSTAIN-6 — it is in the same block, not hidden.

How semaglutide controls blood sugar in type 2 diabetes

The glycemic evidence is the foundation. In SUSTAIN 1, once-weekly semaglutide monotherapy (0.5 and 1.0 mg) significantly reduced HbA1c and body weight versus placebo in drug-naive adults with type 2 diabetes at 30 weeks ^[8]. In SUSTAIN 7, once-weekly semaglutide produced greater reductions in HbA1c and body weight than once-weekly dulaglutide ^[11]. SUSTAIN FORTE then showed that a 2.0 mg dose lowers HbA1c more than the 1.0 mg dose, supporting an approved higher diabetes dose ^[12].

The oral formulation carries the same effect into a tablet. PIONEER 1 established the efficacy and safety of once-daily oral semaglutide (3, 7, 14 mg) versus placebo as monotherapy in type 2 diabetes, with dose-dependent HbA1c and body-weight reductions at 26 weeks ^[10]. The oral program's glycemic and weight benefits, with a cardiovascular profile consistent with the GLP-1 receptor agonist class, are reflected across the PIONEER trials ^[10].

Semaglutide and cardiovascular outcomes

The cardiovascular trials reframed semaglutide. In SUSTAIN-6, once-weekly semaglutide (0.5 or 1.0 mg) reduced the composite of cardiovascular death, nonfatal heart attack and nonfatal stroke (HR 0.74; 95% CI 0.58-0.95) in type 2 diabetes at high cardiovascular risk; the same trial recorded a higher rate of diabetic-retinopathy complications (HR 1.76; 95% CI 1.11-2.78), concentrated in people with pre-existing eye disease undergoing rapid glucose correction ^[2].

SELECT extended the benefit to people without diabetes. In adults with established cardiovascular disease and overweight or obesity but no diabetes (n=17,604), once-weekly semaglutide 2.4 mg reduced major adverse cardiovascular events versus placebo (HR 0.80; 95% CI 0.72-0.90; a 20% relative risk reduction) ^[3]. Together these trials anchor the cardiovascular-risk-reduction indication.

Semaglutide weight loss

The weight evidence is led by STEP 1. In that randomized trial, once-weekly subcutaneous semaglutide 2.4 mg produced a mean body-weight change of -14.9% from baseline to week 68, versus -2.4% with placebo, in adults with overweight or obesity without diabetes — a treatment difference of about 12.4 percentage points ^[1].

The central mechanism behind that loss has been mapped in animals: semaglutide lowered body weight in rodents by acting through distributed brain pathways — reaching the brainstem, area postrema, hypothalamic arcuate nucleus and parabrachial nucleus — reducing food intake and modifying food preference without lowering energy expenditure ^[4]. In other words, the weight comes off because people eat less, not because the body burns more. The body-composition caveat — that part of the lost weight is lean mass — is covered on the Semaglutide effects page ^[16].

Semaglutide vs tirzepatide

The most direct comparison is SURMOUNT-5, a head-to-head trial. In adults with obesity (n=751), tirzepatide produced greater mean weight loss than semaglutide at 72 weeks (-20.2% vs -13.7%; difference statistically significant, P<0.001) — about a 6.5 percentage-point advantage for tirzepatide ^[7].

The difference is consistent with their pharmacology: tirzepatide engages two receptors (GIP and GLP-1) while semaglutide engages one (GLP-1). On weight, the head-to-head favors tirzepatide; that does not erase semaglutide's own large effect, nor its independently established cardiovascular and kidney outcome benefits, which is why both molecules occupy the field. No brand names are used here — only the generic compound names.

Kidney, liver and the newest outcomes

The kidney evidence comes from FLOW. In type 2 diabetes with chronic kidney disease (n=3,533), once-weekly semaglutide 1.0 mg reduced major kidney-disease events — kidney failure, a 50% or greater fall in eGFR, or kidney or cardiovascular death — versus placebo (HR 0.76; 95% CI 0.66-0.88), a 24% lower risk ^[6].

The liver evidence is the newest. The ESSENCE phase-3 trial established histologic benefit in metabolic dysfunction-associated steatohepatitis (MASH), a progressive fatty-liver disease with inflammation and scarring, underpinning the 2025 MASH indication ^[13]. Across all of these, the glycemic effect is the common thread — the same GLP-1 receptor agonism that steadies blood sugar also drives the weight, heart, kidney and liver benefits.

Compounded semaglutide

Compounded semaglutide is semaglutide prepared by a compounding pharmacy rather than the approved manufactured product. During the federally declared shortage (roughly 2022 to early 2025), compounding pharmacies were permitted to produce it ^[3]. The FDA documented dosing errors, adverse events requiring hospitalization, and products with unverified or non-pharmaceutical active ingredients during that period. After the shortage was declared resolved in 2025, the compounding pathways were curtailed, leaving ongoing regulatory and telehealth uncertainty. Compounded or non-pharmaceutical sources fall outside the approved-product evidence base summarized on this site and carry documented quality and safety concerns.