The studies, in order
Semaglutide research, walked trial by trial.
From the cardiovascular-outcome trials outward to weight, kidney, liver and the head-to-head comparison — the figures logged where the studies confirm them.
Before the details
Semaglutide has one of the deepest trial records of any modern metabolic medicine, and the studies break into a few clusters: heart-outcome trials, weight-loss trials, blood-sugar trials, a kidney trial, a liver trial, and direct comparisons with another drug. The clearest way through is to read the heart data first, because that is the lens this notebook keeps — and because the cardiovascular finding turned out to be the one that reframed the whole drug class. Below, each major finding gets its own heading. The shorthand to know: a hazard ratio (HR) compares how quickly events accumulate in two groups; below 1 favors the drug. A confidence interval (CI) is the range the true effect is likely to sit in. MACE is a common bundled endpoint — cardiovascular death, nonfatal heart attack, and nonfatal stroke counted together. With those three terms, the trial figures read cleanly.
Cardiovascular outcomes: the lens
The largest cardiovascular trial is SELECT. In 17,604 adults with established cardiovascular disease and overweight or obesity but without diabetes, once-weekly semaglutide 2.4 mg reduced the MACE composite by 20% versus placebo (HR 0.80; 95% CI 0.72-0.90; P<0.001) [3]. A 2024 secondary analysis confirmed the reduction and reported it as independent of starting blood-sugar status [13].
The root finding came earlier, in diabetes. SUSTAIN-6 randomized 3,297 people with type 2 diabetes at high cardiovascular risk; semaglutide cut the same MACE endpoint by about a quarter (HR 0.74; 95% CI 0.58-0.95) — a result that, in that population, came alongside more diabetic-retinopathy complications (HR 1.76; 95% CI 1.11-2.78) [2]. The oral formulation was tested for safety in PIONEER 6, where once-daily oral semaglutide was noninferior to placebo for MACE with a numerically lower event rate [8]. And in obesity-related heart failure with preserved ejection fraction, STEP-HFpEF showed once-weekly 2.4 mg improved heart-failure symptoms and physical limitation and produced greater weight loss than placebo [9]. The dedicated semaglutide heart failure page reads that trial in full.
Semaglutide weight loss
The headline weight figure comes from STEP 1: once-weekly semaglutide 2.4 mg produced a mean body-weight change of -14.9% from baseline to week 68, against -2.4% on placebo — a treatment difference of about 12.4 percentage points [1]. That semaglutide weight loss did not arrive alone; exploratory analyses of the STEP program found accompanying improvements across cardiometabolic risk factors, including waist circumference, blood pressure, lipids, glucose and inflammatory markers [10].
The weight effect tracks the mechanism. In rodents, semaglutide reduced food intake and shifted food preference by acting on central appetite circuits — without lowering energy expenditure [4]. In other words, the studies frame the loss as eating less, not burning more. Durability and discontinuation are covered honestly on the effects page: stopping the drug is followed by substantial regain [21].
What is semaglutide
What is semaglutide, structurally? It is a 31-amino-acid acylated analogue of human GLP-1, the gut hormone released after eating, sharing roughly 94% of GLP-1's sequence and carrying a molecular formula of C187H291N45O59. Two deliberate modifications define it: an alpha-aminoisobutyric-acid substitution at position 8 that blocks the enzyme DPP-4 from degrading it, and a C18 fatty di-acid side chain that binds reversibly to albumin in the blood, slowing clearance [17]. Functionally it is a long-acting GLP-1 receptor agonist — a drug that activates the GLP-1 receptor to mimic the natural hormone, potentiating glucose-dependent insulin release, suppressing glucagon, slowing gastric emptying, and reducing appetite [5]. The kidney and metabolic reach of the molecule is broad: in type 2 diabetes with chronic kidney disease, the FLOW trial (n=3,533) found once-weekly 1.0 mg reduced major kidney-disease events by 24% versus placebo (HR 0.76; 95% CI 0.66-0.88) [6].
Semaglutide vs tirzepatide
The most direct comparison is SURMOUNT-5, a head-to-head trial of 751 adults with obesity. At 72 weeks, tirzepatide produced greater mean weight loss than semaglutide (-20.2% vs -13.7%; difference statistically significant, P<0.001) — roughly a 6.5 percentage-point advantage for tirzepatide [7]. The semaglutide vs tirzepatide question therefore has a clear weight-loss answer in that trial: tirzepatide led on the scale.
The comparison is narrower than it looks, though. Tirzepatide acts on two receptors (GIP and GLP-1) where semaglutide acts on one, and the long-term cardiovascular-outcome evidence base is far deeper for semaglutide, anchored by SELECT [3] and SUSTAIN-6 [2]. A weight-loss edge in one obesity trial is not the same as a matched cardiovascular-outcome comparison, which is why this notebook keeps the two endpoints distinct.
Semaglutide peptide: the engineering, and where the data is human vs animal
As a semaglutide peptide, the molecule is a piece of deliberate protein engineering, and it helps to mark which findings are human and which are animal. The pivotal efficacy and outcome trials — STEP for weight [1], SUSTAIN-6 and SELECT for cardiovascular events [2][3], FLOW for kidney outcomes [6], SURMOUNT-5 for the head-to-head [7] — are large human randomized trials. The fine-grained mechanism, by contrast, is largely rodent: the mapping of how semaglutide reaches the brainstem and hypothalamus to suppress intake comes from mouse and rat work [4]. Both layers matter, and conflating them is the usual error online. The drug-interaction record is reassuring and human: semaglutide had no clinically relevant effect on the pharmacokinetics of metformin, warfarin, atorvastatin or digoxin, consistent with low interaction risk despite slowed gastric emptying [12]. Higher oral doses (25 mg and 50 mg) improved glycemic control and weight versus the 14 mg dose in PIONEER PLUS [11].