Semaglutide: The STEP Trials, SELECT, and the Clinical Case for Weekly GLP-1

What semaglutide is

Semaglutide is a synthetic analog of GLP-1, engineered to resist the rapid enzymatic degradation that limits the native hormone to a plasma half-life of roughly two minutes. Novo Nordisk achieved this by making two amino acid substitutions at positions 8 and 34 of the native GLP-1 sequence and attaching a C18 fatty diacid chain to a modified lysine residue at position 26. The fatty acid chain allows semaglutide to bind reversibly to albumin in plasma, which blocks renal filtration and slows enzymatic cleavage. The result is a half-life of approximately seven days, making once weekly subcutaneous dosing practical.

The compound is available under three brand names targeting different uses. Ozempic, at doses of 0.5mg, 1mg, and 2mg weekly by subcutaneous injection, is indicated for type 2 diabetes and for reduction of major cardiovascular events in adults with type 2 diabetes and established cardiovascular disease. Wegovy, at 2.4mg weekly, is indicated for chronic weight management. Rybelsus, an oral formulation at doses of 3mg, 7mg, and 14mg daily, is indicated for type 2 diabetes and represents the first oral GLP-1 receptor agonist approved by the FDA.

FDA approval for Ozempic came in December 2017 for type 2 diabetes, followed by Rybelsus in September 2019 and Wegovy in June 2021. The SELECT cardiovascular outcomes approval for Ozempic in adults with obesity or overweight without diabetes was granted in March 2024, expanding the compound's approved indications beyond glycemic management.

How GLP-1 receptor agonism works

GLP-1 (glucagon-like peptide-1) is secreted by L cells in the small intestine when nutrients reach that part of the gut. It acts on multiple organ systems simultaneously. In the pancreas, it triggers insulin secretion from beta cells in a glucose-dependent manner, meaning it amplifies insulin release only when blood glucose is already elevated rather than forcing insulin secretion unconditionally. It also suppresses glucagon from alpha cells, which reduces hepatic glucose output. In the stomach, it slows gastric emptying, blunting the post-meal glucose spike. In the hypothalamus and brainstem, GLP-1 receptors mediate satiety signaling, and activation in those areas reduces appetite and food intake.

• •Glucose-dependent insulin secretion from pancreatic beta cells, active only when blood glucose is elevated
• •Glucagon suppression from alpha cells, reducing hepatic glucose output between meals
• •Delayed gastric emptying, slowing nutrient absorption and blunting post-meal glucose excursions
• •Hypothalamic and brainstem satiety signaling, reducing appetite and caloric intake
• •Cardiovascular effects including reductions in systolic blood pressure, inflammation markers, and potentially direct cardioprotective signaling through GLP-1 receptors on cardiac tissue

The SUSTAIN program: type 2 diabetes approvals

The SUSTAIN (Semaglutide Unabated Sustainability in Treatment of Type 2 Diabetes) program comprised eight Phase 3 trials that supported Ozempic's FDA approval and its growing label. The trials tested semaglutide at 0.5mg and 1mg weekly against placebo and against active comparators including exenatide extended release, dulaglutide, sitagliptin, and insulin glargine.

The SUSTAIN program also included the PIONEER sub-program for oral semaglutide (Rybelsus), which ran 10 trials across multiple doses and comparators. PIONEER-6, published in NEJM in 2019, was the cardiovascular safety study for the oral formulation, which met non-inferiority criteria for cardiovascular safety in adults with type 2 diabetes at high cardiovascular risk.

The STEP program: obesity and weight management

The STEP (Semaglutide Treatment Effect in People with Obesity) program evaluated semaglutide 2.4mg weekly in adults with obesity or overweight across eight trials. These trials were the evidentiary basis for Wegovy's FDA approval and represented a step change in the expected magnitude of pharmacological weight loss. Participants in the STEP program were typically adults without type 2 diabetes who had a BMI of 30 or above, or 27 or above with at least one weight-related comorbidity such as hypertension, dyslipidemia, or obstructive sleep apnea.

STEP 1, published in NEJM in March 2021, was the pivotal trial for Wegovy's obesity approval. It enrolled 1,961 adults without type 2 diabetes and randomized them to semaglutide 2.4mg weekly or placebo for 68 weeks, with lifestyle intervention in both groups. Mean body weight reduction was 14.9 percent in the semaglutide group versus 2.4 percent with placebo. Approximately 86 percent of semaglutide participants achieved at least 5 percent weight loss, and roughly one third achieved at least 20 percent body weight reduction.

Across the STEP program, the consistency of the weight reduction results across different trial populations and comparator conditions was notable. The average weight loss in adults without type 2 diabetes clustered around 15 percent at 68 weeks across multiple trials, with some participants reaching 20 percent or more. These results established a new reference point for what pharmacological obesity treatment could achieve before tirzepatide and the next generation of agents produced even larger average reductions.

SELECT: cardiovascular outcomes in obesity without diabetes

The SELECT (Semaglutide Effects on Cardiovascular Outcomes in People with Overweight or Obesity) trial addressed a question distinct from weight or glucose management: could semaglutide reduce the risk of serious cardiovascular events in adults with obesity or overweight who did not have diabetes? This was a meaningful question because obesity itself is a cardiovascular risk factor, and the GLP-1 class had already shown cardiovascular benefits in trials enrolling people with type 2 diabetes. Whether those benefits extended to people without diabetes was unknown.

SELECT enrolled 17,604 adults with a BMI of 27 or above and established cardiovascular disease, specifically a history of prior myocardial infarction, prior stroke, or symptomatic peripheral arterial disease, but no diabetes at enrollment. Participants were randomized to semaglutide 2.4mg weekly or placebo. The primary endpoint was the time to first occurrence of a major adverse cardiovascular event: cardiovascular death, non-fatal myocardial infarction, or non-fatal stroke.

Results were published in NEJM in November 2023. Over a median follow-up of approximately 40 months, semaglutide reduced the risk of the primary cardiovascular endpoint by 20 percent compared with placebo (hazard ratio 0.80, 95 percent confidence interval 0.72 to 0.90, p less than 0.001). The event rates were 6.5 percent with semaglutide versus 8.0 percent with placebo. All three components of the composite endpoint were numerically reduced, with cardiovascular death and non-fatal myocardial infarction showing clearer individual-component signals than non-fatal stroke.

• •17,604 participants with obesity or overweight and established cardiovascular disease but no diabetes
• •20 percent relative risk reduction in the primary MACE composite at approximately 40 months median follow-up
• •Hazard ratio 0.80 (95% CI 0.72 to 0.90) for the primary endpoint
• •Cardiovascular benefit appeared early in the trial timeline, before the full magnitude of weight loss was realized, raising questions about mechanisms beyond weight reduction
• •The SELECT approval expanded Ozempic's US label to include cardiovascular risk reduction in adults with obesity and cardiovascular disease without diabetes, effective March 2024

Safety profile and prescribing context

The safety profile of semaglutide across the STEP, SUSTAIN, and SELECT programs is consistent with the GLP-1 receptor agonist class. Gastrointestinal adverse events, primarily nausea, vomiting, diarrhea, and constipation, are the most frequently reported. These events are most prominent during dose escalation and typically diminish after several weeks at a stable dose. The multi-week titration schedule built into the Wegovy prescribing information was designed specifically to reduce the burden of early gastrointestinal intolerance.

The prescribing label carries a boxed warning for thyroid C-cell tumors based on findings in rodent studies at supraphysiological exposures. This warning is shared across the GLP-1 receptor agonist class. Clinical relevance for humans has not been established, and epidemiological studies in large patient cohorts have not found a definitive association. The label contraindicates semaglutide in individuals with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.

Other labeled risks include acute pancreatitis, gallbladder disease, acute kidney injury from severe dehydration related to gastrointestinal adverse events, and diabetic retinopathy complications in people with pre-existing retinopathy. Hypoglycemia risk as monotherapy is low because of the glucose-dependent mechanism, but rises meaningfully when semaglutide is combined with insulin or sulfonylureas; dose reductions of those agents are generally recommended when initiating semaglutide in people already on them.

Muscle mass preservation during semaglutide-induced weight loss is a topic that has received increasing clinical attention. Lean mass loss accompanies fat loss during caloric restriction from any cause, and pharmacologically mediated weight loss is no exception. The degree of lean mass loss relative to total weight loss with GLP-1 agents appears broadly similar to what is observed with dietary restriction or lifestyle intervention producing equivalent weight changes, but this remains an area of active investigation. Resistance exercise during treatment is widely discussed in clinical practice as a strategy to attenuate lean mass loss, though randomized trial data specifically on this combination are limited.

Compound reference pages