Semaglutide (Wegovy, Ozempic): complete evidence guide

Semaglutide is a once-weekly GLP-1 receptor agonist, sold as Ozempic for type 2 diabetes and Wegovy for obesity. At the 2.4 mg Wegovy dose, adults in the STEP 1 trial lost 14.9% of their body weight on average over 68 weeks, a dose reached by starting at 0.25 mg and stepping up every 4 weeks through 0.5, 1 and 1.7 mg. It was the first once-weekly GLP-1 agonist to show more than 10% average weight loss in obesity trials, and its brand names have become shorthand for the whole class. Understanding semaglutide is understanding the baseline everything else in the GLP-1 conversation compares against.

What semaglutide is#

Semaglutide is a synthetic peptide analogue of native glucagon-like peptide-1 (GLP-1). Native GLP-1 is broken down in minutes by the enzyme dipeptidyl peptidase-4 (DPP-4). Semaglutide is engineered with modifications that resist DPP-4 degradation and bind albumin, extending the half-life to approximately one week, which is what enables once-weekly subcutaneous dosing.

Mechanistically, semaglutide activates GLP-1 receptors in three key locations:

• Hypothalamus and brainstem: suppresses appetite signaling, promotes satiety
• Gastrointestinal tract: slows gastric emptying, reduces post-meal glucose spikes
• Pancreatic beta cells: stimulates glucose-dependent insulin release (no hypoglycemia in normal-glucose states)

This is single-receptor pharmacology. Semaglutide does not activate GIP, glucagon, or any other incretin receptor. That's the pharmacological difference from tirzepatide and retatrutide.

Semaglutide GLP-1 receptor activation

The efficacy data#

Obesity: STEP 1. In the STEP 1 trial (Wilding et al., NEJM 2021), semaglutide 2.4 mg weekly produced 14.9% mean body-weight reduction at 68 weeks in adults with obesity. Placebo: 2.4%. Approximately 86% of participants achieved ≥5% weight loss; 50% achieved ≥15%; 32% achieved ≥20%.

Type-2 diabetes: SUSTAIN program. Across the SUSTAIN trials, semaglutide consistently reduced HbA1c by 1.0–1.8 percentage points depending on dose and comparator. It also produced meaningful weight loss as a secondary outcome in T2D patients.

Cardiovascular outcomes: SELECT. The landmark SELECT trial (Lincoff et al., NEJM 2023) enrolled 17,604 adults with overweight or obesity and established cardiovascular disease, without diabetes. Studies have shown semaglutide reduces the risk of major adverse cardiovascular events (non-fatal myocardial infarction, non-fatal stroke, or cardiovascular death) by 20% over placebo. This is the first obesity drug where cardiovascular benefit has been shown in a dedicated outcomes trial, a finding that reshapes how obesity pharmacotherapy is positioned in clinical practice.

MASH. In part 1 of the Phase 3 ESSENCE trial, 62.9% of people on semaglutide 2.4 mg had resolution of steatohepatitis without worsening fibrosis at 72 weeks, versus 34.3% on placebo. On that basis the FDA granted accelerated approval of Wegovy for noncirrhotic MASH with moderate to advanced fibrosis (F2 to F3) on August 15, 2025. Part 2, on liver-related clinical outcomes, is expected to read out in 2029.

Dosing and titration#

Semaglutide for obesity (Wegovy) follows a standardized 16-week escalation before reaching target dose:

For type-2 diabetes (Ozempic), target doses are lower (0.5 mg, 1.0 mg, or 2.0 mg weekly), with at least 4 weeks at each step. The Klarovel titration calculator builds a custom schedule for any target dose.

Reconstitution for research-grade vials: a 5 mg semaglutide vial reconstituted with 2 mL of bacteriostatic water yields 2.5 mg/mL. A 0.25 mg starting dose equals 0.1 mL = 10 syringe units. The Klarovel semaglutide calculator handles any vial-and-dose combination.

Side effects and tolerability#

Gastrointestinal events dominate, as they do across the GLP-1 class:

• Nausea (~44% at the 2.4 mg dose, pooled weight-management trials on the Wegovy label)
• Diarrhea (~30%)
• Vomiting (~24%)
• Constipation (~24%)
• Abdominal pain (~20%)

Most events are mild to moderate, concentrate during dose escalation, and diminish over time. Discontinuation due to adverse events was 7.0% on semaglutide versus 3.1% on placebo in STEP 1, and 6.8% versus 3.2% across the label's pooled trials. At the 7.2 mg dose, dysesthesia (altered skin sensation) becomes common, reported by 22% of patients versus 6% on 2.4 mg.

Less common but class-relevant signals:

• Gallbladder disease, associated with elevated risk as a class effect
• Pancreatitis (rare; class warning)
• Injection-site reactions
• Diabetic retinopathy worsening in pre-existing retinopathy (T2D population)
• Thyroid C-cell tumors in rodents (not confirmed in humans; absolute contraindication in patients with personal or family history of MTC/MEN 2)

Regulatory status and access#

Semaglutide is approved and marketed globally. In the US, Wegovy now comes as a weekly injection (0.25 mg to 2.4 mg, plus 7.2 mg Wegovy HD since March 2026) and, since December 2025, as a once-daily 25 mg tablet. In Norway, a secondary market for this site, both brands hold EU marketing authorizations:

• Ozempic: type-2 diabetes indication
• Wegovy: chronic weight management in adults with obesity or overweight with weight-related comorbidities

Norwegian reimbursement through blå resept for Ozempic in type-2 diabetes requires an individual application to Helfo, with Ozempic used alongside metformin, a sulfonylurea or insulin. Wegovy for obesity is generally not reimbursed: DMP found it cost-effective at a negotiated price in November 2025 for a BMI of 35 or more with two weight-related conditions, but the funding decision rests with the Storting and had not been made as of October 2026. Full out-of-pocket cost via hvit resept is the norm for obesity-only prescriptions.

Shortages affected semaglutide supply from 2022 onwards. The FDA declared the US semaglutide injection shortage resolved on February 21, 2025, which also ended the shortage-based allowance for compounded copies.

Semaglutide vs tirzepatide vs retatrutide#

Semaglutide sits at the first receptor tier of the GLP-1 class:

For newcomers to GLP-1 pharmacology, semaglutide remains the most common first choice: longest real-world track record, best-established cardiovascular benefit, broadest clinical familiarity. Tirzepatide's efficacy advantage is real but comes with less long-term real-world safety data. For the detailed head-to-head, see semaglutide vs tirzepatide.

Direct comparisons: Semaglutide vs retatrutide, Tesofensine vs semaglutide.

Practical considerations#

Three things matter for anyone considering a semaglutide protocol:

1. Titration discipline. The GLP-1 class rewards patience. Staying at each step for the full 4 weeks before escalating is the single best predictor of tolerability. The titration calculator builds the schedule.
2. Bloodwork baseline and follow-up. HbA1c, lipid panel, TSH, and liver enzymes at baseline. Repeat at 12 and 24 weeks. Any new retinopathy symptoms (in T2D patients) warrant an ophthalmology check.
3. Realistic weight-loss expectations. Average weight loss is 14–15% at the highest dose over 14–16 months. Individual variation is wide: some people lose more than 20%, some lose less than 8%. Response is not fully predictable from baseline characteristics.

For the class-level mechanism, see the GLP-1 class explained. For the pharmacology ceiling of the class, see the retatrutide guide. For how semaglutide sits among the alternatives, see the broader guide to peptides for weight loss. Klarovel's questionnaire is the right starting point: it screens for class-level contraindications and maps results against the approved-drug profile.

Semaglutide is the drug that changed the conversation. For well-screened patients, it works. The discipline is in the screening, the titration, and the bloodwork.