Key takeaways

Mean weight reduction of −14.9% over 68 weeks at the 2.4mg dose (STEP-1 trial).
SELECT trial demonstrated a 20% relative reduction in MACE in patients with established CVD and overweight/obesity — the first such evidence for any obesity drug.
Largest body of long-term and post-marketing data of any GLP-1; >15 years of clinical use.
GI side effects (nausea, reflux) are common during titration; usually transient.
UK: POM. Available on the NHS for eligible patients (Wegovy) and via regulated private clinics. Counterfeit pens documented in 2024–2025.

What it is

Semaglutide is a 31-amino-acid GLP-1 receptor agonist that mimics the action of native glucagon-like peptide-1. Modifications to the peptide backbone resist DPP-4 enzymatic breakdown, and a fatty-acid side chain binds reversibly to albumin — together extending the half-life to ~7 days and enabling once-weekly subcutaneous dosing.

It was first approved for type-2 diabetes (Ozempic) in 2017, then for chronic weight management (Wegovy) in 2021. An oral formulation (Rybelsus) is also licensed for diabetes. UK MHRA approval for obesity followed in 2021.

How it works

Semaglutide activates GLP-1 receptors in the pancreas, gut, and brain. The downstream effects compound across systems — improved insulin sensitivity, slowed gastric emptying, and central appetite suppression each contribute to the observed weight reduction.

Step 1

Subcutaneous injection

Weekly dose absorbed into circulation; albumin-bound for slow release.

Step 2

GLP-1 receptor binding

Activates receptors in pancreas, GI tract, and CNS — particularly the hypothalamus.

Step 3

Insulin & glucagon control

Glucose-dependent insulin secretion; glucagon suppressed.

Step 4

Satiety & gastric emptying

Caloric intake falls; slowed gastric emptying produces early satiety.

Weight reduction is driven primarily by reduced caloric intake — most studies report a sustained ~30–35% reduction in food consumption at the maintenance dose.

Benefits

Weight reduction (high confidence)

STEP-1 (n=1,961, 68 weeks) reported a mean weight reduction of −14.9% on 2.4mg vs −2.4% on placebo. Approximately 50% of participants achieved ≥15% weight reduction. Effects begin within 4 weeks of initiation and continue accumulating through ~60 weeks.

Cardiovascular outcomes (high confidence)

The SELECT trial (n=17,604, mean follow-up 39.8 months) demonstrated a 20% relative reduction in MACE (death from CV causes, non-fatal MI, non-fatal stroke) in adults with established cardiovascular disease and overweight/obesity but without diabetes. This is the first hard-outcome trial to show cardiovascular benefit from an obesity drug in a non-diabetic population.

Glycaemic control (high confidence)

Across the SUSTAIN program in type-2 diabetes, semaglutide produced HbA1c reductions of −1.3% to −1.9% across doses, with simultaneous weight reduction. Effective both as monotherapy and add-on to metformin/insulin regimens.

Effect plateau and rebound

Mean weight reduction plateaus around 60 weeks. Discontinuation typically results in regain of approximately two-thirds of lost weight within 12 months (STEP-4 extension data). Maintenance dosing should be considered a long-term rather than time-limited intervention.

Research summary

Once-weekly semaglutide in adults with overweight or obesity (STEP-1)

2021

Phase-3 RCT, double-blind, placebo-controlled·n = 1,961

Mean change in body weight from baseline at 68 weeks: −14.9% with 2.4mg semaglutide vs −2.4% with placebo. 86.4% achieved ≥5% reduction; 50.5% achieved ≥15%.

N Engl J Med · doi:10.1056/NEJMoa2032183

Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT)

2023

Phase-3 RCT, primary CV outcomes·n = 17,604

20% relative risk reduction in primary MACE composite endpoint (HR 0.80, 95% CI 0.72–0.90, p<0.001). First obesity-drug trial to demonstrate cardiovascular benefit in non-diabetic patients.

N Engl J Med · doi:10.1056/NEJMoa2307563

Continued vs discontinued semaglutide for weight maintenance (STEP-4)

2021

Phase-3 RCT, withdrawal design·n = 803

Participants who switched to placebo after 20 weeks regained ~7% body weight by week 68, vs continued reduction (−7.9%) in those maintained on 2.4mg. Confirms long-term-therapy framing.

JAMA · doi:10.1001/jama.2021.3224

Dosage & administration

Semaglutide for chronic weight management is administered as a weekly subcutaneous injection, with a 16–20 week titration schedule. The titration is mandatory for tolerance — skipping steps produces near-universal severe GI symptoms.

Wegovy titration schedule per UK SmPC. Ozempic (diabetes) maxes at 2.0mg.
Phase	Dose	Duration	Notes
Initiation	0.25 mg / week	4 weeks	Tolerance phase only
Step 1	0.5 mg / week	4 weeks	Diabetes maintenance starts here
Step 2	1.0 mg / week	4 weeks	Common diabetes maintenance dose
Step 3	1.7 mg / week	4 weeks	Obesity titration step
Maximum	2.4 mg / week	—	Highest approved obesity dose

Educational summary, not a prescription

Always escalate under prescriber supervision. The MHRA has issued counterfeit warnings — verify pen authenticity through your prescribing clinician.

Side effects & safety

GI symptoms dominate the side-effect profile; tolerability improves substantially after the first 8–12 weeks of titration.

Effect	Frequency	Severity
Nausea	30–44%	Mild–moderate
Diarrhoea	17–30%	Mild
Constipation	11–24%	Mild
Vomiting	10–24%	Mild–moderate
Headache	14%	Mild
Acute pancreatitis	<0.5%	Serious — discontinue

Contraindications: Personal/family history of medullary thyroid carcinoma or MEN-2. Active gallbladder disease. Pregnancy.

UK legal status

Prescription-only medicine (POM) in the UK

Both Wegovy (obesity) and Ozempic (diabetes) are licensed POMs under the Human Medicines Regulations 2012. NHS access for Wegovy follows NICE TA875 criteria — typically BMI ≥35 with at least one weight-related comorbidity. The MHRA has issued counterfeit pen warnings; the Counter Fraud Authority confirmed seizures in 2024 of pens containing insulin or saline rather than semaglutide.

Summary

Semaglutide is the most established and broadly evidenced compound in modern weight management. Its mean weight-reduction signal is smaller than tirzepatide's — but the long-term safety dataset, post-marketing experience, and unique cardiovascular outcome data (SELECT) give it advantages in specific clinical situations. Discontinuation produces predictable rebound, so framing should be long-term not "course-based". UK supply via regulated private clinics is widely available; the grey market is documented to contain counterfeit pens and should be avoided.

References (8)

Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP-1). N Engl J Med. 2021;384(11):989-1002. doi:10.1056/NEJMoa2032183
Lincoff AM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023;389(24):2221-2232.
Rubino D, et al. Effect of continued weekly subcutaneous semaglutide vs placebo on weight loss maintenance (STEP-4). JAMA. 2021;325(14):1414-1425.
Davies M, et al. Semaglutide 2·4 mg once a week in adults with overweight or obesity, and type 2 diabetes (STEP-2). Lancet. 2021;397(10278):971-984.
Wadden TA, et al. Effect of subcutaneous semaglutide vs placebo as adjunct to intensive behavioural therapy on body weight (STEP-3). JAMA. 2021;325(14):1403-1413.
NICE TA875. Semaglutide for managing overweight and obesity. March 2023.
MHRA. Wegovy 0.25/0.5/1/1.7/2.4 mg solution for injection — Summary of Product Characteristics. 2024.
MHRA Drug Safety Update. Counterfeit GLP-1 injector pens: clinician alert. January 2025.

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