Key facts
- Class: GLP-1 receptor agonist — a 31-amino-acid analogue of human glucagon-like peptide-1, modified with a fatty-acid chain so it survives roughly a week in circulation.
- Brand names: Ozempic (weekly injection, type 2 diabetes), Wegovy (weekly injection, weight management), Rybelsus (daily tablet, type 2 diabetes), plus an oral Wegovy 25 mg tablet for weight management.
- Developer: Novo Nordisk. First approved in the United States in December 2017.
- Status: FDA approved and prescription-only. Not a supplement, not a research chemical, not legally sold over the counter.
- Route: Subcutaneous injection once weekly, or oral tablet once daily paired with the absorption enhancer SNAC.
- Evidence level: Very strong. Dozens of phase 3 randomized trials plus a 17,604-person cardiovascular outcomes trial — among the best-studied metabolic drugs in existence.
- Common reported effects: Nausea, diarrhea, vomiting, constipation, abdominal pain, burping; more gallbladder events than placebo; boxed warning for thyroid C-cell tumors observed in rodents.
What is semaglutide and how does it work?
Semaglutide is a synthetic analogue of glucagon-like peptide-1, a gut hormone released after eating. Native GLP-1 slows gastric emptying, stimulates glucose-dependent insulin release, suppresses glucagon and signals satiety in the hypothalamus — but it is destroyed by the enzyme DPP-4 within a couple of minutes, which is why the natural hormone was never a viable drug (PubMed 17928588).
Chemists solved that with two changes: substituting the amino acid at position 8 to block DPP-4 cleavage, and attaching a C18 fatty di-acid via a linker so the molecule binds reversibly to albumin. Bound to albumin, semaglutide is shielded from renal clearance and drifts through circulation with a half-life near a week — which is what makes once-weekly dosing possible (PubMed 31050435). The same engineering trick underpins most modern GLP-1 receptor agonists.
The clinically relevant consequence is appetite. Trial participants consistently report eating less because they want less, not because they are exercising restraint. That mechanism sits inside a broader shift in how medicine treats appetite as a hormonal signal rather than a willpower problem (PubMed 34067710).
Ozempic, Wegovy and Rybelsus: what is the difference?
All three contain the same molecule. The differences are the approved indication, the maximum strength, and the delivery format.
Ozempic is the weekly injection approved for type 2 diabetes in 2017, topping out at 2.0 mg. Wegovy is the weekly injection approved for chronic weight management in 2021, going to 2.4 mg. Rybelsus is the daily tablet approved for type 2 diabetes in 2019 at 7 mg and 14 mg. In December 2025 the FDA added a 25 mg oral Wegovy tablet for weight management — the first oral GLP-1 approved for that use.
This distinction matters more than it looks. Nearly every headline weight-loss figure you see attached to the word Ozempic actually comes from a Wegovy-strength trial. Insurance coverage, pricing and supply also track the brand rather than the molecule, which is much of why the off-label and compounded markets grew the way they did.
Does semaglutide actually work for weight loss?
Yes, and the effect size is large by the standards of obesity pharmacotherapy. STEP 1 randomized 1,961 adults with a BMI of 30 or higher (or 27 with a weight-related condition) and no diabetes to weekly semaglutide 2.4 mg or placebo for 68 weeks, both arms with lifestyle counseling. Mean body weight fell 14.9% on semaglutide versus 2.4% on placebo. Roughly 86% of the treated group lost at least 5%, 69% lost at least 10%, and 50.5% lost at least 15% (PubMed 33567185).
For context, the previous best-in-class injectable, liraglutide 3.0 mg, delivered about 8% mean loss in the SCALE program (PubMed 26132939). When the two were put head to head in STEP 8, semaglutide produced 15.8% versus 6.4% at 68 weeks (PubMed 35015037). That roughly two-and-a-half-fold gap is why semaglutide reset expectations for the whole category.
Durability held in STEP 5, which ran the same comparison for 104 weeks: 15.2% mean loss versus 2.6% on placebo, with the curve plateauing rather than rebounding while treatment continued (PubMed 36216945).
One important caveat: people with type 2 diabetes lose less. Across the STEP program, diabetic cohorts landed nearer 6 to 10% rather than 15%, a pattern seen with every drug in the class (PubMed 33767808).
What happens when you stop semaglutide?
This is the finding that gets left out of most coverage, and it is the single most useful thing to understand before starting.
A prespecified extension of STEP 1 followed 327 participants for a year after both the drug and the structured lifestyle program were withdrawn. They regained roughly two-thirds of the weight they had lost, ending about 5.6% below their original baseline instead of the 17.3% they had reached on treatment. Improvements in blood pressure, lipids and HbA1c reverted toward pre-treatment values along the same curve (PubMed 35441470).
The interpretation is not that the drug failed. It is that semaglutide suppresses appetite signaling only while present, so removing it removes the effect — the same way blood pressure rises again when an antihypertensive is stopped. Obesity medicine now frames GLP-1 therapy as chronic management rather than a course of treatment, and that framing changes how people plan for cost, adherence and long-term monitoring. If you want to see your own trajectory rather than guess at it, keeping a continuous log of weekly doses and weight is the only way to know what happened during a gap.
Does semaglutide protect the heart?
SELECT is the trial that turned semaglutide from a metabolic drug into a cardiovascular one. It enrolled 17,604 adults aged 45 and older with established cardiovascular disease — prior myocardial infarction, prior stroke or symptomatic peripheral artery disease — plus a BMI of 27 or higher and no diabetes, and randomized them to weekly semaglutide 2.4 mg or placebo on top of standard care.
Over a mean follow-up near 40 months, the primary composite endpoint of cardiovascular death, nonfatal myocardial infarction or nonfatal stroke occurred in 6.5% of the semaglutide group versus 8.0% on placebo — a hazard ratio of 0.80, or a 20% relative risk reduction. Mean weight change was 9.4% versus 0.9% (PubMed 37952131).
Two things are worth holding onto. First, this was a secondary-prevention population — people who had already had a cardiac event. It does not establish the same benefit in a healthy 35-year-old taking semaglutide cosmetically. Second, discontinuation for adverse events was roughly twice as high on semaglutide (16.6% versus 8.2%), which is a real-world tolerability signal, not a footnote.
Oral semaglutide vs the injection
Peptides are notoriously hard to deliver orally: stomach acid and proteases destroy them, and intestinal absorption of a 4-kilodalton molecule is poor (PubMed 15984901). Rybelsus solved it by co-formulating semaglutide with SNAC, an absorption enhancer that transiently raises local gastric pH and helps the peptide cross the stomach lining. Bioavailability is still around 1%, which is why oral strengths are an order of magnitude higher than injectable ones.
The OASIS 4 trial tested a 25 mg once-daily tablet in 307 adults with overweight or obesity and no diabetes. Over 64 weeks on treatment, mean weight loss was 13.6% versus 2.2% on placebo, rising to 16.6% among participants who took the drug as directed. Gastrointestinal adverse events occurred in 74.0% versus 42.2% (PubMed 40934115). No head-to-head trial has pitted the tablet against the pen, so cross-trial comparison is the best available evidence — but the ranges overlap substantially.
Semaglutide side effects reported in trials
Gastrointestinal effects dominate and are the main reason people stop. In STEP 1, GI adverse events occurred in about 74% of the semaglutide group versus 48% on placebo: nausea, diarrhea, vomiting, constipation, abdominal pain and burping, mostly mild to moderate and concentrated around dose-escalation steps. Discontinuation for adverse events was 7.0% versus 3.1% (PubMed 33567185).
Gallbladder disorders, including cholelithiasis, were more common on semaglutide — partly a known consequence of rapid weight loss in general, partly a direct effect on gallbladder motility.
Rarer but more serious signals were quantified in a 2023 population study comparing GLP-1 users prescribed the drugs for weight loss against bupropion-naltrexone users. It found elevated hazard ratios for pancreatitis (9.09), bowel obstruction (4.22) and gastroparesis (3.67). Absolute rates remained low — on the order of one gastroparesis case per thousand person-years — but the relative signal contributed to labeling updates that added ileus as a post-marketing event (PubMed 37796527).
Semaglutide also carries a boxed warning for thyroid C-cell tumors, based on rodent studies; it is contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN2. No causal link has been established in humans, and the warning applies across the class.
Semaglutide vs tirzepatide and other GLP-1 drugs
Semaglutide hits one receptor. Tirzepatide hits two — GLP-1 and GIP — and in SURMOUNT-1 it produced up to 20.9% mean weight loss over 72 weeks (PubMed 35658024). SURMOUNT-5 then compared the two directly in 751 adults with obesity and no diabetes: 20.2% on tirzepatide versus 13.7% on semaglutide 2.4 mg at 72 weeks (PubMed 40353578). Our full breakdown of that trial lives in tirzepatide vs semaglutide.
The next step is combination. Coadministered cagrilintide and semaglutide — an amylin analogue paired with the GLP-1 agonist — reached roughly 20% mean weight loss in a phase 3 program, closing most of the gap without adding a second incretin receptor (PubMed 40544433).
| Trial | Drug and weekly dose | Population | Duration | Mean weight change |
|---|---|---|---|---|
| STEP 1 | Semaglutide 2.4 mg | 1,961 adults, obesity, no diabetes | 68 weeks | −14.9% vs −2.4% placebo |
| STEP 5 | Semaglutide 2.4 mg | 304 adults, overweight or obesity | 104 weeks | −15.2% vs −2.6% placebo |
| STEP 8 | Semaglutide 2.4 mg vs liraglutide 3.0 mg daily | 338 adults, no diabetes | 68 weeks | −15.8% vs −6.4% |
| SELECT | Semaglutide 2.4 mg | 17,604 adults, prior CV disease, no diabetes | ~40 months | −9.4% vs −0.9%; MACE 6.5% vs 8.0% |
| OASIS 4 | Oral semaglutide 25 mg daily | 307 adults, overweight or obesity | 64 weeks on treatment | −13.6% vs −2.2% placebo |
| SURMOUNT-5 | Tirzepatide vs semaglutide 2.4 mg | 751 adults, obesity, no diabetes | 72 weeks | −20.2% vs −13.7% |
Is compounded semaglutide legal?
The compounded market existed because of a loophole in United States drug law. When a drug appears on the FDA shortage list, compounding pharmacies may legally prepare copies of it. Semaglutide sat on that list through the demand surge — and telehealth clinics, med spas and online sellers built businesses on the exemption.
Semaglutide was removed from the FDA shortage list in February 2025, which closed the mass-compounding pathway. Patient-specific compounding under a prescription continues in narrower circumstances, and enforcement has tightened further since, including recalls tied to sterility failures at individual facilities. Anything sold as semaglutide without a prescription, marketed as research-use-only, or shipped from an unregistered overseas source falls outside the regulated system entirely.
The safety concern is concrete, not theoretical. A poison control case series documented patients who self-administered ten-fold overdoses of compounded semaglutide, because vials plus generic syringes remove the fixed-dose protection built into a manufactured pen and invite confusion between milliliters, units and milligrams. Symptoms included prolonged vomiting and abdominal pain (PubMed 37392810). For how these categories differ legally, see our overview of peptide legal status.
How long does semaglutide take to work?
Appetite suppression frequently registers within the first two to four weeks, but visible weight change lags well behind that. Every phase 3 protocol escalates the dose in steps over roughly 16 to 20 weeks before reaching the target strength, so early weeks are as much about tolerability as efficacy.
In STEP 1 the weight curve was still descending at week 60 and only began flattening near week 68 (PubMed 33567185). STEP 5 showed the same shape: most of the total loss banked by roughly week 60, then held for the remaining year (PubMed 36216945). Judging the drug at week 8 means judging it during escalation, before it has reached the strength any of the headline numbers were generated at. More on the wider category in peptides for weight loss.
Frequently asked questions
How much weight do people lose on semaglutide?
In STEP 1, adults with obesity and no diabetes lost a mean of 14.9% of body weight over 68 weeks on once-weekly semaglutide 2.4 mg, against 2.4% on placebo. About half reached 15% or more. STEP 5 extended this to two years with 15.2% mean loss. Results in people with type 2 diabetes are consistently smaller, roughly 6 to 10%.
Is semaglutide FDA approved?
Yes. Semaglutide is FDA approved and prescription-only in the United States. Ozempic was approved for type 2 diabetes in 2017, Rybelsus as an oral tablet for type 2 diabetes in 2019, Wegovy for chronic weight management in 2021, and an oral 25 mg Wegovy tablet for weight management in December 2025. It is not a supplement and not legally sold over the counter.
Do you regain weight after stopping semaglutide?
Usually, yes. In the STEP 1 extension, participants who stopped semaglutide and the trial lifestyle program regained roughly two-thirds of the weight they had lost within one year, ending about 5.6% below their original baseline. Blood pressure, lipids and HbA1c also drifted back toward pre-treatment values. Obesity is treated as a chronic condition for this reason.
What are the most common semaglutide side effects?
Gastrointestinal effects dominate: nausea, diarrhea, vomiting, constipation, abdominal pain and burping. In STEP 1 they affected about 74% of the semaglutide group versus 48% on placebo, were mostly mild to moderate, and clustered around dose escalation. Gallbladder disease was more frequent. Rarer signals include pancreatitis and gastroparesis. Semaglutide carries a boxed warning for thyroid C-cell tumors seen in rodents.
Is oral semaglutide as effective as the injection?
The 25 mg oral tablet lands in a similar range. In OASIS 4, participants lost 13.6% of body weight at 64 weeks versus 2.2% on placebo, and 16.6% when treatment was taken as directed, which is close to the 14.9% seen with weekly 2.4 mg injections in STEP 1. The trials were separate, so this is not a head-to-head result.
Is semaglutide better than tirzepatide?
For weight loss, tirzepatide won the only direct comparison. In SURMOUNT-5, 751 adults with obesity and no diabetes lost 20.2% on tirzepatide versus 13.7% on semaglutide 2.4 mg over 72 weeks. Semaglutide has the larger cardiovascular outcome dataset from SELECT and more years of real-world use, so better depends on the outcome that matters to you.
Is compounded semaglutide legal?
The legal basis narrowed sharply. Compounders could mass-produce semaglutide copies while the drug sat on the FDA shortage list, but semaglutide was removed from that list in February 2025. Patient-specific compounding continues in limited circumstances under regulatory pressure. Compounded vials also lack the fixed-dose pen, and a poison control case series documented ten-fold self-administration errors.
How long does semaglutide take to work?
Appetite changes often appear within the first few weeks, but scale movement is slow because trial protocols escalate the dose over roughly 16 to 20 weeks. In STEP 1 the weight curve was still falling at week 60 and only flattened near week 68. STEP 5 showed most of the loss banked by about week 60, then held for the remainder of two years.
Does semaglutide help the heart?
In SELECT, 17,604 adults with established cardiovascular disease, a BMI of 27 or higher and no diabetes were followed for a mean of about 40 months. Major adverse cardiovascular events occurred in 6.5% on semaglutide versus 8.0% on placebo, a 20% relative reduction. That result applies to a high-risk secondary-prevention population, not to everyone taking the drug.
Sources
Every claim above traces to peer-reviewed literature indexed on PubMed:
- Once-Weekly Semaglutide in Adults with Overweight or Obesity — PubMed 33567185
- Weight regain and cardiometabolic effects after withdrawal of semaglutide — PubMed 35441470
- Two-year effects of semaglutide in adults with overweight or obesity: the STEP 5 trial — PubMed 36216945
- Effect of Weekly Subcutaneous Semaglutide vs Daily Liraglutide on Body Weight in Adults With Overweight or Obesity Without Diabetes: The STEP 8 Randomized Clinical Trial — PubMed 35015037
- Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes — PubMed 37952131
- Oral Semaglutide at a Dose of 25 mg in Adults with Overweight or Obesity — PubMed 40934115
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity — PubMed 40353578
- Tirzepatide Once Weekly for the Treatment of Obesity — PubMed 35658024
- Risk of Gastrointestinal Adverse Events Associated With Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss — PubMed 37796527
- Administration errors of compounded semaglutide reported to a poison control center — case series — PubMed 37392810
- GLP-1 receptor agonists: an updated review of head-to-head clinical studies — PubMed 33767808
- The physiology of glucagon-like peptide 1 — PubMed 17928588
- Glucagon-Like Peptide-1 Receptor Agonists and Strategies To Improve Their Efficiency — PubMed 31050435
- Oral delivery of peptide drugs: barriers and developments — PubMed 15984901
- Coadministered Cagrilintide and Semaglutide in Adults with Overweight or Obesity — PubMed 40544433
- A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management — PubMed 26132939
- The Role of Peptide Hormones Discovered in the 21st Century in the Regulation of Appetite and Metabolism — PubMed 34067710