Semaglutide reached the market on the back of one of the larger phase 3 programs in metabolic medicine. Novo Nordisk's drug appears under three brand names in the United States: Ozempic and Rybelsus for type 2 diabetes, and Wegovy for weight management. The trials behind those approvals were not interchangeable. They tested different doses, in different populations, against different primary endpoints. This is a summary of what each program published, with citations. It is reporting of the trial record, not medical or dosing advice.
SUSTAIN: type 2 diabetes, injectable
The SUSTAIN trials (the name stands for Semaglutide Unabated Sustainability in Treatment of type 2 diabetes) were the phase 3a glycemic-control studies that supported Ozempic's 2017 FDA approval. Across SUSTAIN 1 through 5 and 7, the primary endpoint was change in HbA1c, typically at week 30, with subcutaneous doses of 0.5 mg and 1.0 mg weekly tested against placebo or active comparators including sitagliptin, exenatide ER, insulin glargine, and dulaglutide. In SUSTAIN 1 (monotherapy vs placebo, Lancet Diabetes & Endocrinology 2017), HbA1c fell by roughly 1.45 percentage points on 0.5 mg and 1.55 on 1.0 mg at week 30.
The standout in the program is SUSTAIN-6, a dedicated cardiovascular safety trial. It enrolled 3,297 patients at high cardiovascular risk and ran a median 2.1 years. The primary endpoint was time to first major adverse cardiovascular event (MACE): cardiovascular death, nonfatal myocardial infarction, or nonfatal stroke. The composite occurred in 6.6% of the semaglutide group versus 8.9% on placebo, a hazard ratio of 0.74 (Marso et al., *NEJM* 2016). SUSTAIN-6 was powered to show non-inferiority for safety; the risk reduction it found was driven largely by nonfatal stroke and was a pre-specified secondary read, not proof the drug was designed to deliver.
STEP: obesity, higher-dose injectable
The STEP program tested the 2.4 mg weekly dose that became Wegovy. STEP 1 (Wilding et al., NEJM 2021) randomized 1,961 adults with overweight or obesity, without diabetes, to semaglutide 2.4 mg or placebo for 68 weeks alongside lifestyle counseling. The coprimary endpoints were percentage change in body weight and the proportion losing at least 5%.
| Endpoint (STEP 1, NEJM 2021) | Semaglutide 2.4 mg | Placebo |
|---|---|---|
| Mean weight change, wk 68 | -14.9% | -2.4% |
| Lost >=5% of body weight | 86.4% | 31.5% |
The estimated treatment difference was -12.4 percentage points. The trade-off shows up in the safety table: gastrointestinal disorders (nausea, diarrhea, vomiting, constipation) hit 74.2% of the semaglutide group versus 47.9% on placebo, though most were mild to moderate and transient. Discontinuation for GI events was 4.5% versus 0.8%.
SELECT: cardiovascular outcomes in obesity without diabetes
SELECT asked a different question: does the obesity dose prevent cardiac events in people who have heart disease but not diabetes? It enrolled 17,604 participants (mean age 61.6, baseline preexisting cardiovascular disease) and followed them a mean 39.8 months. The primary endpoint was the same MACE composite. It occurred in 6.5% on semaglutide 2.4 mg versus 8.0% on placebo, a hazard ratio of 0.80 (Lincoff et al., *NEJM* 2023). This was a superiority result, and it supported the 2024 FDA label expansion for cardiovascular risk reduction. Worth flagging: the population was 72% male, which limits how far the finding extends to women.
PIONEER: oral semaglutide (Rybelsus)
PIONEER covered the oral tablet, approved as Rybelsus at 3, 7, and 14 mg. The program ran ten phase 3 trials in more than 10,000 adults with type 2 diabetes. Most used change in HbA1c at week 26 as the primary endpoint. PIONEER 1 (Aroda et al., Diabetes Care 2019) tested oral monotherapy against placebo on that endpoint; PIONEER 4 (Lancet 2019) compared it with subcutaneous liraglutide and placebo.
The cardiovascular member of the family is PIONEER 6, an event-driven safety trial in patients with established cardiovascular disease or high risk. Its MACE primary endpoint produced a hazard ratio of 0.79 over a median 16 months (Husain et al., *NEJM* 2019). PIONEER 6 met non-inferiority for safety; it was not powered to claim a benefit, and the confidence interval crossed 1.
Safety: the warning that follows every brand
All three products carry a boxed warning, the FDA's most serious. In rodents, semaglutide caused dose- and duration-dependent thyroid C-cell tumors. Whether this happens in humans is unknown, but the Ozempic label contraindicates the drug in anyone with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. Across the trials, the most common adverse events were gastrointestinal. Labels also note risks of pancreatitis, gallbladder disease, and, with insulin or sulfonylureas, hypoglycemia.
The trials above tested specific salts, doses, and formulations from a single manufacturer under regulatory oversight. Compounded or research-grade semaglutide sold outside that chain has not been through these studies, is not FDA-approved, and is not established as safe for human use. None of this is a recommendation to start, stop, or change any treatment; those decisions belong with a licensed clinician.