SUSTAIN-6: semaglutide and cardiovascular outcomes in diabetes

What was the SUSTAIN-6 trial?

SUSTAIN-6 was a long-term, randomized, double-blind, placebo-controlled cardiovascular outcomes trial of once-weekly semaglutide in patients with type 2 diabetes, sponsored by Novo Nordisk and published in the New England Journal of Medicine in 2016 (PMID 27633186).

Its purpose was regulatory as much as scientific. Guidance for new diabetes therapies requires evidence that a drug does not raise cardiovascular risk, and at the time the cardiovascular effects of semaglutide — a GLP-1 analogue with a half-life of roughly one week — were unknown. So SUSTAIN-6 was structured to test whether semaglutide was non-inferior to placebo on hard cardiovascular events: it aimed to rule out excess risk rather than to prove superiority. The trial is registered as NCT01720446 on ClinicalTrials.gov.

This is a distinct question from the weight-focused trials that later made semaglutide a household name. SUSTAIN-6 asked whether the drug was safe for the heart — and, as it turned out, hinted at benefit — in people who already had diabetes and, in most cases, established cardiovascular disease.

Who was in it?

SUSTAIN-6 randomly assigned 3,297 patients with type 2 diabetes who were already on a standard-care regimen. They were split into a semaglutide arm (1,648 patients in the primary-outcome analysis) and a placebo arm (1,649 patients).

This was a high-risk population by design. At baseline, 2,735 of the participants — 83.0% — had established cardiovascular disease, chronic kidney disease, or both. That matters when reading the result: this was largely a secondary-prevention group, people whose risk of a cardiovascular event was already elevated, which is exactly where a cardiovascular safety signal is easiest to detect. The population also frames the trial's scope — everyone enrolled had type 2 diabetes, unlike the later obesity trials that deliberately excluded it.

What did participants receive?

Participants were randomly assigned to receive once-weekly subcutaneous semaglutide at a dose of 0.5 mg or 1.0 mg, or a matching placebo, added on top of standard diabetes care and continued for 104 weeks. These are the same dose levels later marketed for type 2 diabetes as Ozempic; the underlying molecule is covered in our semaglutide research summary.

Describing the assigned regimen is reporting what the trial did — it is not a dosing instruction. Semaglutide is prescription-only, and the dose any individual might use is a clinical decision, not something to infer from a trial protocol.

Primary results

The primary endpoint was the first occurrence of a composite of three hard outcomes: death from cardiovascular causes, non-fatal myocardial infarction (heart attack), or non-fatal stroke — the standard MACE composite.

Over the 104-week follow-up, a first primary-endpoint event occurred in 108 of the 1,648 patients on semaglutide (6.6%) and in 146 of the 1,649 on placebo (8.9%). That corresponds to a hazard ratio of 0.74, with a 95% confidence interval of 0.58 to 0.95 and P<0.001 for non-inferiority — a lower event rate on semaglutide, with the confidence interval sitting entirely below 1.0.

One point deserves emphasis: SUSTAIN-6 was pre-specified as a non-inferiority trial, with a non-inferiority margin of 1.8 for the upper boundary of the hazard ratio's confidence interval. The observed result cleared that bar comfortably and pointed toward benefit, but the trial was framed to establish safety, not to prove superiority.

Secondary and safety findings

Breaking the composite apart, non-fatal myocardial infarction occurred in 2.9% of the semaglutide group versus 3.9% on placebo (hazard ratio 0.74; 95% CI 0.51–1.08; P=0.12), and non-fatal stroke in 1.6% versus 2.7% (hazard ratio 0.61; 95% CI 0.38–0.99; P=0.04). Rates of death from cardiovascular causes were similar in the two groups.

The safety picture was mixed. Rates of new or worsening nephropathy were lower with semaglutide, a favorable kidney signal. But rates of retinopathy complications — vitreous hemorrhage, blindness, or conditions requiring treatment with an intravitreal agent or photocoagulation — were significantly higher in the semaglutide group (hazard ratio 1.76; 95% CI 1.11–2.78; P=0.02). Fewer serious adverse events occurred overall on semaglutide, yet more patients discontinued treatment because of adverse events, mainly gastrointestinal — the familiar tolerability pattern of the GLP-1 class.

How SUSTAIN-6 differs from SELECT

SUSTAIN-6 is often mentioned alongside the later SELECT trial, and it helps to keep them apart:

• Different population. SUSTAIN-6 enrolled 3,297 adults who all had type 2 diabetes. SELECT enrolled 17,604 adults with overweight or obesity and established heart disease but explicitly no diabetes.
• Different dose. SUSTAIN-6 used the lower diabetes doses of 0.5 mg or 1.0 mg weekly. SELECT used the higher 2.4 mg weekly weight-management dose.
• Different intent. SUSTAIN-6 was a cardiovascular safety (non-inferiority) trial. SELECT was a superiority trial and reported a hazard ratio of 0.80 for its primary endpoint.

Read together, the two trials extend semaglutide's cardiovascular evidence across two distinct groups — people with diabetes and people with obesity but no diabetes — rather than repeating the same experiment.

Limitations and what it does not prove

SUSTAIN-6 is an important trial, but its boundaries matter:

• It was a safety trial, not a superiority trial. The design was built to rule out excess risk. The lower event rate is encouraging, but the trial was not powered to prove semaglutide is superior to placebo for preventing cardiovascular events.
• It studied a specific population. Participants had type 2 diabetes and were largely at high cardiovascular risk. The result applies most directly to people like those enrolled, not to lower-risk or non-diabetic populations.
• The retinopathy signal is real. Retinopathy complications were significantly more common on semaglutide, a finding that shaped later scrutiny of the drug and cannot be waved away.
• It does not isolate a mechanism. The trial reports clinical outcomes; it cannot say how much of the cardiovascular result came from glucose control, weight change, blood-pressure effects, or direct vascular action.

Why the SUSTAIN-6 trial matters

SUSTAIN-6 was one of the cardiovascular outcomes trials that reshaped how GLP-1 receptor agonists are viewed. By showing that a weekly GLP-1 drug did not raise cardiovascular risk in high-risk diabetes — and in fact came with a lower event rate — it helped move the class from purely glucose-lowering agents toward drugs assessed on hard cardiovascular endpoints.

It also set the stage for the questions that later trials answered in different populations: the diabetes-focused branding of Ozempic, the higher-dose weight-management work summarized in our pillar guide on peptides for weight loss, and the obesity-without-diabetes evidence of SELECT. SUSTAIN-6 is the earlier, diabetes-anchored piece of that larger evidence map — and a reminder that the same molecule can behave differently at different doses and in different patients.

Frequently asked questions

Sources

This summary traces to the peer-reviewed publication and trial registration:

1. Semaglutide and Cardiovascular Outcomes in Patients with Type 2 Diabetes (SUSTAIN-6) — New England Journal of Medicine, 2016;375(19):1834–1844; PubMed 27633186.
2. SUSTAIN-6 (semaglutide cardiovascular and long-term outcomes) — ClinicalTrials.gov NCT01720446.
3. Drug reference record: semaglutide.

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