Outcome trial — Semaglutide (Ozempic)
SUSTAIN-6: trial summary, limitations and UK relevance
SUSTAIN-6 evaluated semaglutide against placebo in adults with type 2 diabetes at high cardiovascular risk over 104 weeks. The trial was designed to test non-inferiority for MACE-3 and reported a result consistent with cardiovascular benefit rather than harm, contributing evidence that later informed the larger SELECT programme in non-diabetic obesity. SUSTAIN-6 is also notable for an unexpected safety signal: participants receiving semaglutide had a higher reported rate of diabetic retinopathy complications than placebo, a finding not observed to the same degree in later semaglutide trials and thought to relate to the rapidity of glycaemic improvement rather than a direct drug effect. Readers should consult the primary NEJM 2016 publication for exact reported figures.
Key findings
SUSTAIN-6 evaluated semaglutide against placebo in adults with type 2 diabetes at high cardiovascular risk over 104 weeks. The trial was designed to test non-inferiority for MACE-3 and reported a result consistent with cardiovascular benefit rather than harm, contributing evidence that later informed the larger SELECT programme in non-diabetic obesity. SUSTAIN-6 is also notable for an unexpected safety signal: participants receiving semaglutide had a higher reported rate of diabetic retinopathy complications than placebo, a finding not observed to the same degree in later semaglutide trials and thought to relate to the rapidity of glycaemic improvement rather than a direct drug effect. Readers should consult the primary NEJM 2016 publication for exact reported figures.
Safety summary
GI adverse events (nausea, vomiting, diarrhoea) were more frequent with semaglutide, consistent with the GLP-1 class profile. The most distinctive safety finding was a higher rate of diabetic retinopathy complications (including vitreous haemorrhage, blindness, or conditions requiring treatment with an intravitreal agent or photocoagulation) reported with semaglutide than placebo, concentrated in participants with pre-existing retinopathy and rapid HbA1c reduction at trial entry.
Limitations
- Small absolute number of retinopathy events limits precise quantification of the risk and its generalisability.
- T2D-only population at high cardiovascular risk; does not establish benefit in non-diabetic populations (see SELECT).
- Trial was designed and powered for non-inferiority rather than superiority, complicating direct efficacy claims.
- Two-year duration does not establish long-term cardiovascular or retinal safety beyond the trial period.
UK relevance
Key evidence underpinning MHRA/NICE consideration of semaglutide (Ozempic) for cardiovascular risk reduction in UK adults with type 2 diabetes, and central to UK clinical guidance on retinal screening intensity when initiating rapid HbA1c-lowering therapy.
- Does not establish cardiovascular benefit in adults without type 2 diabetes (see SELECT for the non-diabetic obesity population).
- Does not prove the diabetic retinopathy signal generalises to all patients — the observed imbalance was concentrated in those with pre-existing retinopathy and rapid glycaemic improvement.
- Non-inferiority design means the trial does not establish superiority over placebo to the standard a dedicated superiority trial would.
- Does not establish equivalence with oral semaglutide's cardiovascular profile (see PIONEER 6).
Frequently asked questions
What did SUSTAIN-6 find about cardiovascular risk?
SUSTAIN-6 tested whether semaglutide was non-inferior to placebo for MACE-3 (cardiovascular death, non-fatal myocardial infarction, non-fatal stroke) in adults with type 2 diabetes at high cardiovascular risk. The trial reported a result consistent with cardiovascular benefit rather than harm over 104 weeks, though it was designed and powered for non-inferiority rather than superiority.
What was the retinopathy signal reported in SUSTAIN-6?
SUSTAIN-6 reported a higher rate of diabetic retinopathy complications with semaglutide than placebo, concentrated among participants with pre-existing retinopathy and rapid HbA1c reduction. This unexpected signal has informed UK clinical guidance on retinal screening when starting therapies that rapidly lower blood glucose.
Does SUSTAIN-6 apply to people without type 2 diabetes?
No. SUSTAIN-6 enrolled only adults with type 2 diabetes at high cardiovascular risk. Its findings do not establish cardiovascular benefit in non-diabetic populations — that evidence base comes from the separate SELECT trial.
How does SUSTAIN-6 compare with PIONEER 6?
SUSTAIN-6 studied injectable semaglutide (Ozempic) over 104 weeks, while PIONEER 6 studied oral semaglutide (Rybelsus) over a shorter median follow-up of approximately 15.9 months. Both were non-inferiority-designed cardiovascular safety trials in high-risk type 2 diabetes populations; direct cross-trial comparison of magnitude should be made cautiously given the differing follow-up durations.
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Sources are tagged by class. Regulatory documents, peer-reviewed publications and trial registry records are preferred. Manufacturer disclosures and conference material are accepted where peer-reviewed publication is not yet available; preprints are flagged.
- Peer-reviewed publicationMarso SP et al., Semaglutide and cardiovascular outcomes in patients with type 2 diabetes (SUSTAIN-6). NEJM 2016.
- Regulatory document (MHRA/EMA/FDA)EMA EPAR — Ozempic
- Trial registry recordClinicalTrials.gov NCT01720446