Semaglutide and HbA1c: Trajectories at 3, 6, 12 Months

One of the most common questions from people starting semaglutide for type 2 diabetes isn't about weight or nausea — it's about the HbA1c number. Specifically: how fast does it move, how much should it move, and when should you and your doctor start wondering if something else needs to change?

The SUSTAIN trial program gives us about as good a dataset as exists for answering these questions. SUSTAIN-1 through SUSTAIN-10 enrolled tens of thousands of people with type 2 diabetes across a range of baseline HbA1c levels and treatment histories, and the A1c curves from those trials follow a shape that's reproducible and predictable.

Here's what the data actually shows about semaglutide A1c reduction at each time point, and how to use that to interpret your own results.

The shape of the curve: front-loaded improvement

The most important thing to understand about semaglutide's effect on HbA1c is that it's not linear. The reduction curve is steep in the first three months and flattens progressively after that. Most of the achievable reduction happens early.

From the SUSTAIN trials:

These ranges are from participants at the maintenance dose (0.5 mg or 1.0 mg weekly) with baseline HbA1c of roughly 8.0–8.5%. Your starting point matters — people who start higher tend to see larger absolute reductions. Someone starting at HbA1c 10.5% will see a larger drop in absolute terms than someone at 7.8%, even if the percentage change is similar.

The plateau is real: in SUSTAIN-1, the placebo-subtracted HbA1c reduction was relatively stable from week 26 through week 30, with minimal further decline. This means that if you're checking in at 12 months and the number has barely changed since your 6-month check, that's expected — not a sign the drug has stopped working.

What to expect at 3 months

Your first significant HbA1c recheck is typically at 3 months. At this point, most people are either finishing the 0.5 mg step or just starting 1.0 mg. A few things to know:

The titration matters. If you're still at 0.25 mg or 0.5 mg at week 12, you haven't reached the full maintenance dose. Your A1c reduction at 3 months is a partial-dose result. Don't interpret a smaller-than-expected drop as evidence that semaglutide isn't working for you — wait until you've been at 1.0 mg for at least 4 weeks before drawing conclusions.

Rough benchmarks at 3 months (on 0.5–1.0 mg, baseline HbA1c ~8.0–8.5%):

• A reduction of 0.8–1.2% is on-target
• Less than 0.5% is below average and worth discussing with your prescriber
• Greater than 1.5% is strong and often correlates with higher starting HbA1c

A note on starting point: if you began with an HbA1c of 9.0% or above, the numbers above don't quite apply. Higher baseline values consistently associate with larger absolute reductions — a 1.0% drop from a baseline of 10.0% means you've used only a fraction of the drug's capacity; expect continued improvement.

What to expect at 6 months

The 6-month mark is the clearest signal. By this point, most people have been at their maintenance dose for 8–12 weeks, the drug has reached pharmacokinetic steady state, and the majority of the achievable A1c reduction is visible.

From SUSTAIN-7 (semaglutide 0.5 mg and 1.0 mg vs. dulaglutide, 40 weeks):

• Semaglutide 0.5 mg: -1.5% HbA1c from baseline
• Semaglutide 1.0 mg: -1.8% HbA1c from baseline
• Both significantly outperformed dulaglutide at equivalent doses

From SUSTAIN-2 (vs. sitagliptin, 56 weeks):

• 0.5 mg: -1.3% at 26 weeks
• 1.0 mg: -1.6% at 26 weeks

What to do at 6 months if you're not hitting goal:

The American Diabetes Association considers an HbA1c below 7.0% appropriate for most adults with T2D (individual targets vary — your clinician may be aiming for 6.5% or 7.5% depending on your history). If you're at 6 months on 1.0 mg semaglutide and your HbA1c is still above target:

• Confirm you're getting the full dose. Dose errors with compounded semaglutide (miscalculation, vial reconstitution errors) are surprisingly common. Our reconstitution calculator helps.
• Discuss dose escalation. In Europe, semaglutide is available up to 2.0 mg/week for T2D. If you're on Ozempic, 2.0 mg is an option that's been available since 2022.
• Consider adding an SGLT-2 inhibitor. For people with HbA1c above target despite semaglutide, adding empagliflozin, dapagliflozin, or canagliflozin is supported by current ADA guidelines as a next step — especially if cardiovascular or kidney protection is also a goal.

What to expect at 12 months

By 12 months, the SUSTAIN data shows that HbA1c improvement is essentially complete. The plateau reached at ~26 weeks is maintained through 52 weeks. This is the new steady state on semaglutide.

If your 12-month HbA1c is higher than your 6-month result — or if you've seen the number drift back up — these are worth investigating:

1. Dose consistency. Missing doses, changes in injection timing, or storage errors can blunt the effect
2. Disease progression. Type 2 diabetes is progressive; beta-cell function declines over time in many people even on GLP-1 therapy
3. Weight regain. If you've regained weight (possible if side effects led to stopping and restarting), the metabolic improvements may have partially reversed
4. Non-adherence. The most common reason for loss of glycemic control on any T2D medication

A rising HbA1c at 12 months, despite consistent semaglutide use, usually signals that combination therapy is needed. An SGLT-2 inhibitor is the most common add-on; some patients also benefit from adding low-dose metformin if they're not already on it.

Tirzepatide for comparison

If you're weighing semaglutide against tirzepatide for glucose control, the SURPASS trial program provides a direct comparison. In SURPASS-2 (tirzepatide vs. semaglutide 1.0 mg, 40 weeks):

• Tirzepatide 5 mg: -2.0% HbA1c
• Tirzepatide 10 mg: -2.2%
• Tirzepatide 15 mg: -2.3%
• Semaglutide 1.0 mg: -1.9%

The difference at 5 mg tirzepatide vs. 1.0 mg semaglutide is modest (-2.0 vs. -1.9%). At higher tirzepatide doses, the gap opens. For people whose A1c target requires more aggressive reduction, tirzepatide's higher ceiling may be relevant. See our tirzepatide vs. semaglutide cluster for the full comparison.

The relationship between HbA1c and weight loss

Worth noting: semaglutide's A1c effect and its weight loss effect are partially but not entirely linked. You can have strong weight loss with modest A1c improvement (if your glucose was already relatively well-controlled), and you can have meaningful A1c improvement with modest weight loss (if your glucose was driven more by insulin resistance than by excess body fat).

The SUSTAIN trials enrolled people with T2D, many of whom had limited excess weight. The average weight loss in SUSTAIN-1 (not designed as a weight loss trial) was roughly 3–4 kg. The STEP trials (weight management program, higher doses) showed much larger weight losses but enrolled people without diabetes. The A1c improvement in the STEP program came from weight loss plus direct glycemic effects.

For most people with both T2D and obesity, both pathways are active simultaneously — which is part of why semaglutide's clinical effect is often greater than adding up the separate glucose and weight effects would predict.

See also:

• Semaglutide dosing schedule explained
• How long semaglutide takes to work
• Semaglutide side effects timeline
• Reconstitution calculator for compounded semaglutide

Related reading

• Semaglutide complete guide
• Tirzepatide vs. semaglutide
• Weight loss: realistic timelines