Mounjaro and Type 2 Diabetes: How Tirzepatide Controls Blood Sugar

Mounjaro (tirzepatide) is now widely recognised as a weight-loss treatment, yet its original and primary indication is type 2 diabetes. In fact, the MHRA first approved Mounjaro for type 2 diabetes in September 2022, more than a year before the weight-management licence followed. For the millions of people in the UK living with type 2 diabetes, tirzepatide represents a genuinely new class of treatment. At CutKilo, many of our patients have type 2 diabetes alongside obesity, and understanding how the medication works on blood sugar is just as important as understanding its effect on body weight.

This guide explains how Mounjaro controls blood sugar, what the clinical trial data shows, and what patients with type 2 diabetes should expect during treatment.

Mounjaro is a dual GIP and GLP-1 receptor agonist. It works by stimulating two incretin hormones that your body naturally uses to regulate blood sugar after meals. In clinical trials (the SURPASS programme), tirzepatide reduced HbA1c by 1.87% to 2.07% at the highest dose, bringing the majority of participants below the widely accepted 6.5% target. Furthermore, because it also produces significant weight loss, tirzepatide addresses two of the most important drivers of type 2 diabetes simultaneously.

Type 2 diabetes occurs when your body either does not produce enough insulin or cannot use the insulin it makes effectively. Over time, persistently elevated blood glucose damages blood vessels, nerves, kidneys, and the retina. In the UK, approximately 4.3 million people are living with a diabetes diagnosis, and roughly 90% of those cases are type 2.

The standard measure of long-term blood sugar control is HbA1c (glycated haemoglobin). NICE recommends a target of 48 mmol/mol (6.5%) for most adults with type 2 diabetes managed with lifestyle changes and a single drug, or 53 mmol/mol (7.0%) for those on treatments that carry a risk of hypoglycaemia. Achieving and maintaining these targets reduces the risk of diabetic complications significantly.

Unlike older GLP-1 receptor agonists such as semaglutide (Ozempic), tirzepatide activates two receptors rather than one. It targets both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. This dual action is why Mounjaro is sometimes called a “twincretin.”

In practical terms, this means Mounjaro stimulates insulin secretion when blood glucose is elevated (reducing the risk of hypoglycaemia compared to sulphonylureas), suppresses glucagon release (which otherwise raises blood sugar), slows gastric emptying (reducing post-meal glucose spikes), and acts on appetite centres in the brain to reduce caloric intake. Additionally, the GIP component appears to enhance fat metabolism and may contribute to greater weight loss than GLP-1 activation alone, according to data published in the New England Journal of Medicine.

The SURPASS clinical trial programme is the largest evidence base for tirzepatide in type 2 diabetes. It includes five major Phase III trials, each comparing tirzepatide at 5 mg, 10 mg, and 15 mg doses against either placebo or an active comparator.

SURPASS-1 (monotherapy): Over 40 weeks, tirzepatide reduced HbA1c by 1.87% (5 mg), 1.89% (10 mg), and 2.07% (15 mg) compared to a 0.04% increase with placebo. Between 81% and 86% of participants reached an HbA1c below 6.5%.

SURPASS-2 (vs semaglutide 1 mg): Tirzepatide at all doses produced greater HbA1c reductions than semaglutide. Mean changes from baseline were 2.01% (5 mg), 2.24% (10 mg), and 2.30% (15 mg) for tirzepatide compared to 1.86% for semaglutide 1 mg.

SURPASS-3 (vs insulin degludec): Tirzepatide achieved superior HbA1c reduction and weight loss compared to titrated insulin degludec over 52 weeks, without the weight gain typically associated with insulin therapy.

SURPASS-4 (vs insulin glargine): In patients with established cardiovascular disease or high cardiovascular risk, tirzepatide was non-inferior for cardiovascular safety and superior for glycaemic control and weight reduction over 52 weeks.

SURPASS-5 (add-on to insulin glargine): Adding tirzepatide to basal insulin produced HbA1c reductions of up to 2.59% and weight loss rather than the weight gain commonly seen when intensifying insulin regimens.

Consequently, across the full SURPASS programme, tirzepatide consistently delivered HbA1c reductions of 1.9% to 2.6% and body weight reductions of 6.2 to 13.9 kg over 40 to 52 weeks.

The MHRA licensed Mounjaro for the treatment of type 2 diabetes in September 2022. According to the Summary of Product Characteristics (SmPC), the approved indication covers adults, adolescents, and children aged 10 years and above with insufficiently controlled type 2 diabetes as an adjunct to diet and exercise. It can be used as monotherapy when metformin is inappropriate, or in combination with other diabetes medications.

NICE has also published guidance supporting tirzepatide’s use in weight management (TA1026, December 2024), recommending it for adults with a BMI above 35 and at least one weight-related comorbidity. For CutKilo patients who have both obesity and type 2 diabetes, this means tirzepatide can address both conditions with a single weekly injection. Similarly, patients who are already taking metformin alongside Mounjaro may see improved glycaemic control beyond what either medication achieves alone.

Mounjaro is administered as a once-weekly subcutaneous injection. The starting dose is 2.5 mg, which is a dose-titration step rather than a therapeutic dose. After four weeks, your prescribing doctor will increase the dose to 5 mg. Depending on your response and tolerability, the dose can be escalated further to 7.5 mg, 10 mg, 12.5 mg, or the maximum of 15 mg at four-weekly intervals.

Most patients begin to see measurable HbA1c improvements within the first 8 to 12 weeks. In particular, the SURPASS trial data showed that a substantial proportion of the HbA1c reduction occurred in the first 12 weeks, with further gradual improvement through to week 40.

Common side effects include nausea, reduced appetite, and gastrointestinal symptoms such as diarrhoea or constipation. These are typically mild to moderate and tend to settle as your body adjusts. For practical guidance on managing specific side effects, our articles on managing nausea on Mounjaro and reducing bloating offer detailed strategies.

In a doctor-led service like CutKilo, suitability is assessed on an individual basis. Generally, tirzepatide may be appropriate for adults with type 2 diabetes whose HbA1c remains above target despite lifestyle modifications and first-line therapy (usually metformin). It is also an option for patients who need to lose weight alongside improving blood sugar control, those who want to avoid or reduce insulin therapy, and individuals who have not achieved adequate control with other GLP-1 receptor agonists.

However, Mounjaro is not suitable for type 1 diabetes, and it should be used with caution in patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. Your doctor will review your full medical history before prescribing.

Tirzepatide is the first dual GIP/GLP-1 receptor agonist approved for type 2 diabetes in the UK, and the SURPASS trial data demonstrates HbA1c reductions that are consistently larger than those seen with existing treatments, including semaglutide and basal insulin. For patients with type 2 diabetes and co-existing obesity, Mounjaro offers the possibility of meaningful improvement in both blood sugar control and body weight with a single weekly injection.

Is Mounjaro only for weight loss, or does it treat diabetes too? Mounjaro was originally developed and licensed for type 2 diabetes. The MHRA approved it for this indication in September 2022. The weight-management licence came later, in November 2023. It treats both conditions effectively.

How much does Mounjaro reduce HbA1c? In the SURPASS trials, tirzepatide reduced HbA1c by 1.87% to 2.59% depending on the dose and trial design. Most participants achieved an HbA1c below 7.0%, and a large proportion reached below 6.5%.

Can I take Mounjaro if I am already on metformin? Yes. Tirzepatide can be used alongside metformin. The SURPASS-2 trial enrolled participants already taking metformin, and the combination was both effective and well tolerated.

Will Mounjaro cause low blood sugar? Tirzepatide works in a glucose-dependent manner, meaning it primarily stimulates insulin when blood sugar is elevated. The risk of hypoglycaemia is low unless you are also taking a sulphonylurea or insulin, in which case your doctor may adjust the doses.

How long does it take for Mounjaro to improve blood sugar? Most patients see measurable improvements within the first 8 to 12 weeks. The full effect on HbA1c typically becomes apparent by 24 to 40 weeks of treatment.

Start Your CutKilo Journey

CutKilo is a doctor-led supervised Mounjaro weight-loss service based at 86 Harley Street, London W1G 7HP. Call: 0207 637 8227. Start the CutKilo questionnaire to see if you are suitable for treatment.