Mounjaro

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Mounjaro is a prescription-only medicine containing tirzepatide, a first-in-class dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist.

It is approved for use in the United Kingdom for the treatment of type 2 diabetes mellitus in adults and, subject to regulatory pathway, for chronic weight management.

Mounjaro is manufactured by Eli Lilly and is administered as a once-weekly subcutaneous injection using a single-use pre-filled pen.

It is available in six dose strengths: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg and 15 mg.

Tirzepatide represents a significant advance in incretin-based therapy.

Unlike existing GLP-1 receptor agonists such as semaglutide and liraglutide, which target only the GLP-1 receptor, tirzepatide simultaneously activates both the GIP and GLP-1 receptors.

This dual mechanism produces greater improvements in glycaemic control and body weight reduction than single-receptor agonists.

The introduction of tirzepatide has reshaped the treatment landscape for type 2 diabetes and obesity in the United Kingdom.

This page provides a comprehensive clinical overview of Mounjaro, including how it works, dosage, side effects, safety warnings, and how to obtain a prescription in the UK.

Important safety information about Mounjaro

• Mounjaro is a prescription-only medicine and must be used under medical supervision.
• Do not use Mounjaro if you have type 1 diabetes or diabetic ketoacidosis.
• The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting and constipation, particularly during dose escalation.
• Cases of acute pancreatitis have been reported with GLP-1 receptor agonists. Seek urgent medical attention if you experience severe persistent abdominal pain.
• Mounjaro should not be used during pregnancy. Women of childbearing potential should use effective contraception.

Understanding GIP/GLP-1 dual receptor agonism

To appreciate how Mounjaro works, it is helpful to understand the incretin system.

Incretins are hormones released from the gut after eating that stimulate insulin secretion in a glucose-dependent manner.

The two principal incretins are GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). In type 2 diabetes, the incretin effect is impaired, contributing to poor blood sugar control.

GLP-1 receptor agonists such as semaglutide, liraglutide and dulaglutide have transformed the management of type 2 diabetes over the past decade.

They lower blood glucose, promote weight loss, and have cardiovascular benefits. However, they target only one arm of the incretin system.

Tirzepatide was designed to activate both GIP and GLP-1 receptors using a single synthetic peptide molecule.

In preclinical and clinical studies, this dual activation produced additive and potentially synergistic effects on insulin secretion, glucagon suppression, appetite regulation and energy metabolism.

The GIP receptor is highly expressed in pancreatic beta cells and adipose tissue. GIP signalling enhances beta-cell function and may improve fat metabolism.

By activating the GIP receptor alongside the GLP-1 receptor, tirzepatide achieves greater glucose lowering and weight reduction than GLP-1 receptor agonists alone.

This is supported by the results of the SURPASS clinical trial programme, which compared tirzepatide with semaglutide, insulin, and placebo in adults with type 2 diabetes.

Mechanism of action of tirzepatide

Tirzepatide exerts its effects through several complementary mechanisms.

It enhances glucose-dependent insulin secretion from pancreatic beta cells, meaning it promotes insulin release primarily when blood glucose is elevated, which reduces the risk of hypoglycaemia.

It suppresses glucagon secretion from pancreatic alpha cells during hyperglycaemia, reducing hepatic glucose output.

It delays gastric emptying, slowing the rate at which food passes from the stomach into the small intestine, which reduces postprandial glucose excursions and contributes to satiety.

It acts on appetite-regulating centres in the hypothalamus, reducing hunger and food intake.

These effects combine to lower HbA1c, reduce fasting and postprandial blood glucose, and promote clinically significant weight loss.

Tirzepatide is a 39-amino-acid peptide based on the native GIP sequence, with modifications to enable GLP-1 receptor activation and to extend the half-life.

A C20 fatty diacid moiety is attached, enabling binding to albumin and prolonging the circulating half-life to approximately 5 days. This supports once-weekly dosing.

Peak plasma concentrations are reached 8 to 72 hours after injection, and steady state is achieved after 4 weeks of once-weekly administration.

Clinical evidence: the SURPASS and SURMOUNT programmes

The efficacy of tirzepatide in type 2 diabetes has been established through the SURPASS clinical trial programme, which included more than 12,000 participants across multiple phase 3 trials.

SURPASS-1 compared tirzepatide (5 mg, 10 mg and 15 mg) with placebo in treatment-naive adults.

At 40 weeks, HbA1c reductions ranged from 1.87 to 2.07 percentage points, and weight reductions ranged from 7.0 to 9.5 kg.

SURPASS-2 compared tirzepatide head-to-head with semaglutide 1 mg in adults already taking metformin.

All three tirzepatide doses achieved statistically superior HbA1c reductions and weight loss compared with semaglutide 1 mg.

At the 15 mg dose, tirzepatide reduced HbA1c by 2.46 percentage points compared with 1.86 for semaglutide, and weight loss was 12.4 kg compared with 6.2 kg.

SURPASS-3 compared tirzepatide with insulin degludec, demonstrating superior glycaemic control and weight reduction with tirzepatide. SURPASS-4 demonstrated cardiovascular safety in a high-risk population.

The SURMOUNT programme evaluated tirzepatide for chronic weight management in adults without type 2 diabetes.

SURMOUNT-1, published in the New England Journal of Medicine in 2022, showed that tirzepatide 15 mg produced a mean weight reduction of 22.5 percent at 72 weeks compared with 2.4 percent for placebo.

SURMOUNT-2 studied tirzepatide in adults with obesity and type 2 diabetes, showing mean weight reductions of 12.8 to 14.7 percent.

These results position tirzepatide as one of the most effective pharmacological weight management agents studied to date.

UK regulatory and NICE guidance

Tirzepatide received marketing authorisation from the MHRA for the treatment of type 2 diabetes mellitus in adults as an adjunct to diet and exercise.

NICE technology appraisal TA924 (2023) recommends tirzepatide as an option for treating type 2 diabetes in adults, either as monotherapy when metformin is contraindicated or not tolerated, or in combination with other glucose-lowering medicines when existing treatment has not achieved adequate glycaemic control.

The NICE appraisal positions tirzepatide alongside other GLP-1 receptor agonists and reflects the clinical trial evidence of superior efficacy.

For weight management, tirzepatide is licensed under the brand name Zepbound in the United States.

In the UK, Eli Lilly has sought approval for the weight management indication, and NICE appraisal is ongoing.

Availability for weight management on the NHS may depend on the outcome of this evaluation. Some UK clinics offer tirzepatide for weight management through private prescriptions.

Dosage and administration

Mounjaro is administered as a once-weekly subcutaneous injection. The recommended starting dose is 2.5 mg once weekly for at least 4 weeks.

This is an initiation dose to reduce gastrointestinal side effects and is not intended as a maintenance dose.

After 4 weeks, the dose should be increased to 5 mg once weekly.

Depending on individual response and tolerability, the dose may be further increased in 2.5 mg increments at intervals of at least 4 weeks, up to a maximum of 15 mg once weekly.

Inject into the skin of the abdomen, thigh or upper arm.

Choose a different injection site within the same region each week and rotate between regions to reduce the risk of injection site reactions.

The pre-filled pen has a hidden needle and is designed for ease of use.

Your prescriber, diabetes specialist nurse or pharmacist will demonstrate the injection technique at your first appointment.

The injection can be given at any time of day, with or without food, but it should be administered on the same day each week.

If you miss a dose and there are at least 72 hours (3 days) until your next scheduled dose, administer the missed dose as soon as possible.

If fewer than 72 hours remain, skip the missed dose and administer the next dose on your regular day.

The day of weekly administration may be changed as long as at least 3 days have elapsed since the last dose.

Side effects of Mounjaro

Common side effects

Gastrointestinal adverse effects are the most frequently reported side effects of tirzepatide.

In clinical trials, nausea was reported by approximately 12 to 18 percent of participants, diarrhoea by 12 to 17 percent, decreased appetite by 7 to 11 percent, vomiting by 5 to 9 percent, and constipation by 6 to 8 percent.

These effects were generally mild to moderate in severity, occurred most frequently during dose escalation periods, and diminished with continued treatment.

Eating smaller meals, avoiding high-fat or greasy foods, and ensuring adequate hydration can help manage symptoms.

Injection site reactions (redness, itching, swelling or pain) were reported in approximately 3 to 5 percent of participants and were typically mild. Rotating the injection site with each dose reduces the likelihood of local reactions.

Hypoglycaemia

Hypoglycaemia (low blood sugar) is uncommon when Mounjaro is used alone or with metformin.

The risk is increased when Mounjaro is used in combination with a sulphonylurea or insulin. Symptoms include sweating, tremor, hunger, dizziness, confusion, palpitations and difficulty concentrating.

If hypoglycaemia occurs, treat with fast-acting glucose (such as glucose tablets or a sugary drink) followed by a longer-acting carbohydrate.

Your prescriber may need to reduce the dose of your sulphonylurea or insulin when starting Mounjaro.

Other side effects

Dyspepsia, gastro-oesophageal reflux, flatulence, eructation (belching), abdominal distension, fatigue, dizziness, and hair thinning have been reported.

Hair thinning is observed during periods of rapid weight loss and is generally temporary, resolving as weight stabilises.

Small mean increases in resting heart rate (2 to 4 beats per minute) were observed in clinical trials. The clinical significance is uncertain.

Acute pancreatitis and gallbladder events (cholelithiasis, cholecystitis) have been reported rarely. Allergic reactions, including urticaria, angioedema and anaphylaxis, are rare.

When to seek urgent medical advice

Stop Mounjaro and seek urgent medical attention or call 999 if you experience severe persistent abdominal pain (possible pancreatitis), difficulty breathing or swelling of the face, lips, tongue or throat (possible anaphylaxis), or severe allergic reaction.

Contact your GP or call NHS 111 if you experience persistent vomiting or diarrhoea causing dehydration, signs of hypoglycaemia that do not respond to treatment, or new or worsening mood changes.

Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Contraindications

Mounjaro must not be used in patients with type 1 diabetes, diabetic ketoacidosis, known hypersensitivity to tirzepatide or any excipient, or a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2.

Pancreatitis

Acute pancreatitis has been reported with GLP-1 receptor agonists, including tirzepatide. If pancreatitis is suspected, Mounjaro should be discontinued immediately and should not be restarted.

Patients with a history of pancreatitis should be monitored with particular care. Symptoms include severe persistent abdominal pain, sometimes radiating to the back, with nausea and vomiting.

Thyroid tumours

In rodent studies, tirzepatide caused dose-dependent and treatment-duration-dependent thyroid C-cell tumours at clinically relevant exposures.

It is unknown whether tirzepatide causes thyroid C-cell tumours, including medullary thyroid carcinoma, in humans.

Patients should be counselled to report symptoms such as a lump or swelling in the neck, difficulty swallowing, shortness of breath, or persistent hoarseness.

Diabetic retinopathy

Rapid improvement in glycaemic control has been associated with a temporary worsening of diabetic retinopathy. Patients with proliferative retinopathy or retinopathy requiring treatment should be monitored closely during dose escalation.

Renal function and dehydration

Gastrointestinal side effects such as vomiting and diarrhoea can cause dehydration, which may worsen kidney function, particularly in patients with pre-existing renal impairment. Maintain adequate fluid intake. Monitor renal function in patients reporting persistent gastrointestinal symptoms.

Drug interactions

Because Mounjaro delays gastric emptying, the absorption of oral medications taken at the same time may be affected.

This is most relevant for medicines with a narrow therapeutic index, such as warfarin and levothyroxine. Monitor INR closely when initiating or adjusting tirzepatide in patients on warfarin.

Patients on oral contraceptives should use barrier contraception for 4 weeks after starting Mounjaro and for 4 weeks after each dose escalation, as delayed gastric emptying may reduce contraceptive absorption.

Pregnancy, breastfeeding and fertility

Mounjaro should not be used during pregnancy. Animal studies have shown adverse effects on embryo-fetal development.

Women of childbearing potential should use effective contraception during treatment and for at least 4 weeks after discontinuation. Discontinue Mounjaro at least 2 months before a planned pregnancy.

It is not known whether tirzepatide passes into human breast milk. A decision to discontinue breastfeeding or treatment should be made considering the benefit to the mother.

Mounjaro compared with other treatments

The UK treatment landscape for type 2 diabetes includes several classes of glucose-lowering medicines. Metformin remains first-line therapy.

When additional treatment is needed, options include sulphonylureas, SGLT2 inhibitors (such as dapagliflozin, empagliflozin and canagliflozin), DPP-4 inhibitors (sitagliptin, linagliptin), GLP-1 receptor agonists (semaglutide, liraglutide, dulaglutide, exenatide) and insulin.

NICE guidelines recommend considering an SGLT2 inhibitor early in the treatment pathway, particularly for patients with cardiovascular disease or heart failure.

GLP-1 receptor agonists, including tirzepatide, are typically positioned as add-on therapy when earlier treatments have not achieved target HbA1c.

In the SURPASS-2 trial, tirzepatide 15 mg achieved greater HbA1c reduction (2.46 percentage points) and weight loss (12.4 kg) than semaglutide 1 mg (1.86 percentage points and 6.2 kg) at 40 weeks.

These results suggest tirzepatide may offer clinical advantages over existing GLP-1 receptor agonists, though direct comparison with semaglutide 2 mg (the higher dose approved for diabetes) was not conducted in SURPASS-2.

For weight management, tirzepatide 15 mg produced mean weight loss of 22.5 percent in SURMOUNT-1, compared with approximately 15 percent reported with semaglutide 2.4 mg (Wegovy) in the STEP programme.

Cardiovascular outcome data for tirzepatide are still maturing.

How to get Mounjaro in the UK

Mounjaro is a prescription-only medicine.

For type 2 diabetes, it can be prescribed by your GP, hospital diabetes specialist, or an authorised online prescriber registered with the General Pharmaceutical Council (GPhC).

It is available on the NHS in accordance with NICE TA924 guidance.

The standard NHS prescription charge of 9.90 pounds per item applies in England; prescriptions are free in Scotland, Wales and Northern Ireland.

For weight management, availability on the NHS depends on the outcome of NICE appraisal. Some private clinics and online prescribers in the UK offer tirzepatide for weight management.

Ensure any online service is registered with the Care Quality Commission (CQC) and the GPhC. Beware of unregulated or counterfeit products, which pose serious health risks.

The MHRA has issued warnings about the dangers of purchasing prescription weight-loss injections from unregulated sources.

Lifestyle advice while taking Mounjaro

Mounjaro works best as part of a comprehensive approach that includes dietary modification and regular physical activity.

A balanced diet rich in lean protein, vegetables, whole grains and fibre supports glycaemic control and sustainable weight management.

Eating smaller, more frequent meals may help manage gastrointestinal side effects during dose escalation. Avoid skipping meals, as this may increase the risk of nausea.

Aim for at least 150 minutes of moderate-intensity physical activity per week, as recommended by the Chief Medical Officer. If you smoke, stopping is strongly advised.

Limit alcohol to no more than 14 units per week. Attend regular diabetes reviews, including HbA1c, blood pressure, renal function, and retinal screening.

If you are using Mounjaro for weight management, your prescriber should monitor progress and review the continued need for treatment at regular intervals.

Sources

• Mounjaro, Summary of Product Characteristics (EMC)
• Tirzepatide, British National Formulary (BNF)
• NICE TA924: Tirzepatide for treating type 2 diabetes
• Type 2 diabetes, NHS
• MHRA Yellow Card Scheme