Mounjaro vs Wegovy: Which GLP-1 Medication Triggers Better Weight Loss?

Mounjaro vs Wegovy is one of the most common comparisons people make when exploring medical weight management options in the UK. Both are once-weekly injectable medicines approved for obesity, yet they differ in how they work, their clinical evidence, side effect profiles, and NHS eligibility criteria. Mounjaro (tirzepatide) activates two hormonal pathways, while Wegovy (semaglutide 2.4 mg) targets one. Understanding these differences can help you have a more informed conversation with your GP or specialist about which option may be appropriate for your circumstances.

How Mounjaro and Wegovy Work in the Body

Wegovy activates GLP-1 receptors to reduce appetite and slow gastric emptying, while Mounjaro activates both GIP and GLP-1 receptors simultaneously, a dual mechanism hypothesised to produce greater weight loss.

Both Mounjaro and Wegovy are injectable medicines used in the management of obesity, but they work through distinct pharmacological mechanisms. Understanding how each drug acts in the body helps to explain their differences in efficacy and tolerability.

Wegovy (semaglutide 2.4 mg) is a glucagon-like peptide-1 (GLP-1) receptor agonist. It mimics the action of the naturally occurring GLP-1 hormone, which is released from the gut after eating. By activating GLP-1 receptors — particularly in the hypothalamus — Wegovy reduces appetite, slows gastric emptying, and promotes a feeling of fullness. It also stimulates insulin secretion in a glucose-dependent manner, helping to regulate blood sugar levels. Wegovy is administered as a once-weekly subcutaneous injection using a single-dose pre-filled pen. The dose is titrated gradually over 16 weeks (starting at 0.25 mg and increasing to the maintenance dose of 2.4 mg) to minimise gastrointestinal side effects, as described in the UK Summary of Product Characteristics (SmPC).

Mounjaro (tirzepatide) takes a dual-action approach. It is a GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptor co-agonist, meaning it activates two distinct hormonal pathways simultaneously. GIP receptors are found in adipose tissue and the brain, and their activation is hypothesised to enhance the weight-reducing effects of GLP-1 signalling — though the precise mechanisms by which dual agonism produces greater weight loss than GLP-1 receptor agonism alone are still being investigated. Mounjaro is administered as a once-weekly subcutaneous injection using a multi-dose pre-filled pen (requiring manual injection), not an auto-injector. The dose is titrated from 2.5 mg weekly, increasing at four-weekly intervals up to a maximum of 15 mg, as per the UK SmPC.

While the end goals of both medicines are similar — reducing caloric intake and supporting sustainable weight loss — their differing mechanisms and delivery devices are relevant considerations when discussing treatment options with a healthcare professional.

Comparing Clinical Evidence and Weight Loss Outcomes

Tirzepatide 15 mg achieved approximately 20.9% average weight loss in SURMOUNT-1, compared with approximately 14.9% for semaglutide 2.4 mg in STEP 1; the head-to-head SURMOUNT-5 trial also reported greater weight loss with tirzepatide.

The clinical evidence base for both medicines is robust, drawn from large-scale randomised controlled trials that have informed regulatory approvals by the European Medicines Agency (EMA) and the Medicines and Healthcare products Regulatory Agency (MHRA).

Wegovy was evaluated in the STEP clinical trial programme. In the landmark STEP 1 trial (Wilding et al., NEJM, 2021), adults with obesity (BMI ≥30 kg/m²) or overweight with at least one weight-related comorbidity achieved an average weight loss of approximately 14.9% of body weight over 68 weeks with semaglutide 2.4 mg, compared with 2.4% with placebo. The STEP 4 trial (Rubino et al., NEJM, 2021) further demonstrated that continuing treatment was necessary to maintain weight loss, with participants who switched to placebo regaining a significant proportion of lost weight.

Mounjaro was assessed in the SURMOUNT trial programme. In SURMOUNT-1 (Jastreboff et al., NEJM, 2022), participants without type 2 diabetes receiving the highest dose of tirzepatide (15 mg) achieved an average weight reduction of approximately 20.9% over 72 weeks. Even at lower doses (5 mg and 10 mg), weight loss exceeded that typically observed with semaglutide in comparable populations. It is important to note that average weight loss tends to be somewhat lower in people with type 2 diabetes than in those without, for both medicines.

A direct head-to-head randomised controlled trial, SURMOUNT-5, has compared tirzepatide with semaglutide 2.4 mg; results have been reported and suggest greater average weight loss with tirzepatide. Readers should refer to the published trial data for full details, as findings continue to be reviewed in the context of clinical practice. Network meta-analyses also consistently suggest that tirzepatide produces greater average weight loss than semaglutide 2.4 mg at a population level.

Clinicians should interpret population-level data cautiously when advising individual patients. Individual responses vary considerably, and factors such as baseline weight, the presence of type 2 diabetes, adherence, diet, and physical activity all influence outcomes. Both medicines represent a significant advance over previously available pharmacological options for obesity management.

Side Effects, Risks, and Safety Considerations

Both medicines commonly cause nausea, vomiting, and diarrhoea, particularly during dose escalation; additional cautions include pancreatitis, gallbladder disease, and dehydration, with tirzepatide requiring extra contraceptive precautions for oral contraceptive users.

Both Mounjaro and Wegovy share a broadly similar side effect profile, largely attributable to their shared GLP-1 receptor activity. Patients and prescribers should be aware of these effects before initiating treatment.

Common side effects for both medicines include:

• Nausea and vomiting

• Diarrhoea or constipation

• Abdominal discomfort or bloating

• Reduced appetite (which is, in part, the intended effect)

• Injection site reactions

These gastrointestinal effects are most pronounced during dose escalation and typically improve over time. Starting at a low dose and titrating slowly — as recommended in the prescribing information — helps to minimise their severity.

Additional cautions that apply to both medicines include:

• Pancreatitis: Both medicines should be used with caution in patients with a history of pancreatitis. Treatment should be discontinued if pancreatitis is confirmed.

• Gallbladder disease: Gallstones and cholecystitis have been reported; patients should be advised of the symptoms.

• Dehydration and acute kidney injury: Significant gastrointestinal losses (vomiting or diarrhoea) can lead to dehydration and, in some cases, acute kidney injury. Patients should maintain adequate fluid intake and seek medical advice if symptoms are severe or prolonged.

• Severe gastrointestinal disease: Both medicines should be used with caution in patients with gastroparesis or other severe gastrointestinal conditions, as they slow gastric emptying.

• Hypoglycaemia: When used alongside insulin or sulfonylureas in people with type 2 diabetes, there is an increased risk of hypoglycaemia. Dose adjustments of the concomitant medicine may be required.

• Diabetic retinopathy: Rapid improvement in blood glucose control has been associated with worsening of diabetic retinopathy in people with diabetes. Patients with pre-existing retinopathy should be monitored appropriately.

• Thyroid C-cell tumours: Rodent studies have identified thyroid C-cell tumours with GLP-1 receptor agonists; the clinical relevance in humans is uncertain.^[1][3] The UK SmPCs for both medicines include this as a precautionary warning rather than a formal contraindication. Patients with symptoms such as a neck lump, hoarseness, or difficulty swallowing should seek medical advice.

Contraception: For tirzepatide, the UK SmPC advises that oral contraceptive exposure may be reduced during the first four weeks after initiation and after each dose increase, due to delayed gastric emptying.^[3][12] During this period, patients should use a non-oral contraceptive method or add a barrier method. This interaction has not been demonstrated to be clinically relevant with injectable semaglutide (Wegovy).

Pregnancy and breastfeeding: Neither medicine is recommended during pregnancy or breastfeeding. People who could become pregnant should use effective contraception during treatment. Based on UK SmPC guidance, semaglutide should be stopped at least two months before a planned pregnancy, and tirzepatide at least one month before planned conception, due to their respective half-lives.^[1][3] Patients should discuss family planning with their prescriber.

Patients should be advised to contact their GP or seek urgent medical attention if they experience:

• Severe or persistent abdominal pain (possible pancreatitis)

• Symptoms of gallbladder problems such as upper right abdominal pain, fever, or jaundice

• Signs of a serious allergic reaction

• Symptoms of severe dehydration

Suspected side effects should be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. The MHRA continues to monitor the safety profiles of both medicines through post-marketing surveillance.

Eligibility and Prescribing Criteria on the NHS

Wegovy is recommended under NICE TA875 and Mounjaro under NICE TA1026; both require a BMI threshold, at least one weight-related comorbidity, and engagement with a specialist weight management service.

Access to Mounjaro and Wegovy on the NHS is governed by NICE guidance and NHS England commissioning policies, which set out specific eligibility criteria to ensure these medicines are targeted at those most likely to benefit. It should be noted that NICE guidance applies to England; commissioning arrangements may differ in Scotland, Wales, and Northern Ireland, where separate guidance from SMC, AWMSG, and relevant health authorities applies.

Wegovy (semaglutide 2.4 mg) received a positive NICE technology appraisal (TA875) in March 2023. It is recommended for adults who meet all of the following criteria:

• BMI of at least 35 kg/m² (or 32.5 kg/m² for people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family backgrounds, who are at equivalent risk at a lower BMI)

• At least one weight-related comorbidity (such as type 2 diabetes, hypertension, obstructive sleep apnoea, or cardiovascular disease)

• Engagement with a specialist weight management service that provides dietary advice, physical activity support, and behavioural interventions

• No contraindications to the medicine

NICE TA875 specifies that Wegovy should be used as part of a comprehensive weight management programme. NHS commissioning currently supports treatment for a maximum of two years, reflecting the importance of sustained lifestyle change.^[15]

Mounjaro (tirzepatide) received a positive NICE technology appraisal (TA1026) in January 2025 for weight management in adults.^[10] Patients should confirm the exact eligibility criteria with their GP or specialist, as the full commissioning details — including BMI thresholds, comorbidity requirements, and service specifications — are set out in the final NICE guidance and associated NHS England commissioning policy. As with Wegovy, treatment is expected to be delivered within a specialist weight management service.

NHS rollout for both medicines is being phased, and availability through specialist services may vary by region. Patients should speak with their GP in the first instance to discuss eligibility and referral pathways. Neither medicine is currently recommended for use in children or adolescents under NHS commissioning.

Cost and Availability in the UK

Both medicines are available on the NHS via specialist referral and privately through CQC-registered clinics; supply shortages and phased rollout mean access varies by region and waiting times can be significant.

The cost and availability of Mounjaro and Wegovy differ depending on whether they are accessed through the NHS or privately, and both have experienced supply challenges since their launch.

On the NHS, both medicines are available only through specialist weight management services following a formal referral. In England, NHS prescriptions are subject to the standard prescription charge (check the current charge at nhs.uk, as this is updated periodically), though many patients with qualifying conditions may be exempt. Prescription charges do not apply in Scotland or Wales, and arrangements differ in Northern Ireland. The phased rollout means that not all eligible patients will have immediate access, and waiting times for specialist services can be considerable in some areas.

Privately, both medicines are available through registered online and in-person clinics. Private costs vary by dose and provider and may change over time; patients should obtain up-to-date pricing directly from their chosen provider. Patients should ensure they are using a CQC-registered service (in England) and receiving prescriptions from a qualified prescriber. Equivalent registration requirements apply in other UK nations.

The MHRA has issued alerts about falsified and counterfeit versions of GLP-1 receptor agonist medicines being sold online. Patients should never purchase these medicines from unregulated sources and should report suspected counterfeit products via the MHRA Yellow Card scheme.

Supply shortages have affected both products since their launch, partly due to high global demand. NHS England and the Department of Health and Social Care (DHSC) have issued guidance advising that, during periods of supply constraint, GLP-1 receptor agonists should be prioritised for patients using them for their licensed diabetes indications (for example, Ozempic for type 2 diabetes).^[17] Wegovy and Mounjaro are licensed specifically for weight management and are separate products; patients and prescribers should follow current DHSC and NHS England supply disruption guidance. Patients already established on treatment should not stop abruptly without medical advice, as this can lead to rapid weight regain.

Which Treatment May Be More Suitable for You

Mounjaro may suit those seeking greater average weight loss or who have type 2 diabetes, while Wegovy may be preferred where cardiovascular risk reduction is a priority, as the SELECT trial demonstrated reduced major cardiovascular events with semaglutide 2.4 mg.

Choosing between Mounjaro and Wegovy is not a straightforward decision, and it should always be made in consultation with a qualified healthcare professional who can assess your individual medical history, preferences, and treatment goals.

Mounjaro may be worth discussing with your clinician if:

• You are seeking the greatest possible degree of weight loss, as the clinical evidence — including the SURMOUNT-5 head-to-head trial — suggests higher average reductions in body weight with tirzepatide compared with semaglutide 2.4 mg

• You have type 2 diabetes alongside obesity, as tirzepatide also has a licensed indication for blood glucose management

• You have not responded adequately to a GLP-1 receptor agonist, though evidence specifically supporting switching after non-response is limited and this decision should be made on a case-by-case basis with your prescriber

Wegovy may be worth discussing with your clinician if:

• Cardiovascular outcomes are a particular concern — the SELECT trial demonstrated that semaglutide 2.4 mg significantly reduced the risk of major adverse cardiovascular events in adults with obesity and established cardiovascular disease; patients should check the current UK SmPC for the licensed indications and any cardiovascular claims

• You have previously tolerated semaglutide at lower doses (for example, Ozempic for diabetes) and wish to continue with a familiar medicine

• Access, device preference, or other practical considerations favour it in your circumstances

Practical considerations that may influence shared decision-making include:

• Device handling: Wegovy uses a single-dose pre-filled pen; Mounjaro uses a multi-dose pre-filled pen requiring manual injection

• Contraception: people using tirzepatide who take oral contraceptives should use an additional or alternative contraceptive method for four weeks after initiation and after each dose increase (see the Side Effects section above)

• Side-effect tolerance and titration schedules, which differ between the two medicines

Both medicines require a long-term commitment to lifestyle change to achieve and maintain results. Neither is a standalone solution, and both are intended to complement — not replace — dietary modification and increased physical activity. Weight regain is common if treatment is discontinued without sustained behavioural changes in place.

If you believe you may be eligible for either treatment, the recommended first step is to speak with your GP, who can assess your BMI, comorbidities, and suitability for referral to a specialist weight management service. Self-prescribing or purchasing these medicines without medical supervision carries significant safety risks and should be avoided.

Frequently Asked Questions