Switching from Mounjaro to Wegovy is a decision that requires careful medical supervision and individualised planning. Both medications are injectable treatments used for weight management, but they differ in their mechanisms of action, dosing schedules, and regulatory status in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) works solely through GLP-1 pathways. Patients may consider switching due to tolerability issues, supply constraints, inadequate weight loss response, or changes in NHS commissioning arrangements. This article explores the key differences between these medications, reasons for switching, and the safe transition process under medical guidance.

Understanding Mounjaro and Wegovy: Key Differences

Mounjaro (tirzepatide) and Wegovy (semaglutide) are both injectable medications used for weight management, though their licensed indications in the UK differ. Wegovy is specifically licensed for weight management in adults with obesity or overweight with weight-related comorbidities, while Mounjaro is primarily licensed for type 2 diabetes, with weight management use potentially being off-label depending on current regulatory status.

These medications work through different mechanisms. Mounjaro is a dual agonist, activating both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors. This dual action influences insulin secretion, reduces appetite, and slows gastric emptying. Wegovy, by contrast, is a selective GLP-1 receptor agonist, working primarily through GLP-1 pathways to achieve similar metabolic effects.

Key practical differences include:

• Dosing frequency: Both are given once weekly, but starting doses and maintenance regimens vary

• Titration schedules: Mounjaro typically increases every 4 weeks in 2.5 mg increments from 2.5 mg to 5–15 mg as tolerated, whilst Wegovy uses a longer titration from 0.25 mg to a maintenance dose of 2.4 mg over 16–20 weeks

• Regulatory status: Mounjaro received UK marketing authorisation more recently than Wegovy, and availability through the NHS may differ

• Clinical trial data: Both demonstrate significant weight loss in clinical trials, with semaglutide showing approximately 10-15% mean weight loss in the STEP trials and tirzepatide up to 20% in the SURMOUNT trials

Importantly, these medications should never be used together – patients should only take one GLP-1 receptor agonist at a time. The choice between Mounjaro and Wegovy should be individualised, taking into account efficacy, tolerability, availability, and patient preference. Healthcare professionals will assess whether switching is clinically appropriate based on treatment response and any adverse effects experienced.

Why People Switch from Mounjaro to Wegovy

Several clinical and practical factors may prompt a switch from Mounjaro to Wegovy. Understanding these reasons helps patients and clinicians make informed decisions about medication changes.

Tolerability and adverse effects represent a common reason for switching. Both medications share similar gastrointestinal side effects—including nausea, vomiting, diarrhoea, and constipation—but individual responses vary considerably. Some patients experience better tolerability with one agent over the other. If Mounjaro causes persistent or severe gastrointestinal symptoms despite dose adjustments and supportive measures, switching to Wegovy may be considered. There is no official link suggesting one medication universally causes fewer side effects, as tolerability is highly individual.

Supply and availability issues have affected both medications in the UK. Periodic shortages have necessitated switches to ensure continuity of treatment. NHS England has issued specific guidance during shortage periods, sometimes recommending alternative GLP-1 receptor agonists to maintain therapeutic benefit.

Inadequate weight loss response may also prompt switching. Whilst both medications demonstrate robust efficacy in clinical trials, real-world responses vary. According to the Wegovy SmPC and NICE Technology Appraisal (TA875), treatment should be discontinued if patients do not achieve at least 5% weight loss after 6 months at the maintenance dose. If Mounjaro fails to meet this threshold despite optimal dosing and lifestyle modifications, clinicians may consider switching to Wegovy.

Cost and funding considerations can influence prescribing decisions. NHS commissioning arrangements, local formulary restrictions, and individual Integrated Care Board (ICB) policies may favour one medication over another. Private prescription costs also differ, potentially affecting patient choice. Additionally, some patients may switch due to changes in private medical insurance coverage or participation in specific weight management programmes that preferentially support one medication.

When switching, clinicians should review and potentially adjust doses of insulin or sulfonylureas to reduce the risk of hypoglycaemia, particularly in patients with diabetes.

How to Switch from Mounjaro to Wegovy Safely

Switching from Mounjaro to Wegovy requires careful planning and medical supervision to ensure safety and maintain therapeutic benefit. Never attempt to switch medications without consulting your prescribing clinician, as individualised guidance is essential.

Timing and washout considerations: Do not use two GLP-1 receptor agonists concurrently. The first Wegovy dose should be given at the next scheduled injection time when your Mounjaro dose would have been due. There is no official requirement for a washout period when switching between these medications, as both have similar mechanisms and safety profiles.

Dose equivalence and titration: Direct dose equivalence between Mounjaro and Wegovy is not established, as they are different molecules with distinct potencies. Most clinicians will restart Wegovy at the initial titration dose (0.25 mg weekly) regardless of your Mounjaro maintenance dose. This approach minimises gastrointestinal side effects and allows your body to adjust to the new medication. The standard Wegovy titration schedule involves:

• Weeks 1–4: 0.25 mg once weekly

• Weeks 5–8: 0.5 mg once weekly

• Weeks 9–12: 1 mg once weekly

• Weeks 13–16: 1.7 mg once weekly

• Week 17 onwards: 2.4 mg once weekly (maintenance dose)

Your prescriber may modify this schedule based on tolerability and clinical response. Monitoring during the switch should include regular weight measurements, assessment of side effects, and review of any comorbidities such as type 2 diabetes. Blood glucose levels may fluctuate during the transition, particularly if you take other diabetes medications. If you use insulin or sulfonylureas, your doctor may reduce these doses to prevent hypoglycaemia.

Managing side effects: Even if you tolerated Mounjaro well, gastrointestinal symptoms may emerge with Wegovy. Practical strategies include eating smaller meals, avoiding high-fat foods, staying well-hydrated, and taking the injection on the same day each week. If you experience severe or persistent vomiting or diarrhoea, your kidney function may need monitoring. Contact your GP or prescriber if you experience severe or persistent vomiting, signs of dehydration, severe abdominal pain, or any concerning symptoms.

Important safety considerations: These medications are not suitable during pregnancy or breastfeeding. If planning pregnancy, Wegovy should be discontinued at least 2 months before conception, while Mounjaro should be stopped at least 1 month before. People with diabetes should be aware of potential diabetic retinopathy complications, particularly if blood glucose improves rapidly. Report any suspected side effects to your healthcare professional or via the MHRA Yellow Card Scheme.

NHS Guidance on Switching Weight Loss Medications

The NHS provides structured guidance on the use of weight loss medications, though specific protocols for switching between Mounjaro and Wegovy continue to evolve as clinical experience accumulates.

NICE Technology Appraisal (TA875) addresses the use of semaglutide (Wegovy) for managing overweight and obesity in adults. This guidance recommends that Wegovy should only be prescribed as part of a specialist weight management service, with ongoing support for diet, physical activity, and behavioural interventions. Treatment should be discontinued if patients do not achieve at least 5% weight loss after 6 months at the maintenance dose.

NHS commissioning arrangements for weight management medications vary across Integrated Care Boards (ICBs). Some areas have established specialist weight management clinics that oversee medication switches, whilst others manage these through primary care with specialist input. Eligibility criteria for NHS-funded Wegovy typically require:

• BMI ≥35 kg/m² (or ≥32.5 kg/m² for people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds)

• BMI ≥30 kg/m² with at least one weight-related comorbidity (such as type 2 diabetes, hypertension, or obstructive sleep apnoea)

Additionally, NICE TA875 specifies that treatment should be provided within a specialist weight management service and is time-limited.

Prescribing responsibilities: Switching between weight loss medications should be undertaken by clinicians experienced in their use—typically specialists in endocrinology, diabetes, or obesity medicine. GPs may continue prescriptions following specialist initiation, but the decision to switch medications generally requires specialist review. During medication shortages, NHS England has issued specific guidance, with the Specialist Pharmacy Service (SPS) providing advice on switching between GLP-1 receptor agonists.

Patient safety considerations: The UK SmPCs for these medications list specific contraindications and precautions. Patients should be advised to report any symptoms of pancreatitis (severe abdominal pain), gallbladder problems, or changes in mood, and to seek immediate medical attention if these occur. Regular follow-up appointments are essential to monitor efficacy, tolerability, and any emerging safety concerns. Patients with diabetes should be monitored for hypoglycaemia if also taking insulin or sulfonylureas, and for diabetic retinopathy, particularly if blood glucose control improves rapidly.

Frequently Asked Questions