Mounjaro vs Wegovy

Mounjaro and Wegovy are the two most-prescribed GLP-1 drugs for weight loss in the UK right now. They look similar, work similarly, but they're different molecules with different efficacy and slightly different safety histories. Here's the actual head-to-head.

Tirzepatide activates two gut hormone receptors instead of one. GLP-1 is the same target as semaglutide. GIP is an additional gut hormone that influences insulin release, fat storage, and central appetite signalling. Activating both at once produces stronger effects on appetite suppression and metabolic improvement than GLP-1 alone. The trade-off was that tirzepatide is newer with less long-term safety data, but the data accumulating is reassuring so far.

• Published 2025. 751 adults with obesity, no diabetes.
• 72 weeks of treatment.
• Average weight loss: tirzepatide 20.2%, semaglutide 13.7%.
• Tirzepatide users were more than twice as likely to lose 25% or more of body weight (about 32% vs 16%).
• Side effect rates were broadly similar: nausea 46% on tirzepatide vs 44% on semaglutide, with GI issues most common during dose escalation.
• Fewer people stopped tirzepatide because of gut side effects (2.7% vs 5.6%).
• First randomised comparison; clear winner on weight loss.

• Schedule: both weekly injections.
• Pen design: very similar.
• Storage: refrigerated, room temperature once in use.
• Dose ladder pace: both titrate up over 16-20 weeks to limit side effects.
• Side effect profile: nausea, constipation, reflux, fatigue. In SURMOUNT-5, diarrhoea was identical (25% on both); Wegovy had slightly more constipation (31% vs 27%), more vomiting (21% vs 15%) and more reflux events.
• Rare serious risks: pancreatitis, gallstones, severe gastroparesis on both. Severe, persistent abdominal pain needs same-day medical assessment, not a wait-and-see approach (see below).

• Maximum weight loss is the primary goal: lean Mounjaro on the SURMOUNT-5 evidence.
• Longer track record matters more to you: Wegovy has 3+ extra years of post-launch use.
• You've already responded well to Wegovy and tolerate it: probably no reason to switch.
• You've not responded to Wegovy after 6+ months at 2.4 mg: switching to Mounjaro is reasonable.
• You can't tolerate the GI effects of Wegovy: Mounjaro often (but not always) causes similar issues.
• Cardiovascular outcomes evidence matters most (you have prior heart disease): Wegovy currently has more CV outcomes data; tirzepatide trials are still maturing.

This article is about comfort and everyday patterns, not medical guidance. Your doctor and pharmacist know you and your medicine; if anything about how you feel worries you, speak to them promptly, and if you ever feel seriously unwell, treat it as an emergency. The medicine's own patient information leaflet carries the important safety information and is worth reading in full.

• NHS, Wegovy: through specialist weight-management services under NICE TA875 (BMI ≥35 plus at least one weight-related condition, or 30-34.9 meeting specialist referral criteria; maximum 2 years). Capacity is limited.
• NHS, Mounjaro: NICE TA1026 (BMI ≥35 plus at least one weight-related condition) with a phased 3-year rollout that includes primary care, starting with the highest-need groups.
• Both appraisals apply lower BMI thresholds (usually reduced by 2.5) for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds.
• Private clinics: both around £150-300 per month depending on dose. Mounjaro often slightly cheaper at the lower-equivalent doses.
• Pharmacy supply has been intermittent for both during the global demand surge. Mounjaro supply has been more stable since mid-2025.