Quick Answer
Both are weekly injections, but they work differently. Mounjaro targets two hormone receptors (GIP and GLP-1), while Wegovy targets only GLP-1. In clinical trials, Mounjaro produced greater average weight loss (22.5% vs 14.9%). Both are NICE-approved for weight management.
Detailed Answer
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both injectable prescription medications approved for weight management in the UK, but they have important differences in how they work and their clinical profiles.
The most fundamental difference is their mechanism of action. Wegovy is a GLP-1 receptor agonist — it mimics the GLP-1 hormone, which reduces appetite, slows gastric emptying, and signals satiety to the brain. Mounjaro is a dual GIP/GLP-1 receptor agonist — it targets both the GLP-1 receptor and the GIP (glucose-dependent insulinotropic polypeptide) receptor. This dual mechanism is believed to provide enhanced metabolic effects beyond those of GLP-1 alone.
In terms of clinical efficacy, Mounjaro showed greater average weight loss in its pivotal trial: 22.5% at the 15mg dose over 72 weeks (SURMOUNT-1) compared to Wegovy's 14.9% at 2.4mg over 68 weeks (STEP 1). However, these were separate trials with different patient populations, so direct comparison has limitations.
Both medications are injected once weekly using a pre-filled pen. Both follow a gradual dose escalation schedule to minimise side effects. The side effect profiles are similar, with gastrointestinal symptoms (nausea, diarrhoea, constipation) being the most common for both.
Wegovy received NICE approval for weight management via TA875 in 2023, while Mounjaro received approval via TA1026 in December 2024. Both can be prescribed in specialist weight management services and, for Mounjaro, also in primary care under the NICE guidance.