Mounjaro vs Wegovy in the UK

At a glance

Mechanism

Both activate the GLP-1 receptor. Tirzepatide also activates the GIP (glucose-dependent insulinotropic polypeptide) receptor — a second incretin pathway. The dual mechanism is the basis for the larger trial-effect size; in practice both drugs achieve their effect through appetite suppression, slowed gastric emptying, and improved glycaemic control.

Trial outcomes

Reported trial outcomes (SURMOUNT for tirzepatide, STEP for semaglutide):

• Tirzepatide SURMOUNT-1 (adults with obesity, no diabetes, 72 weeks): mean weight loss of around 16% (5 mg), 21.4% (10 mg), and 22.5% (15 mg) vs ~2% placebo.
• Semaglutide STEP 1 (adults with overweight or obesity, no diabetes, 68 weeks): mean weight loss of around 14.9% at 2.4 mg vs ~2.4% placebo.

A direct head-to-head trial (SURMOUNT-5) reported tirzepatide 15 mg producing greater weight loss than semaglutide 2.4 mg in adults with obesity.

Trial averages don’t predict individual response. Some patients lose more on semaglutide than the trial average; some lose less on tirzepatide. The trial numbers are a useful prior, not a guarantee.

NHS access

Both are NICE-recommended for NHS use. In practice, NHS access isn’t typically a patient choice between the two — local formularies, specialist-service capacity, and supply availability determine which product a patient is offered. Both pathways require BMI thresholds and specialist-service involvement.

Side-effect picture

The GI profile is broadly similar between the two. The dose-response curve for tirzepatide is steeper — patients moving from 5 mg to 15 mg often need to slow titration to manage escalating nausea. The 12.5 mg and 15 mg tirzepatide steps are where most discontinuations cluster.

Practical factors that usually decide

• Availability. Whatever the NHS service or private clinic actually has in stock.
• Tolerability. Individual response varies; switching is a normal clinical conversation.
• Comorbidities. Patients with T2DM are often steered toward whichever product their diabetes pathway uses.
• Cost (private). Pricing changes; don't fixate on a single number.
• Trial-effect size. A meaningful prior for ‘which to try first’ but not a guarantee.