The short answer
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections licensed by the MHRA for chronic weight management in adults. Both are prescription-only medicines and both must be prescribed in the context of a reduced-calorie diet and increased physical activity. The most important difference is mechanism: Wegovy is a single GLP-1 receptor agonist, while Mounjaro acts on both GLP-1 and GIP receptors.
Mechanism of action
Semaglutide (Wegovy) mimics GLP-1, a gut hormone that slows gastric emptying, suppresses appetite signalling and improves glucose-dependent insulin secretion. Tirzepatide (Mounjaro) mimics both GLP-1 and GIP. GIP is a second gut hormone that, in combination with GLP-1 receptor activation, appears to produce a larger effect on appetite and on metabolic function than GLP-1 alone.
Weight loss data
In the SURMOUNT-1 trial, tirzepatide produced average body weight reductions of around 15% at the 5 mg maintenance dose, around 19% at 10 mg, and around 21% at 15 mg, over 72 weeks. In the STEP-1 trial, semaglutide 2.4 mg produced average body weight reductions of around 15% over 68 weeks. The SURMOUNT-5 head-to-head trial reported greater average weight loss on tirzepatide than on semaglutide at the doses tested. Trial populations and protocols differ, so results are not directly comparable between trials, but the head-to-head signal is the relevant one for most clinical decisions.
Side effects
Side-effect profiles are very similar. Both medicines cause gastrointestinal symptoms — nausea, vomiting, diarrhoea, constipation and reduced appetite — most commonly during the first weeks at each new dose. Serious effects that apply to both classes include acute pancreatitis, severe allergic reaction, gallbladder problems, and (in patients with type 2 diabetes) worsening of diabetic retinopathy. Suspected side effects should be reported via the MHRA Yellow Card scheme.
Dosing
Both medicines start at a low tolerability step and escalate every four weeks. Mounjaro starts at 2.5 mg and steps in 2.5 mg increments to a maximum of 15 mg. Wegovy starts at 0.25 mg and steps through 0.5, 1.0, 1.7 and 2.4 mg. In both cases the prescriber decides on escalation based on response and tolerability, not on patient preference.
UK availability
Both are licensed by the MHRA. Both are available through specialist NHS weight-management services where eligibility criteria are met, and through private prescribers including online pharmacies regulated by the GPhC. Waiting lists in the NHS are long in many areas; many patients access tirzepatide privately for that reason.
Which one is right for me?
Choice between Mounjaro and Wegovy depends on your full medical history, your previous experience with weight management medicines, any contraindications you carry, and the prescriber's clinical judgement. Both are appropriate first-line options for the right patient. Neither is suitable for everyone.
Sources: SmPC for Mounjaro and Wegovy; NICE TA875 (semaglutide); NICE TA1026 (tirzepatide); SURMOUNT-1, SURMOUNT-5 and STEP-1 trial publications; BNF tirzepatide and semaglutide monographs.