The headline numbers

The two pivotal trials had different durations and dose escalation, but here's the apples-to-apples comparison:

• Mounjaro 15mg (SURMOUNT-1, 72 weeks): ~21% mean body weight loss
• Wegovy 2.4mg (STEP-1, 68 weeks): ~15% mean body weight loss
• Placebo (both trials): ~2.4–3% mean weight loss

That 21% vs 15% difference is real and meaningful — but it's a mean. Individual results vary widely on both drugs. Some patients lose more on Wegovy than they would have on Mounjaro because of how their body responds to one molecule vs the other.

How they work — the key mechanistic difference

Wegovy (semaglutide) mimics one gut hormone: GLP-1. One pathway, one signal to your brain saying "you're full".

Mounjaro (tirzepatide) mimics two gut hormones at once: GLP-1 and GIP. Two pathways, more pronounced effect. The dual mechanism is why Mounjaro tends to produce greater weight loss — but it's also why side effects can be more pronounced, particularly in the first 8 weeks.

Side effects compared

Both drugs have similar side effect profiles — gastrointestinal symptoms (nausea, diarrhoea, constipation, indigestion) dominate, particularly in the first 4–8 weeks and after dose increases. The frequency and intensity differs:

• Nausea: Mounjaro ~25%, Wegovy ~20%
• Diarrhoea: Mounjaro ~20%, Wegovy ~30%
• Constipation: Both ~15–20%
• Gallbladder problems: Both ~1–2%
• Pancreatitis: Rare on both (~0.2%)

Mounjaro's faster dose escalation (every 4 weeks vs Wegovy's slower ramp) means side effects can hit harder when stepping up. Most patients on either drug find side effects settle by week 8–12.

Cost — the practical reality

Pricing varies by dose and supplier and changes regularly. Both medications cost a similar amount per month at equivalent therapeutic doses — generally Mounjaro is slightly more expensive at maintenance doses. We give exact monthly figures at your free consultation. We charge for the medication only — consultation and monthly reviews are free at Preston Clinic.

Stock availability

Mounjaro has had supply constraints in the UK through 2024–25 because demand has outstripped Eli Lilly's manufacturing scale-up. Wegovy has been more reliably in stock. As of 2026, both are generally available, but if you start treatment and your dose isn't in stock when you need to escalate, that's worth knowing in advance.

When to choose Mounjaro

• You want the highest probability of achieving significant weight loss (more than 20% of body weight)
• You can tolerate a faster dose escalation
• You haven't responded as well as hoped to Wegovy in the past
• You have type 2 diabetes (Mounjaro improves both glucose and weight)

When to choose Wegovy

• You're sensitive to side effects and want a gentler escalation
• You have established cardiovascular disease (the SELECT trial showed semaglutide reduces major adverse cardiovascular events)
• You've tried Mounjaro and the side effects were too much
• You want a longer track record — semaglutide has been used for diabetes since 2017, more long-term real-world data

Can you switch from one to the other?

Yes, in either direction. We'd typically restart the new drug at its lowest starting dose (Mounjaro 2.5mg, or Wegovy 0.25mg) to let your body adjust. The switch is straightforward but should be done under guidance — we manage this regularly at Preston Clinic.

Our default recommendation

Most patients at Preston Clinic start on Mounjaro because the weight loss numbers are better. We switch to Wegovy if side effects are intolerable, if cardiovascular disease is a major factor, or if the patient prefers a slower, gentler approach. The free initial consultation covers your specifics and we'll explain which we'd recommend and why.

NHS routes (both drugs)

Both Mounjaro and Wegovy are available on the NHS via Tier 3 weight management services. Waiting lists run 12–24 months. If you have time and meet the criteria, ask your GP about referral — that's the free route. If you want to start this month, private is the practical option.