Wegovy vs Mounjaro: Which Weight Loss Injection Is Right for You?

If you've spent any time researching weight loss injections in the UK, you've almost certainly landed on the same question: Wegovy or Mounjaro? Both are prescription GLP-1 medications. Both can produce significant weight loss. And both come with a list of possible side effects that probably made you pause mid-scroll.

But they're not the same medication, and the differences matter more than most comparison articles let on. The right choice depends on your body, your health history, and — frankly — what you can access and afford right now in the UK.

How Wegovy and Mounjaro actually work

Both medications belong to a class of drugs that mimic GLP-1, a hormone your gut releases after eating. GLP-1 slows gastric emptying, signals fullness to your brain, and helps regulate blood sugar. That's why both Wegovy and Mounjaro reduce appetite so effectively — they're amplifying a system your body already uses.

The difference is that Mounjaro doesn't stop at GLP-1. It also mimics a second hormone called GIP (glucose-dependent insulinotropic polypeptide). GIP has its own effects on appetite, fat metabolism, and insulin sensitivity. This dual-agonist approach is why Mounjaro is sometimes called a "twincretin" — it's working two levers where Wegovy works one.

Does that automatically make Mounjaro better? Not necessarily. More mechanisms can mean more side effects. And Wegovy's single-target approach has a longer track record and more clinical data behind it, particularly around heart health.

Weight loss results: what the trials actually show

In the SURMOUNT-1 trial, people taking the highest dose of Mounjaro (15 mg) lost an average of 22.5% of their body weight over 72 weeks. That's roughly 24 kg for someone starting at 105 kg. The lower doses (5 mg and 10 mg) produced 15% and 19.5% weight loss respectively.

Wegovy's pivotal STEP 1 trial showed average weight loss of about 14.9% over 68 weeks at the standard 2.4 mg dose. Substantial, but noticeably less than Mounjaro's top-end results.

Here's where it gets more nuanced, though. In January 2026, the MHRA approved a higher Wegovy dose — 7.2 mg — based on trial data showing around 21% body weight reduction. That narrows the gap with Mounjaro considerably.

Head-to-head: the SURMOUNT-5 data

The study everyone was waiting for — a direct comparison. SURMOUNT-5 pitted tirzepatide against semaglutide 2.4 mg (the standard Wegovy dose, not the newer 7.2 mg) over 72 weeks.

Results: tirzepatide produced about 20.2% weight loss versus 13.7% for semaglutide. A meaningful margin. We don't yet have head-to-head data comparing Mounjaro 15 mg against Wegovy 7.2 mg. When that trial happens, the results will likely be much closer.

Side effects compared

Nausea, constipation, diarrhoea, and reduced appetite are the most commonly reported with either drug. These tend to be worst during the first 4–8 weeks and during dose increases. Most people find they settle significantly once they've been on a stable dose for a month or so.

Mounjaro's dual mechanism does, however, increase the likelihood of GI side effects in some patients. Anecdotally, we see slightly more reports of sulfur burps with tirzepatide — an unpleasant but harmless side effect that tends to resolve within the first couple of months.

Side effects worth knowing about

Common (both medications):

• Nausea (affects roughly 20–30% of patients, usually temporary)
• Constipation or diarrhoea
• Injection site reactions
• Fatigue during the titration phase

Less common but important:

• Gallbladder issues — both GLP-1 medications carry a small increased risk of gallstones, particularly during rapid weight loss
• Pancreatitis — rare, but both carry warnings. Persistent severe abdominal pain needs urgent medical attention
• Changes in mood or energy — some patients report this, though it's not well-studied yet

If you're concerned about managing GLP-1 side effects, we've written a detailed guide covering each one and what actually helps.

Cardiovascular benefits: Wegovy's advantage

This is one area where Wegovy currently has a clear edge.

The SELECT trial — involving over 17,600 adults with obesity and established cardiovascular disease — found that semaglutide reduced the risk of major cardiac events (heart attack, stroke, cardiovascular death) by 20% compared to placebo. That's a significant finding, and it's why the MHRA has approved Wegovy specifically for cardiovascular risk reduction in eligible patients.

Mounjaro doesn't have equivalent data yet. The SURPASS-CVOT trial is ongoing, but results aren't expected for some time. Early signals from metabolic markers (blood sugar, lipids, blood pressure) are encouraging, but we can't say with confidence that tirzepatide offers the same heart protection until the trial data is in.

For patients with existing heart disease or significant cardiovascular risk factors, this distinction matters. It's worth discussing with your prescriber.

Cost in the UK: what you'll actually pay

Neither medication is cheap privately, and NHS access remains severely limited for both.

NHS availability: Wegovy is recommended by NICE (TA875) for adults with BMI ≥35 (or ≥30 with weight-related conditions) through specialist weight management services. In practice, waiting lists are long — often 12–24 months. Mounjaro has NHS approval via NICE (TA1026) under similar criteria, but supply and access vary enormously by region.

Private costs: Most UK online pharmacies charge between £149 and £299 per month depending on the medication, dose, and what's included in the service (consultations, ongoing support, blood tests).

At heySlim, both medications are available through our doctor-led service, and pricing is transparent — no hidden consultation fees or surprise costs at higher doses.

If cost is a major facto, the practical question often becomes "which one can I access and sustain for 12+ months?" rather than which one is theoretically superior.

Who is each medication best suited for?

There's no universal answer, but here are some patterns we see in practice.

Mounjaro may be a better fit if:

• You have significant weight to lose (BMI 35+) and want the highest possible efficacy
• You have type 2 diabetes or insulin resistance — tirzepatide's dual action on GIP provides additional metabolic benefits
• You've previously tried semaglutide (Ozempic or Wegovy) and plateaued or didn't respond well
• You tolerate GI side effects reasonably well

Wegovy may be a better fit if:

• You have existing cardiovascular disease or significant heart risk factors — the SELECT trial data gives Wegovy a proven edge here
• You want a medication with a longer safety track record (semaglutide has been prescribed since 2017)
• You've had difficulty tolerating Mounjaro's side effects and want to try a single-agonist approach
• The new 7.2 mg dose is available to you, narrowing the efficacy gap

Either medication works well for:

• Adults with BMI ≥30 (or ≥27 with weight-related health conditions)
• People committed to combining medication with dietary changes and regular activity

What about Ozempic?

You might be wondering where Ozempic fits in. Ozempic is the same active ingredient as Wegovy — semaglutide — but it's licensed for type 2 diabetes, not weight management. The maximum Ozempic dose (2 mg) is lower than Wegovy's weight loss doses (2.4 mg and 7.2 mg).

Some GPs prescribe Ozempic off-label for weight loss, and it does work. But if weight management is your primary goal, Wegovy at the approved dose is the more appropriate choice.

Switching between Wegovy and Mounjaro

Switching is possible, and more common than you might think. Some reasons patients switch:

• Side effect intolerance — if one medication causes persistent nausea or GI issues that don't settle after adequate titration time, the other may agree with you better
• Plateau — some patients find their weight loss stalls on one medication and responds to the other
• Supply issues — stock shortages have affected both medications in the UK, particularly Wegovy in 2024–2025
• Cost changes — Eli Lilly's 2025 price increase on Mounjaro has prompted some patients to reconsider

If you're thinking about switching, it should always be done under medical supervision. The titration schedules are different, and starting doses need to be carefully matched to avoid unnecessary side effects. We've covered the process in detail in our switching guide.

The bottom line

Both Wegovy and Mounjaro are effective weight loss medications backed by strong clinical evidence. Mounjaro's dual mechanism gives it a slight edge on average weight loss, but Wegovy's cardiovascular data is something Mounjaro can't yet match. The new Wegovy 7.2 mg dose has made the efficacy comparison much tighter than it was even six months ago.

The most important factor isn't which medication is theoretically "best" — it's which one you can access, afford, and stick with long enough to see real results. Both work. Both require commitment. And both work best when paired with proper clinical support, not just a monthly prescription.

If you're unsure which is right for you, a consultation with a prescribing clinician — someone who can review your medical history, metabolic profile, and goals — is the best next step.

Frequently asked questions

Is Mounjaro more effective than Wegovy for weight loss?

On average, yes — Mounjaro at 15 mg produced about 22.5% weight loss in trials, versus 14.9% for standard-dose Wegovy. However, the newer Wegovy 7.2 mg dose achieves around 21%, making the gap much smaller. Individual responses vary significantly.

Can I switch from Wegovy to Mounjaro (or vice versa)?

Yes, but it should be done under medical supervision. The medications use different titration schedules and dosing structures, so your clinician needs to plan the transition carefully to minimise side effects.

Which has fewer side effects — Wegovy or Mounjaro?

Both cause similar GI side effects (nausea, constipation, diarrhoea). Mounjaro's dual-agonist mechanism can cause slightly more stomach-related symptoms in some people, but this varies. Slow titration helps with both.

Is Wegovy or Mounjaro available on the NHS?

Both have NICE approval, but NHS access is extremely limited due to eligibility criteria and long waiting lists. Most patients in the UK currently access these medications through private prescriptions.

How long do I need to take Wegovy or Mounjaro?

Weight loss medications work for as long as you take them. Most clinical guidance suggests at least 12 months of treatment, with ongoing reassessment. Stopping abruptly typically leads to weight regain — which is why a structured maintenance plan matters.

This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before starting any treatment.