Mounjaro vs Wegovy: Tirzepatide vs Semaglutide Compared Honestly

A balanced, evidence-informed comparison of Mounjaro (tirzepatide) and Wegovy (semaglutide) — how they work, weight loss data, side effects, cost in the UK, and which one might suit you.

Quick comparison table

Before we get into the detail, here's the headline summary. Both drugs are effective. The differences are real but narrower than most internet debates suggest.

Now let's unpack what actually matters.

What they actually are

Both Mounjaro and Wegovy are injectable medications originally developed for type 2 diabetes that turned out to be remarkably effective for weight loss. They're both GLP-1 receptor agonists — they mimic a gut hormone called GLP-1 that tells your brain you're full and slows down how fast your stomach empties.

The key difference: Mounjaro is a dual agonist. It targets both GLP-1 and a second hormone called GIP (glucose-dependent insulinotropic polypeptide). Wegovy targets GLP-1 only.

In plain English: Mounjaro pulls two levers. Wegovy pulls one. Both levers lead to appetite suppression, slower gastric emptying, and better blood sugar control — but the dual mechanism appears to produce a stronger combined effect.

Worth knowing: "Ozempic" is the same active ingredient as Wegovy (semaglutide) but at a lower dose and licensed specifically for type 2 diabetes, not weight management. If someone tells you they're on "Ozempic for weight loss," they're either on an off-label prescription or they mean Wegovy. For this article, when we say semaglutide we mean the full weight-management dose (Wegovy 2.4 mg).

How they work differently

Both drugs do the same core things: suppress appetite, slow gastric emptying, improve insulin sensitivity, and reduce "food noise" — that constant background chatter about what to eat next. Most people on either drug describe the same subjective experience: food just stops being as interesting.

Where they diverge is at the receptor level:

• Semaglutide (Wegovy)binds to GLP-1 receptors in the gut, pancreas and brain. It's a pure GLP-1 agonist — one target, one mechanism.
• Tirzepatide (Mounjaro) binds to both GLP-1 and GIP receptors. GIP is another incretin hormone involved in fat metabolism and energy balance. The dual action appears to enhance fat loss beyond what GLP-1 alone achieves, and may improve how the body processes and stores fat.

The practical upshot: researchers believe the GIP component is why Mounjaro tends to produce slightly more weight loss in head-to-head comparisons. It's not that Wegovy doesn't work — it clearly does — but the second mechanism gives Mounjaro an edge in clinical trials.

Weight loss: what the trials actually show

The two landmark trial programmes are SURMOUNT (tirzepatide) and STEP (semaglutide). Both are large, well-designed, Phase III studies. Here's what they found:

SURMOUNT trials (Mounjaro / tirzepatide)

• SURMOUNT-1 (72 weeks): participants lost an average of 15% body weight at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg versus 3.1% on placebo.
• That 15 mg figure equates to roughly 24 kg (3.7 stone) for someone starting at 115 kg.
• Over a third of participants at the highest dose lost more than 25% of their body weight.

STEP trials (Wegovy / semaglutide)

• STEP 1 (68 weeks): participants lost an average of 14.9% body weight at 2.4 mg versus 2.4% on placebo.
• STEP 2 (in people with type 2 diabetes): average loss of 9.6% — lower because diabetes makes weight loss harder.
• About a third of participants lost more than 20% of their body weight.

The SURMOUNT-5 trial (2024) was the first proper head-to-head: tirzepatide 15 mg vs semaglutide 2.4 mg over 72 weeks. Tirzepatide produced 20.2% weight loss versus 13.7% for semaglutide — a statistically significant and clinically meaningful gap of about 6.5 percentage points.

It's also worth noting: trial results are averages. Individual responses vary enormously. Some people lose 30% on Wegovy. Some people lose 10% on Mounjaro. Your genetics, adherence, diet, movement, sleep and stress all feed into your personal result. The drug is the catalyst — what you do alongside it determines the outcome. Our approaches guide covers how to stack the deck in your favour.

Side effect comparison

The honest answer: side effects are very similar between the two drugs. Both primarily cause gastrointestinal issues because they're both slowing your stomach down.

Common side effects (both drugs)

• Nausea — the most frequently reported, worst in the first 2-3 days after each dose and after dose increases
• Diarrhoea or constipation — often one or the other, sometimes alternating
• Reduced appetite — technically the intended effect, but it can feel extreme at first
• Fatigue — especially in the first few weeks as your body adjusts to lower calorie intake
• Headaches — usually related to dehydration or reduced food intake
• Reflux and heartburn — food sitting in the stomach longer means more acid exposure

Where they differ slightly

• Injection site reactions — slightly more common with Mounjaro. Some people report redness, itching or a small lump at the injection site. Usually mild and short-lived.
• Nausea severity— SURMOUNT-5 reported broadly similar nausea rates between the two drugs, though some real-world reports suggest Mounjaro's titration schedule (larger dose jumps) can make the first few weeks choppier.
• Gallbladder issues — rapid weight loss on either drug increases gallstone risk. This is a weight-loss side effect, not drug-specific.
• Cardiovascular benefit— semaglutide has stronger cardiovascular outcome data (the SELECT trial showed a 20% reduction in major cardiac events). Tirzepatide's CV outcome trial (SURPASS-CVOT) is still ongoing.

Most side effects on both drugs peak in the first 2-4 weeks and after each dose increase, then settle. Our nausea-friendly meal ideas work equally well for both medications — the underlying mechanism is the same.

Dosing and titration

Both drugs start low and titrate up over several months. The schedules differ quite a bit:

Mounjaro titration

• Start: 2.5 mg weekly for 4 weeks
• Step up to 5 mg for at least 4 weeks
• Then 7.5 mg, 10 mg, 12.5 mg, 15 mg — each step held for at least 4 weeks
• Minimum time to reach max dose: about 20 weeks (5 months)
• Many people find their effective dose at 5 mg, 7.5 mg or 10 mg and never need to go higher

Wegovy titration

• Start: 0.25 mg weekly for 4 weeks
• Step to 0.5 mg for 4 weeks
• Then 1 mg for 4 weeks, then 1.7 mg for 4 weeks
• Maintenance dose: 2.4 mg
• Minimum time to reach max dose: about 16-20 weeks
• The titration is more gradual with smaller incremental jumps, which some people find easier to tolerate

A key difference: Mounjaro has more dose flexibility. With six possible maintenance doses (5 mg through 15 mg), your prescriber has more room to find the sweet spot — enough appetite suppression to be effective without intolerable side effects. Wegovy is more binary: you're aiming for 2.4 mg, and if you can't tolerate it, there's less room to adjust.

Cost and availability in the UK

This is where it gets messy, and it changes frequently. Here's the situation as of early 2026:

NHS access

Both Mounjaro and Wegovy are NICE-approved for weight management in the UK, but approval doesn't mean easy access. NHS prescribing criteria are strict:

• BMI of 35+ (or 30+ with a weight-related health condition)
• Referral through a specialist weight management service
• Maximum treatment duration of 2 years on the NHS
• Waiting lists for specialist services can be 6-18 months depending on your area

In practice, NHS supply of both drugs has been patchy. Wegovy had severe stock issues through much of 2024-2025. Mounjaro supply has generally been more stable, though this varies by region. Your GP can't just hand you a prescription — it has to come through the specialist pathway.

Private access

Most people in the UK currently access these medications privately, either through online prescribing services or private clinics. Costs vary:

• Mounjaro: typically £150-220/month at lower doses, rising to £250-300/month at higher doses
• Wegovy: typically £150-250/month, though availability has been inconsistent
• Some services charge a separate consultation fee (£20-50) on top of the medication cost

At the time of writing, Mounjaro tends to be slightly easier to get hold of in the UK private market. Wegovy supply has improved since 2024 but still experiences periodic shortages. Check our medications page for the most up-to-date availability information.

So which one is "better"?

The honest answer: it depends on you, and anyone who gives you a definitive answer without knowing your medical history is guessing.

Here's a framework for thinking about it:

Mounjaro might suit you better if...

• You have a significant amount of weight to lose (the extra efficacy matters more with higher starting weight)
• You want more dose flexibility — six maintenance dose levels give you and your prescriber more room to fine-tune
• You have type 2 diabetes or insulin resistance (the dual GLP-1/GIP mechanism may offer additional metabolic benefits)
• Availability matters — Mounjaro supply has been more consistent in the UK private market

Wegovy might suit you better if...

• You have cardiovascular risk factors — semaglutide has proven CV outcome benefits that tirzepatide doesn't yet
• You prefer a more gradual titration with smaller dose jumps
• You've already tried semaglutide at a lower dose (e.g., Ozempic for diabetes) and know you tolerate it well
• Your prescriber has more experience with semaglutide — it's been available longer and there's more long-term safety data

In reality, many people don't get to choose. Your prescriber recommends one based on your health profile, what's in stock, and what they're experienced with. If the first one doesn't work for you, switching to the other is a reasonable conversation to have.

Can you switch between them?

Yes, and it's more common than you'd think. People switch for several reasons:

• Side effect intolerance— can't tolerate one but do fine on the other
• Plateaued results — weight loss has stalled on one and switching can restart progress
• Supply issues— your current drug isn't available and you need an alternative
• Cost — one becomes more affordable than the other through your provider

The switch is straightforward in principle: stop one, start the other. Your prescriber will decide whether to restart at the lowest dose or begin at a moderate dose since your body is already adapted to a GLP-1 agonist. Most prescribers start the new drug within a week of the last dose of the old one.

Expect a short adjustment period — possibly 1-2 weeks of mild side effects as your body adapts to the new drug's specific receptor profile. It's usually less intense than your original first week.

What to do alongside either drug

Regardless of which medication you're on, the evidence is clear: outcomes are significantly better when you combine the drug with lifestyle changes. Here's the short list:

1. Protein first, every meal. Aim for 1.2-1.6 g per kg of your target body weight daily. This is non-negotiable for preserving muscle. Our meal library is built around this.
2. Strength training 2-3 times per week. Without resistance exercise, up to 40% of the weight you lose can be muscle. Even bodyweight exercises count. See our training guide for beginner-friendly options.
3. Stay hydrated. 2-3 litres of water daily. Dehydration worsens every GI side effect.
4. Don't skip meals.Even when you're not hungry, eat something. A protein shake counts.
5. Track your progress. Weight, measurements, how clothes fit, energy levels. The scale is one data point, not the whole picture.

Frequently asked questions

What to do next

If you're comparing these two medications, you're probably at the start of your GLP-1 journey — or thinking about switching. Either way:

• Talk to your prescriber about which suits your specific health profile. Bring your questions — they'd rather you asked than guessed.
• Don't over-optimise the drug choice. Both work. The bigger variable is what you do alongside it.
• Get your nutrition and training dialled in from day one. The first 30 days set the trajectory.

If you want a structured plan for those first 30 days — meal plans, protein targets, a nausea playbook, GP question scripts, and a printable tracker — that's exactly what the Trimsy Starter Pack is. It works with both Mounjaro and Wegovy because the principles are the same.