What Is More Effective: Semaglutide or Tirzepatide? (October 2026)

01 · The verdict

Which is more effective for weight loss?

Tirzepatide produced greater average weight loss than semaglutide in SURMOUNT-5, the first large head-to-head trial between them. In SURMOUNT-5, 751 adults took tirzepatide or semaglutide 2.4mg for 72 weeks. Average loss was 22.8 kg vs 15.0 kg.¹⁵

That gap is large, but it is an average across a trial group. Both are effective, licensed treatments; which suits you depends on your health profile, tolerance and prescriber advice, not the headline number alone.¹⁵ Our related guide, which is better, semaglutide or tirzepatide, covers the wider decision.

02 · Side by side

Tirzepatide vs semaglutide at a glance

You can see current doses and prices on the Mounjaro pen page or the Wegovy pen page. Prices were checked 11 October 2026, and all strengths of Mounjaro are in stock.

03 · Your decision

How to decide between them

Start with the evidence, then add your own circumstances. Tirzepatide had the greater average result in SURMOUNT-5, so it is a reasonable question to raise with a prescriber. Semaglutide remains a proven option, and some people tolerate it better or prefer it for personal reasons.

The decision is made in a free online consultation reviewed by a GPhC-registered prescriber. If you want to understand your options before that, our weight loss clinic page explains the pathway. Ongoing monthly reviews are included, so the choice is not locked in for good.

04 · SURMOUNT-5 in detail

SURMOUNT-5: the head-to-head results

SURMOUNT-5 is the first large trial that put tirzepatide and semaglutide against each other directly. In the first large head-to-head trial (SURMOUNT-5), adults on tirzepatide lost on average 20.2% of body weight over 72 weeks versus 13.7% on semaglutide 2.4mg (Wegovy).¹⁵

The trial also shows how many people reached each loss threshold. At least 10% of body weight was lost by 81.6% on tirzepatide vs 60.5% on semaglutide; at least 15% by 64.6% vs 40.1%; at least 20% by 48.4% vs 27.3%; at least 25% by 31.6% vs 16.1%.¹⁵

The gap widens at the higher thresholds. Far more people on tirzepatide reached the largest losses, which is where the average difference comes from. Even so, many people on semaglutide lost clinically meaningful weight, and the trial does not say who will respond best to which medicine.

05 · Each drug's own trial

What each medicine achieved in its own trial

Each medicine also has its own large registration trial. These were separate studies with different participants, so their numbers should not be set against each other as a comparison. Use SURMOUNT-5 for that.

For tirzepatide: in the SURMOUNT-1 trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages — individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

For semaglutide: in the STEP 1 trial, adults on semaglutide 2.4mg (Wegovy) lost on average 14.9% of body weight over 68 weeks, compared with 2.4% on placebo.⁹ In STEP 1, 86.4% of adults on semaglutide 2.4mg lost at least 5% of body weight over 68 weeks, compared with 31.5% on placebo.⁹

Both medicines clearly beat lifestyle changes alone or placebo. That is why both are licensed. The question "which is more effective" is really "how much more", and SURMOUNT-5 answers it in favour of tirzepatide.

06 · Why tirzepatide may differ

Why tirzepatide and semaglutide differ

The difference starts with mechanism. Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹

In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy/Ozempic).¹

If you want the full picture on tolerability, see whether semaglutide or tirzepatide has more side effects and which is safer.

07 · Why choose semaglutide

Why a doctor might prescribe semaglutide instead

A prescriber may choose semaglutide because the right treatment is the one that suits your health profile and that you can tolerate, not simply the one with the larger average. Both are effective, licensed treatments; which suits you depends on your health profile, tolerance and prescriber advice, not the headline number alone.¹⁵

Side effects are part of that. In STEP 1, gastrointestinal side effects were reported by 74.2% of people on semaglutide 2.4mg, compared with 47.9% on placebo. 7.0% stopped treatment because of side effects, compared with 3.1% on placebo.⁹

Other reasons a prescriber might lean towards semaglutide include your previous experience with it, how well you responded, or practical preferences. Someone already doing well on Wegovy has no need to switch just because tirzepatide averages higher. Wegovy tablets (oral semaglutide) are also now licensed in the UK; see current Wegovy tablet prices on the product page if a non-injection route appeals. They have their own dosing and trial results and are not directly comparable to Mounjaro.

08 · Switching and mixing

Switching between them, and what not to mix

There is no licensed direct conversion between semaglutide and tirzepatide. Switching is done under prescriber supervision: typically the previous medicine is stopped, a washout interval observed, and Mounjaro started at a dose your prescriber judges appropriate — often the 2.5mg starting dose, sometimes adjusted based on your history.

Never run two GLP-1 medicines together. That answers the common question about what not to mix with semaglutide: other GLP-1 medicines are the clear one, along with unlicensed products. Mounjaro must not be combined with other GLP-1 medicines, unlicensed "research" peptides (such as retatrutide, which has no UK licence), or compounded copies. These combinations have no safety data and no regulated supply chain. If something promises faster results outside a licensed pathway, that is the warning sign.

If you are curious about newer, unlicensed compounds, see tirzepatide compared with retatrutide.

09 · Ozempic and licensing

Where Ozempic fits

Ozempic is semaglutide licensed for type 2 diabetes — it is not a UK weight-loss medicine, and prescribing it privately for weight loss is off-label. The licensed weight-management options are Mounjaro (tirzepatide) and Wegovy (semaglutide 2.4mg).

So "Mounjaro vs Ozempic" for weight loss is really Mounjaro vs Wegovy, and SURMOUNT-5 is the head-to-head trial evidence for that comparison. A regulated UK pharmacy will not supply Ozempic for weight loss. For more, read whether Ozempic is semaglutide or tirzepatide, which is more effective, Mounjaro or Ozempic, and our guide on Mounjaro versus Wegovy.

10 · Other questions answered

Injection site, muscle and other common questions

Is Mounjaro more effective in the arm? Injection site does not change how Mounjaro works: it is injected under the skin at the stomach area (at least 5cm from the belly button), the front of the thigh, or the back of the upper arm if someone else gives the injection, and you should rotate the site each dose. Follow the pen's instructions and the patient information leaflet for where and how to inject, and ask your prescriber if unsure.¹³

Muscle loss on semaglutide versus tirzepatide is another frequent question. Raise it with your prescriber, who can discuss protein intake and activity in the context of your own health.

For practical steps, see how to make Mounjaro more effective.

If you notice any new symptom on either medicine, tell your prescriber and report it through the Yellow Card scheme.¹⁹

11 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹

12 · Access and getting started

How to access either treatment

Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting. NHS access to Wegovy runs through specialist weight-management services; ask your GP.

Privately, the route starts with a free online consultation reviewed by a GPhC-registered prescriber (GPhC 9012878). Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full. Dispatch is the same day when ordered by 3pm, with free next-day tracked delivery in temperature-controlled packaging. Supply is official LillyDirect Mounjaro, direct from Eli Lilly's UK distribution.

Mounjaro (tirzepatide) is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber. It is used alongside a reduced-calorie diet and increased physical activity. Individual results vary. This page is information, not a substitute for medical advice.