Tirzepatide vs Semaglutide for Weight Loss: Which Works Better? (October 2026)

01 · The verdict

Tirzepatide vs semaglutide: who each option suits

If the question is which produced more weight loss on average, tirzepatide did in SURMOUNT-5: 20.2% versus 13.7% over 72 weeks.¹⁵ For someone starting at 100 kg, 20.2% is about 20 kg and 13.7% is about 14 kg. Those are trial averages, and the comparison in kilograms shows how the same trial looks in absolute terms. Trial averages do not predict your own result.

Both medicines are effective, licensed treatments. Which suits you depends on your health profile, how you tolerate each one and your prescriber's advice, not the headline number alone.¹⁵

Your prescriber decides which option suits you after reviewing your health questionnaire.

02 · Side by side

Comparison table: Mounjaro and Wegovy

Do not combine them with figures from other trials; what a true head-to-head means explains why.

03 · Deciding

What to weigh up besides the average

Several practical factors matter more than a single percentage. First, tolerance: gastrointestinal side effects affect many people on either medicine, and your prescriber can adjust the pace of dose increases. Second, your health history, which only a prescriber can assess. Third, cost: a Mounjaro first 2.5mg pen is £129.99 with code FIRST40, and a Mounjaro pen at the starting dose has a standard price of £169.99. Every later order is £25 off the standard price (code DOSE25).

Fourth, route to treatment. Our free consultation is a short health questionnaire with no obligation, reviewed by a GPhC-registered prescriber. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full. You can start your free consultation and the prescriber will discuss which option fits. Across the UK pharmacies we track, Mounjaro pens ranged from £143.96 to £339.00 per month as of 11 October 2026, so it is worth comparing like with like.

04 · How they work

How tirzepatide and semaglutide work

Tirzepatide activates 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 Most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹

That dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy and Ozempic).¹

Both are once-weekly injections. Mounjaro comes as a multi-dose KwikPen in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ Wegovy pens are 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg, stepping up after at least 4 weeks to reach the 2.4mg maintenance dose from week 17.¹⁸ Starting low and stepping up is the same principle on both.

05 · What the trials show

Weight loss: what the clinical trials show

The most useful evidence is SURMOUNT-5, where 751 adults took tirzepatide or semaglutide 2.4mg for 72 weeks. Average loss was 22.8 kg on tirzepatide versus 15.0 kg on semaglutide, a gap of 7.8 kg.¹⁵ Our page on the weight loss difference in kg goes through the numbers.

The share of people reaching each milestone also favoured tirzepatide:¹⁵

• At least 10% of body weight: 81.6% on tirzepatide vs 60.5% on semaglutide.
• At least 15%: 64.6% vs 40.1%.
• At least 20%: 48.4% vs 27.3%.
• At least 25%: 31.6% vs 16.1%.

Semaglutide 2.4mg has its own strong evidence. In STEP 1, adults on it lost on average 14.9% of body weight over 68 weeks, compared with 2.4% on placebo.⁹ In that trial 86.4% lost at least 5%, 69.1% at least 10% and 50.5% at least 15%.⁹ These are not a head-to-head with Mounjaro, so read them on their own terms.

For tirzepatide on its own, SURMOUNT-1 (2,539 adults) found average losses of 15% on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, versus 3.1% with lifestyle changes alone.² At 72 weeks, 85% on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5%. Half on 10mg and 57% on 15mg lost at least 20%.² Individual results vary, and treatment works alongside a reduced-calorie diet and more physical activity, not instead of them.²

For a deeper look, see our pages on weight loss percentages and average weight loss compared.

06 · Side effects and safety

Side effects and safety

Gastrointestinal side effects are the main tolerability issue with semaglutide 2.4mg. In STEP 1 they were reported by 74.2% of people on it, compared with 47.9% on placebo, and 7.0% stopped treatment because of side effects, compared with 3.1% on placebo.⁹

Like all medicines, Mounjaro can cause side effects, and most are digestive. The most common, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort. Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts.¹³

If you notice a new symptom on either medicine, tell your prescriber. You can also report suspected side effects through the MHRA Yellow Card scheme.¹⁹ Our monthly reviews include side-effect support from the prescribing team.

07 · Ozempic and other options

Ozempic, tablets and other alternatives

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.¹⁸ The licensed weight-management options are Mounjaro (tirzepatide) and Wegovy (semaglutide 2.4mg), so "semaglutide vs Mounjaro" is really Wegovy vs Mounjaro. A regulated UK pharmacy will not supply Ozempic for weight loss. Our guide to Wegovy, semaglutide and Mounjaro covers the naming in more detail.

There is no tablet form of Mounjaro. Two GLP-1 weight-loss tablets are now licensed in the UK: Wegovy tablets (oral semaglutide), approved by the MHRA in June 2026, and Foundayo (orforglipron), approved in August 2026.²⁰²¹ They have their own dosing, eligibility checks and trial results, and both are prescription-only. Our Foundayo and semaglutide pill comparison explains the tablets.

"Natural GLP-1" supplements such as berberine, sometimes called the "poor man's Ozempic", are not medicines. They are not regulated for this use and have no comparable trial evidence. They are available cheaply without assessment, which is part of the risk, not a benefit.

08 · Switching and combining

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 is observed, and Mounjaro is started at a dose your prescriber judges appropriate. That is often the 2.5mg starting dose, sometimes adjusted based on your history.¹

Never run two GLP-1 medicines together. 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 have no safety data and no regulated supply chain.¹ Anything promising faster results outside a licensed pathway is the warning sign. For more, see how effectiveness compares.

09 · 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

10 · NHS and private access

NHS access and private 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. 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. If you go private, we supply genuine Mounjaro KwikPens through official LillyDirect supply, with free next-day tracked delivery and same-day dispatch when ordered by 3pm. All six doses are in stock. A first year costs £2,711.88 if you stay on 5mg after your 2.5mg starting pen, up to £3,599.88 if you step up through every dose to 15mg, at standard prices; a new customer's first order is £40 less and every later order £25 less.

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. See all weight loss treatments for the full range.