Advantages of Weight Loss Injections: What the Evidence Shows (October 2026)

01 · The pros

The main advantages of weight loss injections

The biggest advantage is the size of the average result. In SURMOUNT-1 (2,539 adults), people on tirzepatide with diet and exercise lost on average 15% on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, against 3.1% with lifestyle changes alone.² For someone starting at 100 kg, 20.9% is about 21 kg. These are trial averages, so individual results vary.

Our advantages of Mounjaro guide covers one medicine in more depth, and our weight loss injections hub covers the wider category.

02 · Getting it

Cost, access and how to start

Access is a practical advantage of 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 a weight-related condition, and the primary-care rollout began in June 2025 with the highest-need groups first.⁴ Many people who meet the licensed criteria choose regulated private treatment rather than wait.

The Mounjaro pen starts at £169.99 standard for 2.5mg; new customers save £40 on a first order at any dose with code FIRST40, and an automatic £25 reorder discount applies to every future order (code DOSE25). All six doses are in stock. Supply is official LillyDirect, with free next-day tracked delivery and same-day dispatch when ordered by 3pm.

See also our guide to buying weight loss injections in the UK.

03 · Beyond the scale

Benefits beyond the number on the scale

Trial data show more than average loss. At 72 weeks in SURMOUNT-1, 85% of people on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight. Half of those on 10mg and 57% on 15mg lost at least 20%.² Results depend on use alongside a reduced-calorie diet and increased physical activity, not instead of them.

04 · Belly fat and pace

Pace and realistic timelines

Pace is gradual by design: the first weeks on 2.5mg are an adjustment phase and losses are usually modest, with most measurable progress building from 5mg onward, month by month.¹ Think in months, not days, and judge by trend rather than single weigh-ins. Early loss includes water weight.

If you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right.¹

05 · Drawbacks to weigh

The other side: cons and limits

Advantages come with trade-offs. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.¹³ It is a prescription-only medicine licensed for adults with a BMI of 30 or above, or 27 to 30 with a weight-related condition, and a prescriber assesses your history and current medicines first.¹ After a final dose it takes roughly 4–5 weeks to clear the system.¹ Report any suspected side effect through the Yellow Card scheme. Our safety and side effects guide covers this in full.

06 · Choosing a medicine

Mounjaro or Wegovy?

Both are licensed once-weekly injections. Wegovy (semaglutide) steps up after at least 4 weeks from 0.25mg, reaching the 2.4mg maintenance dose from week 17.¹⁸ Mounjaro starts at 2.5mg and can rise to 15mg. Which suits you depends on your health profile, tolerance and prescriber advice. See our Wegovy weight loss injections page or all weight loss treatments.

07 · 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 If you are pregnant or planning a pregnancy, Mounjaro must not be used during pregnancy and should be stopped at least one month before trying to conceive. If you use the contraceptive pill, switch to a non-oral method or add a barrier method (such as condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.