What Actually Makes a Weight Loss Injection Programme the Best?

Which injection produces the best results, and what do the numbers mean?

The most rigorous comparison we have is SURMOUNT-5, a 72-week head-to-head trial published in the New England Journal of Medicine in 2025, in which tirzepatide produced greater average weight reduction than semaglutide 2.4 mg in adults with obesity and no diabetes. In the earlier SURMOUNT-1 trial, tirzepatide at 15 mg was associated with around 20–21% average body-weight reduction over 72 weeks. Semaglutide 2.4 mg (Wegovy) produced around 15% average reduction in the STEP 1 trial over 68 weeks. A newer, higher dose of semaglutide (7.2 mg, approved by the MHRA in January 2026 with a dedicated single-dose pen approved on 14 April 2026) has reported around 20.7% average weight loss in trials, narrowing the gap considerably.

Those percentages matter, but they're population averages. Individual responses vary based on genetics, starting weight, adherence, diet, and activity. A programme that achieves slightly less on paper but keeps you safe, supported, and consistent will outperform a higher-dose option that you can't tolerate or can't access reliably. When you're comparing programmes, ask what happens at week six if the nausea is difficult, not just what the trial poster says.

For a side-by-side look at licensed options, our guide to the best weight loss injection for your situation runs through the practical differences in more detail, and if you're still weighing up the options personally, our page on what weight loss injection is best for you can help you think through which choice fits your circumstances.

Does your eligibility change which programme is actually available to you?

Yes, meaningfully. Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, type 2 diabetes, or prediabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. BMI alone, though, has never been the whole picture, a prescriber reviews your health history, current medicines, and any contraindications before anything is approved.

On the NHS, access is phased and tightly criteria-driven. NICE's appraisal of tirzepatide (TA1026) recommends it for adults with a BMI of 35 or above plus at least one weight-related comorbidity; a second cohort covering BMI 35–39.9 plus four qualifying conditions became eligible through NHS commissioning from June 2026. Waiting lists vary considerably by area, and not every GP practice is participating in the new prescribing arrangements.

Private programmes through a regulated online pharmacy work differently: there's no waiting list, and clinical suitability is assessed at the time of consultation. The overview of weight loss injection programmes covers what that assessment typically involves.

What separates a safe injection programme from a risky one?

This is the question the search results tend to skip. Mounjaro and Wegovy are prescription-only medicines; under UK law, supplying them without a valid prescription from a registered prescriber is illegal. The MHRA has issued repeated warnings about fake pens sold via social media and websites, in October 2025, the first UK site manufacturing counterfeit tirzepatide pens was raided, and around 20 million doses of illegally traded weight-loss medicines (roughly £45 million in street value) were seized across 2025 enforcement actions. Some counterfeit pens have contained insulin, with serious consequences for the people who used them.

A legitimate programme should be verifiable. Any online pharmacy dispensing these medicines must be registered with the GPhC, you can check a pharmacy's registration on the GPhC register before you order anything. Red flags include offers without any clinical assessment, prices dramatically below the market range, and discount codes attached to prescription medicines. The guide to choosing a safe online weight loss injection programme sets out what to look for when comparing providers.

Safety also means ongoing clinical contact. The most common side effects (nausea, loose stools, reflux, fatigue) tend to cluster around dose increases. Knowing that a prescriber is reachable seven days a week during those weeks isn't a luxury; it's how problems get caught early. The MHRA also flagged acute pancreatitis as a known, infrequent but potentially serious side effect in a Drug Safety Update in January 2026, severe, persistent stomach pain reaching the back is a reason to get medical help promptly, not to wait for the next scheduled check-in. If you're unsure about the practicalities of administering your medication, our guidance on the best injection site for weight loss explains how to choose the right area and inject safely at home.

How much should a well-run programme cost, and is cheaper ever better?

Private weight-loss injection programmes are priced per pen or per month and vary between providers. Since Eli Lilly raised Mounjaro's UK list price from 1 September 2025, typical private prices have ranged roughly £149–£375 per month depending on the dose and the provider. Some of the lower figures in that range don't include a consultation fee, a prescription fee, or any aftercare, costs that add up quickly once you've started a dose-escalation schedule.

For a prescription medicine, price is the wrong test. Counterfeit pens are cheap; genuine, clinical-grade supply chains are not. The full cost breakdown for weight loss injections explains what a transparent price should include and what to query if it doesn't. One practical note: if you're planning to start, ordering before a bank holiday or long weekend avoids the gap where your pen is in a van and your prescriber is unreachable, something worth factoring in around payday orders in December.

At nume, our GPhC-registered pharmacy uses the licensed UK supply chain, and one price covers the consultation, prescription, treatment, and free next-working-day DPD delivery. There's no subscription, every repeat order is re-reviewed clinically. If you're ready to find out which programme fits your situation, checking your eligibility takes a few minutes and costs nothing.