What's the Best Weight Loss Injection for You?

What the trial evidence says about average results

If you're looking for a straightforward comparison on effectiveness, the clearest evidence comes from SURMOUNT-5, a head-to-head randomised trial published in the New England Journal of Medicine in 2025. Over 72 weeks, adults with obesity who used tirzepatide lost significantly more body weight on average than those using semaglutide 2.4mg. Tirzepatide targets two gut-hormone pathways (GIP and GLP-1) where semaglutide targets one; the prevailing clinical view is that the dual mechanism drives the difference. In the SURMOUNT-1 trial, participants on tirzepatide's highest dose saw average body-weight reductions of around 20–21% over 72 weeks. Semaglutide at 2.4mg produced around 15% in the STEP 1 study. A newer 7.2mg semaglutide dose, approved by the MHRA in early 2026 and described on our weight loss injections overview, closes that gap somewhat, with trials reporting around 20.7% average loss.

On paper, tirzepatide has the stronger numbers at licensed doses. But averages are population-level figures. Your result depends on starting weight, lifestyle factors, how your body responds and whether you stay on the programme. If you want a fuller breakdown of how the options compare before booking, our page covering what's the best weight loss injection walks through the clinical picture in plain language. A number from a trial is a starting point for a conversation, not a personal forecast.

How your health history shapes the decision

This is where the generic "best injection" question runs out of road. Both medicines share a broadly similar eligibility threshold, adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. That said, certain health conditions, existing medications and past medical history can make one option more appropriate than the other, or rule out a particular medicine entirely. If you take oral contraceptives, for instance, guidance on which injection is best for you personally notes that tirzepatide users are advised to add a barrier method for the first four weeks of treatment and after each dose increase, because absorption of the pill may be reduced, something a prescriber will walk you through.

It's worth being honest with yourself here: a lot of people arrive at this question already half-decided based on something they read on social media. That's fine, and understandable. The consultation exists precisely to test whether that instinct holds up against your actual clinical picture, and to catch anything that might make it unsafe or unsuitable. The NHS medicines page on tirzepatide sets out the contraindications and cautions in full.

Side effects: different profiles, same starting principle

Both injections carry a broadly similar side-effect profile, nausea, diarrhoea, constipation, indigestion and fatigue are the most commonly reported, and they tend to be most noticeable in the first few weeks or after a dose increase, then settle for most people. Neither is free of gastrointestinal effects; that's a class characteristic, not a flaw in one medicine over the other. Individual tolerance varies considerably: some people breeze through titration, others find the first weeks uncomfortable. The injection-technique side of things (rotating sites between the abdomen, thigh and upper arm, keeping the pen correctly stored, replacing needles each time) matters for both, and you can read through the specifics on our guide to using weight loss injections at home.

One point worth knowing: the MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis as a known but infrequent side effect across GLP-1 medicines. Severe, persistent abdominal pain radiating to the back (with or without vomiting) needs same-day medical attention, not a wait-and-see. Prescribers will cover this at assessment. If you want to report a side effect, the MHRA Yellow Card scheme handles that directly.

What this means when you're making a choice

On current evidence, tirzepatide produces greater average weight loss than semaglutide at standard licensed doses. That makes it the more effective option statistically. But "statistically more effective on average" and "better for you" are not always the same sentence. A prescriber weighs your history, your other medicines, your tolerability, your goals and the clinical picture as a whole. Sometimes the answer is clear-cut. Sometimes it isn't, and the right move is to start with one and have a structured review at six months.

The cost dimension matters too, and our guide to weight loss injection prices in the UK lays out how private treatment is structured. Since Eli Lilly raised Mounjaro's UK list price from September 2025, with the highest-dose pen rising from around £122 to around £330, price has become a more significant part of the conversation for some people. It's worth factoring in, but it shouldn't be the primary variable when you're making a decision about a prescription medicine. If you're based in the West Midlands, our dedicated page on weight loss injections in Birmingham covers how to access treatment locally, including what to expect from your first consultation. You can also read a broader look at available weight loss treatments to understand the full picture before your consultation. Our prescribers review every consultation personally (the same day, during working hours) and will explain their reasoning if one option is preferable for you over the other.