Injectable drugs for weight loss: which one is right for you?

What the mechanism difference means in practice

Semaglutide mimics a hormone called GLP-1, which the gut releases after eating. It signals the brain to reduce hunger, slows stomach emptying, and helps regulate blood sugar. Tirzepatide does all of that, and also activates a second gut-hormone receptor, GIP. The combined effect appears to produce a stronger appetite signal in most people, which is reflected in the trial data.

In the SURMOUNT-1 study, adults taking tirzepatide at 15mg lost around 20–21% of their body weight on average over 72 weeks. The STEP 1 trial, published in the New England Journal of Medicine, reported roughly 15% average weight loss over 68 weeks with semaglutide 2.4mg. The SURMOUNT-5 head-to-head trial (2025) confirmed that tirzepatide produced greater average loss than semaglutide when the two were compared directly.

That does not mean tirzepatide is automatically the better choice for every person. Individual response varies considerably. Some people do better on semaglutide; for others the side-effect profile of one medicine suits them better than the other. Deciding between them is a clinical conversation, not a league table.

If needles are a concern, it is worth knowing that an oral semaglutide tablet (Wegovy tablets) received UK approval in June 2026, a genuinely different option from the injections, and one worth asking about if the injection route feels like a barrier. You can read more about the full range of weight-loss treatment options at nume when you are ready.

Eligibility: the numbers that matter, and the ones that do not tell the whole story

Both medicines share a broadly similar starting point. The licensed criteria cover adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition, for example, high blood pressure, type 2 diabetes, high cholesterol, obstructive sleep apnoea, or osteoarthritis. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance.

BMI is a starting point, not the whole picture. A prescriber also considers your medical history, current medicines, any contraindications listed in the product information, and whether the expected benefit outweighs individual risk. The NHS medicines page for tirzepatide summarises the main contraindications clearly, including a history of medullary thyroid carcinoma or pancreatitis, which would require careful discussion.

Neither medicine is recommended during pregnancy, breastfeeding, or when actively trying to conceive. They are not licensed for anyone under 18. These are not minor footnotes; a prescriber will ask about them directly.

If you have been researching this topic and wondering whether all weight-loss drugs are injectable, the short answer is no, but the injectable options currently have the strongest clinical evidence for significant weight reduction in adults.

Side effects: what people experience starting out

The most common effects with both injectable drugs for weight loss are gastrointestinal: nausea, loose stools, constipation, reflux, and burping are frequently reported, particularly in the first few weeks or after a dose increase. For most people they settle within days to a couple of weeks as the body adjusts.

Starting at a low dose and increasing gradually (the approach used in clinical practice) is specifically designed to reduce this. The first pen for tirzepatide, for instance, contains the 2.5mg dose, which exists to let the body adjust rather than to produce substantial weight loss. That adjustment period is part of the treatment, not a sign something is wrong.

There are rarer effects that need prompt attention. Severe, persistent stomach pain that radiates towards the back can be a sign of pancreatitis and warrants urgent medical assessment. In January 2026, the MHRA issued a specific Drug Safety Update on GLP-1 medicines highlighting this risk; the full detail is on the Yellow Card reporting site, where suspected side effects can also be reported. For a more thorough overview, our page on the side effects of injectable weight-loss drugs covers both medicines in detail.

Many people find that knowing what to expect (and having someone to call when they are not sure) makes a real difference in those early weeks. That is something a good aftercare service should provide before you ever need it.

How injectable treatment fits alongside lifestyle changes

Neither medicine works independently of what you eat and how you move. Both are licensed as additions to a reduced-calorie diet and increased physical activity, and the clinical trials were run on that basis. The appetite reduction that most people experience creates a window, you feel less compelled to eat, which makes the dietary changes easier to sustain rather than requiring pure willpower.

Protein intake and hydration tend to need deliberate attention on these medicines, because the reduction in overall appetite can mean people unintentionally eat too little of both. A prescriber or dietitian can advise on practical adjustments, and if you are considering tirzepatide specifically, our guide to the Wego weight loss injection explains how it is used in practice. The NHS Live Well healthy weight guidance offers a solid starting framework for the lifestyle side of things.

If you are curious about the specific differences between how each medicine works in the body, the guide to what the injectable weight-loss drugs are goes into more clinical detail. And if you are still deciding whether an injection is the right route for you at all, our overview of what an injectable weight loss drug involves covers the practical and clinical considerations worth knowing before you make that decision. For a broader look at how these treatments are administered, the page on choosing a weight loss injectable sets out what to expect from the process as a whole.

When you feel ready to take that step, you can start a free consultation with our prescribers. A real clinician reads every submission the same day; there is no automated decision at any stage.