Weight Loss Injections: Good or Bad for Your Health?

What the trial data actually shows about effectiveness

The case for weight loss injections begins with the numbers from large, controlled trials, not anecdotes. In the SURMOUNT-1 trial, published in the New England Journal of Medicine, adults treated with tirzepatide (Mounjaro) lost an average of around 20–21% of body weight over 72 weeks. That is a meaningful figure. For context, prior generations of weight-loss medicines rarely reached half of that. STEP 1, the equivalent trial for semaglutide 2.4mg (Wegovy), recorded an average reduction of roughly 15% over 68 weeks.

Beyond the scales, both medicines have shown health benefits that extend further. Semaglutide has UK authorisation to reduce major cardiovascular event risk in eligible adults. NICE reviewed tirzepatide's full evidence base before recommending it for NHS use in England, that appraisal process, TA1026, is worth reading if you want the regulator's own summary of the benefit-risk balance. These are not novelty products. They are medicines that have passed regulatory scrutiny.

It is also worth being honest about what the trials cannot tell you: individual results vary, the trials included diet and lifestyle support alongside the medicine, and long-term data beyond two years is still accumulating. A prescriber helps you read the evidence in relation to your own starting point, not just a population average.

The real side effects, and how common they actually are

Side effects are the most frequent reason people ask whether these injections are bad for them, and the question deserves a straight answer. Gastrointestinal effects (nausea, loose stools, constipation, indigestion, burping) are common, particularly in the weeks after starting or after a dose increase. The NHS's tirzepatide guidance sets out the full picture clearly, and it is worth reading before you start.

For most people these effects are mild to moderate and ease within a few weeks as the body settles. Eating more slowly, keeping meals smaller, and staying well hydrated makes a practical difference. Serious adverse effects are less common but do exist. Acute pancreatitis is one that clinicians watch for: the signal is severe, persistent stomach pain that may spread to the back. In January 2026, the MHRA issued a Drug Safety Update reminding prescribers and patients to be alert to it. If that happens, seek urgent medical help rather than waiting to see if it passes.

People with a personal or family history of medullary thyroid carcinoma, a rare condition called MEN2, or previous pancreatitis will not be prescribed these medicines, those exclusions exist for good reason. Pregnancy and breastfeeding are also situations where these medicines are not recommended; women who may become pregnant while on treatment should discuss contraception with their prescriber, as tirzepatide in particular can reduce the absorption of the oral contraceptive pill in the early weeks of treatment.

Who benefits most (and who the evidence does not support

Weight loss injections are licensed for adults with a BMI of 30 or above, or 27 or above if at least one weight-related condition is present) conditions such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. The evidence base is drawn from people in those categories, which means the benefit-risk balance is most clearly understood for them.

They are not licensed for people under 18. They are not recommended during pregnancy or for anyone trying to conceive. They are not cosmetic tools, and the MHRA has been explicit that they are not assessed for quick or superficial weight reduction. That framing matters because it shapes who is most likely to see meaningful, lasting benefit. If you are curious about which licensed injection might suit your situation, this comparison of the options breaks down the differences in mechanism and evidence.

The short version: the injections are designed for people for whom excess weight is a health issue, and the benefits documented in trials are concentrated in that group. Used outside that frame (without proper assessment, purchased from unverified sellers, or without clinical oversight) the picture changes entirely. One question our prescribers hear regularly is whether cost makes safer sources impractical; if that is a concern for you, understanding what the costs include is a good starting point.

The verdict the evidence actually supports

Good or bad is not quite the right frame, because the answer depends on clinical context. For an adult with obesity and a weight-related health condition, assessed by a qualified prescriber, using a genuine licensed medicine from a GPhC-registered pharmacy, the evidence supports a positive benefit-risk balance. That is the conclusion NICE reached, and it is the conclusion that licensed these medicines in the UK. For someone assessed as unsuitable (or using an unverified product bought without clinical oversight) the calculation is very different.

The question is not really whether the injections are good or bad in the abstract. It is whether they are right for you, under proper supervision. That is exactly the purpose of a clinical assessment. If you have read this far and are thinking through whether to explore treatment, a free consultation with one of our clinical team is the right next step. Our prescribers review consultations personally (not through automated scoring) the same day you submit, including order by 12pm if you want a decision before the afternoon. You can read more about the specific risks in more detail, explore whether weight loss injections are likely to suit your circumstances, or find out which injection is considered the most effective option if you want guidance on how the available medicines compare. You can also explore how the medicines work if you want more background first. When you are ready, speak to our prescribers and let the consultation guide the decision properly.