Can you have weight loss injections? What the eligibility criteria actually mean

Picture this: you've done the research and you think you qualify, so what happens next?

You've looked up your BMI. It's above 30. You've read about Mounjaro and Wegovy. You're wondering whether a prescriber would actually say yes. That's exactly the right question to ask, and the honest answer is that the number on the scales is only part of what a clinician considers.

Both tirzepatide (Mounjaro) and semaglutide (Wegovy) are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside a weight-related condition. The conditions that count include type 2 diabetes, high blood pressure, dyslipidaemia, obstructive sleep apnoea, and cardiovascular disease, among others. UK guidance also recognises that for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, BMI thresholds are typically 2.5 kg/m² lower, because the health risks associated with excess weight can arise at a lower measured BMI. You can read more about how these thresholds apply in practice on our eligibility overview.

Once you meet the threshold on paper, the prescriber looks further: your current medicines, any conditions that interact with GLP-1 treatment, your medical history, and whether the goals of treatment are realistic and appropriate for you. BMI opens the door; clinical judgement decides whether you walk through it.

What can actually stop you being prescribed one of these medicines?

There are situations where a prescriber will not be able to approve treatment, and knowing them upfront avoids disappointment. Neither tirzepatide nor semaglutide is licensed for use during pregnancy, and both are not recommended while breastfeeding or when actively trying to conceive. Treatment must stop before attempting pregnancy; the MHRA advises using effective contraception throughout and for a wash-out period after stopping. For people using oral contraceptive pills alongside tirzepatide specifically, NHS guidance recommends adding a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced.

Neither medicine is licensed for anyone under 18. A personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 (MEN2) is a contraindication, as is a history of pancreatitis, though your prescriber will discuss this based on individual circumstances. Certain gastrointestinal conditions also warrant careful prescriber discussion before treatment starts.

Some medicines interact with GLP-1 treatments too, mainly because both slow gastric emptying, which affects how quickly other drugs are absorbed. Your prescriber will review everything you take. For a more detailed look at what can rule treatment out, our contraindications page goes through the full picture. The NHS medicines page for tirzepatide also lists the key circumstances where caution or avoidance applies.

How do the two licensed injections compare on results, and does it matter for eligibility?

The short answer is that eligibility criteria for the two medicines are very similar, so the clinical decision between them comes down to your health profile and your prescriber's assessment, not which one has the bigger trial number.

That said, the evidence does differ. In SURMOUNT-1, participants using tirzepatide at the highest dose lost an average of around 20–21% of their body weight over 72 weeks. In STEP 1, semaglutide 2.4mg produced an average reduction of around 15% over 68 weeks. The SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, found greater average weight loss with tirzepatide than with semaglutide 2.4mg. A newer 7.2mg semaglutide dose has since been approved in the UK, with trial data suggesting results that narrow that gap considerably. None of this determines whether you personally will be approved, it informs the conversation with your prescriber about which route suits you best. You can compare the licensed injections side by side to see the full breakdown.

Cost is often a factor in that decision too. Private treatment is priced per month, and the price varies by medicine and dose. Our cost overview explains what the figures you'll see quoted online typically do and don't include, useful context before you commit to anything.

How long would you need to stay on a weight loss injection?

This is the question our prescribers hear most often after eligibility, and the honest answer is that it varies, but these are not short-course treatments. Both tirzepatide and semaglutide work over months, with dose titration meaning the first pen or two are spent at a lower starting dose while your body adjusts. Appetite suppression tends to build as the dose increases, and most of the clinical trial results reported above were measured at 68–72 weeks.

Stopping too early typically means weight returns, because these medicines work by modulating appetite hormones rather than permanently changing metabolism. Many people find they are on treatment for at least a year; some continue longer under clinical review. NICE's appraisal of semaglutide specifies a maximum of two years within specialist NHS services, though private prescribing follows different parameters reviewed with your prescriber. If you're planning around timing (say, starting after a holiday or before a busy period at work, rather than mid-January when motivation runs high but routine is patchy) that's worth mentioning during your consultation. How long treatment typically lasts covers this in more depth, including what to expect at the review points along the way.

Side effects are most common at the start and after each dose increase. They are usually gastrointestinal (nausea, loose stools, constipation, bloating) and tend to settle as your body adapts. Our side effects page explains what to expect and when to seek medical help. The NICE technology appraisal for tirzepatide (TA1026) also gives full context on both the benefits and the safety profile as assessed by the committee. If you'd like to talk through your specific situation with a clinician, starting a free consultation is the most direct route to a genuine answer.