Do weight loss injections work for everyone?

The decision starts with whether you're medically eligible

The licensed weight loss injections available in the UK (Mounjaro (tirzepatide) and Wegovy (semaglutide)) are approved for adults with a BMI of 30 or above, or from 27 with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. That licensing threshold is a floor, not a guarantee of approval. A prescriber also considers your full medical history, current medicines, and any conditions that would make these treatments risky.

Hard exclusions exist. Neither medicine is recommended during pregnancy, while breastfeeding, or for anyone actively trying to conceive. They are not licensed for under-18s. A personal or family history of medullary thyroid carcinoma, a rare thyroid cancer, or a condition called MEN2, rules out GLP-1 medicines entirely. Pancreatitis history requires careful prescriber discussion. That is why a genuine clinical consultation (not an automated questionnaire) matters. The mechanism behind these medicines is well understood; whether that mechanism is safe for a specific person is the question a prescriber has to answer individually.

Lower BMI thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds under current UK guidance, so the numbers alone don't tell the whole story. If you are uncertain where you stand, the most common questions our prescribers field cover many of these edge cases.

How biology, dose and adherence shape the result

Even among people who are eligible and start treatment, outcomes differ. The SURMOUNT-1 trial (published in the New England Journal of Medicine) reported average body-weight reductions of around 20–21% at the 15mg tirzepatide dose over 72 weeks, which is striking. But that is a mean across thousands of participants; individuals within the trial ranged from very high responders to modest ones. Genetics, baseline insulin resistance, gut motility and how consistently someone reaches their maintenance dose all feed into where a person lands on that distribution.

Dose progression matters too. Treatment begins at a low starter dose specifically to let the body adjust, then titrates upward over months under prescriber oversight. Someone who stays at a lower dose due to side effects will typically see a smaller reduction than someone who reaches and tolerates the top dose. That is not a failure (it is the medicine working as designed) but it is worth understanding before you start. How long results take to appear is another question that gets asked a lot, and the honest answer is: weeks for early changes, months for the full picture.

Adherence to the lifestyle elements also matters. These medicines reduce appetite significantly, but the clinical guidance from NICE's appraisal of tirzepatide ties treatment to a reduced-calorie diet and increased activity. The people in trials who did best combined the medicine with those changes consistently.

Side effects and tolerability: the practical filter

Not everyone who is eligible will find these medicines comfortable to take. The most common side effects are gastrointestinal, nausea, loose stools, constipation, reflux, and burping feature in both the tirzepatide and semaglutide trial data. For most people these are worst in the first few weeks of a new dose and settle as the body adjusts. For some, they are persistent enough to prompt a prescriber to slow the titration schedule or, in a small number of cases, stop treatment.

Serious side effects are less common but real. The MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis as an infrequent but potentially serious risk: severe stomach pain that reaches into the back, particularly with vomiting, needs prompt medical attention. Gallbladder symptoms are also flagged in prescribing information. Patients can report any suspected side effects directly to the MHRA via the Yellow Card scheme.

Tolerability is one reason the cost conversation is more nuanced than it first appears. If you are trying to decide whether treatment makes financial sense, the cost of weight loss injections page breaks down what legitimate private pricing typically covers, and why headline figures vary so much between providers.

What 'working' actually means, and when to review it

Clinicians use a practical threshold: if you have not lost at least 5% of your body weight after six months at your highest tolerated dose, current NICE guidance recommends reviewing whether continuing is appropriate. That benchmark is not a verdict on the person, it is a clinical signal that this particular treatment may not be the right fit.

For people who do respond, the evidence supports meaningful, sustained reduction when treatment continues alongside lifestyle changes. If you want to understand what options are currently generating the most interest before committing, our page covering the trending weight loss injection options gives a useful overview of where prescribing attention is focused right now. Most people notice appetite changes relatively early; the scale takes longer to reflect them.

One practical note: the timing of when you start can matter more than people expect. Orders placed before 12pm on a weekday at nume are dispatched the same day once clinically approved, which means if you've been putting it off until after a holiday or payday, starting promptly is genuinely straightforward. Our prescribers review every consultation personally, and if you've also been wondering whether B12 injections work for weight loss as an alternative, that question is worth answering before you decide which route suits you. If you'd like to understand whether you're a candidate, check your eligibility with a free consultation and a named clinician will assess your full picture.