Weight Loss by Injection: What It Actually Involves

What does the injection itself actually involve?

People often picture injections as something done in a clinic with a syringe. These are different. Both Mounjaro and Wegovy come in pre-filled, single-use pens (devices roughly the size of a thick marker) which deliver a fixed dose at the press of a button. The needle is very fine and retracts automatically after use.

You inject into the soft tissue just under the skin, most commonly the stomach, outer thigh, or upper arm. Rotating between sites each week helps avoid irritation building up in one spot. The injection itself takes a few seconds. Most people find that, after the first one or two, the process becomes routine rather than daunting.

Timing is flexible: the same day each week, but not tied to meals or a specific time of day. If you want a practical check before your first injection, spend a minute reading through the patient information leaflet that comes with your pen, it covers the injection steps, what the device should look and feel like when it fires correctly, and what a used pen should do when you are ready to dispose of it safely using a sharps container.

Our overview of weight-loss injection treatment covers the pen format and storage requirements in more detail if you want to go deeper before deciding.

Who tends to be suitable for this kind of treatment?

Both medicines are licensed for adults. The starting point for eligibility is a BMI of 30 or above, or 27 or above if you also have a weight-related health condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, prediabetes, or type 2 diabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance.

A BMI figure alone does not determine suitability. A prescriber also considers your current medicines (some interactions matter), your personal and family medical history, and any conditions that would make these treatments unsuitable, a history of medullary thyroid cancer or certain digestive conditions, for example, would prompt a careful conversation. Pregnancy, breastfeeding, and trying to conceive are not compatible with either medicine.

The NHS England guidance on weight-management injections sets out the clinical framework that informs prescribing decisions across the UK. At nume, every assessment is read by a GPhC-registered Independent Prescriber, a real clinician, reviewing your answers the same day you submit them. You can read more about the team behind that process on our clinical team page.

What results does the evidence show, and what shapes them?

In the SURMOUNT-1 trial, participants using the highest dose of tirzepatide (Mounjaro) lost around 20 to 21 percent of their body weight over 72 weeks on average. The STEP 1 trial for semaglutide at 2.4 mg (Wegovy) showed an average of around 15 percent over 68 weeks. These are averages across thousands of adults; individual results varied, and both trials included a structured diet and activity component alongside the medicine.

Several factors influence how well treatment works in practice: consistency with the weekly injection, the lifestyle changes that run alongside it, how well someone tolerates dose increases, and how long they stay on treatment. Results are not immediate, most people notice a meaningful change in appetite before the scales shift significantly, typically over the first few weeks.

A note on expectations: these medicines are not licensed as a fast route to cosmetic weight loss, and the MHRA is clear on that point. They are serious prescription treatments for a health condition, used under ongoing clinical supervision. If you are weighing up the options (including which medicine might be the better fit) our comparison of injectable weight-loss treatments covers the differences in mechanism, evidence, and licensed dosing.

How does someone access these medicines through a regulated route?

The only legal route to either medicine in the UK is a prescription issued after a clinical assessment. That assessment can happen through a GP, a specialist weight management service, or a GPhC-registered online pharmacy like nume.

For people who do not meet the current NHS criteria (which are being phased in gradually and remain relatively strict) or who would rather not wait, a private prescription through a regulated pharmacy is a straightforward alternative. The things worth checking: is the pharmacy listed on the GPhC register? Is the consultation done by a named, qualified prescriber? Are the medicines sourced through the licensed UK supply chain? The MHRA has documented the risks of counterfeit pens sold online or via social media, including cases where fake pens were found to contain insulin rather than the labelled medicine.

Our guide to getting weight-loss injections walks through this step by step, including what to expect from the verification process. If you have broader questions about whether this type of treatment is the right starting point, you can find a full breakdown of how weight loss injections work and what they involve alongside details on weight loss injection requirements and the review process, as well as guidance on using Mounjaro as an injection for weight loss, before the weight-loss treatments overview sets out all the licensed options currently available.

When you are ready to take the next step, you can speak to our prescribers through a free consultation, no obligation, and a qualified clinician reads every response.