The RET Weight Loss Injection: A Step-by-Step Guide to How It Works in the UK

Step 1: Clinical assessment before anything else

The process begins with a consultation, not a prescription. Under UK law, Mounjaro and Wegovy are prescription-only medicines, which means a GPhC-registered Independent Prescriber must assess your health before treatment is issued. At nume, a real clinician reads your answers (not software) and reviews your weight, medical history, and any medicines you already take. Same-day review is how the service works; approval follows if, and only if, you're clinically suitable.

Eligibility broadly follows the licensed criteria: adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, high cholesterol, or type 2 diabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. Our clinical team reviews the whole picture, not just the number on the scales. If you'd like to understand the broader landscape of options before deciding, the treatment overview is a useful place to start.

Once approved, treatment is dispensed from our GPhC-registered pharmacy and dispatched the same day for free next-working-day delivery. The pen arrives in plain, unbranded packaging, discreet, tracked, handled by DPD.

Step 2: Storing and preparing for your first injection

When the pen arrives, it goes straight into the fridge, specifically, 2–8°C, which for most people means slotting it into the fridge door alongside everyday items. There is a limited window during which the pen can be kept at room temperature, but the exact duration differs between Mounjaro and Wegovy; always check the Patient Information Leaflet inside the box rather than relying on any general figure you've read online.

Before your first injection, read the leaflet in full. It covers the pen mechanism in detail and is the authoritative reference for your specific product. Clean the injection site with an alcohol swab and let the skin dry before pressing the pen to the skin. The NHS medicines page for tirzepatide and the equivalent page for semaglutide both include patient-level guidance on injection technique if you want a second reference point.

These are pre-filled, pre-set pens, no drawing up, no dialling. Once you've pressed the pen against the skin at the correct angle and activated the mechanism, the injection is done within seconds. Most people find the first one far more straightforward than they expected.

Step 3: What to expect after injecting, and how to manage it

The most common effects in the days following an injection are gastrointestinal: nausea, loose stools, constipation, indigestion, or a general drop in appetite. These tend to be most noticeable after starting or after a dose increase, and they often settle within a week or two as your body adjusts. Eating smaller, lower-fat meals and staying well-hydrated both help. Fatigue and mild headaches have also been reported.

You'll rotate the injection site each week (abdomen one week, thigh the next, for example) to avoid irritation building up at a single spot. General guidance on weight-loss injections covers site rotation and technique in more detail. If you're weighing up which weight loss injection is the right fit for your circumstances, it's worth reviewing the options before your next clinical check-in.

Pancreatitis is an infrequent but serious risk: the MHRA issued a Drug Safety Update in January 2026 highlighting that severe, persistent stomach pain (particularly pain that spreads to the back) warrants urgent medical attention. The same applies to signs of a serious allergic reaction. For anything that feels urgent, contact 111 or your GP rather than waiting for your next clinical review.

Used pens are sharps waste. A sharps container keeps disposal safe and is required in most local council collections, check your local authority's guidance for how to arrange a collection.

Step 4: Dose titration and ongoing clinical review

Neither Wegovy nor Mounjaro starts at the dose that will eventually do the most work. The opening dose (2.5mg for Mounjaro, 0.25mg for Wegovy) exists to let your system settle, not to produce immediate weight change. The prescriber reviews your progress and tolerance before any move upward, typically in four-week intervals. Moving too fast increases GI side effects; the schedule exists for a reason.

At nume, every repeat order goes through a clinical re-review. There are no auto-renewals, no subscriptions. If a dose increase is being considered, the prescriber looks at evidence of how the current dose has been tolerated. This is one area where the cost structure of weight-loss injections is worth understanding: at higher doses, the per-pen price changes, so planning for titration is sensible from the start. Newer treatments are also entering the market, and our page on weight loss injections including reta explains what is on the horizon for those who want to stay informed. For anyone currently on a retatrutide pathway or exploring it, the dedicated guidance on that weight loss injection sets out the clinical detail in full.

If you have any other health conditions (including epilepsy, for example) it's worth reading about how weight-loss injections interact with specific conditions before your consultation, so your prescriber has the full picture. The FAQs also cover common questions that come up during treatment. Side effects, missed doses, and any concerns about your treatment should go to your prescriber or our aftercare team, available seven days a week.