Weight Loss Injectable Medication: What It Is and How to Use It

Step 1 — Clinical assessment: why the prescription comes before the pen

Injectable weight loss medication cannot be purchased over the counter. Under UK medicines law, tirzepatide and semaglutide are prescription-only medicines (POMs), so the process always starts with a prescriber reviewing your health history, BMI, and any conditions that might affect suitability or safety.

The private route through a regulated online pharmacy removes the waiting list, but not the clinical gate. At nume, a GPhC-registered Independent Prescriber reads every consultation personally on the day it is submitted. If you send yours before noon on a working day, the review happens the same day. Identity and current weight are verified before any medicine is dispensed, and your GP is notified in line with GPhC guidance.

The eligibility threshold for both medicines is broadly adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes, or obstructive sleep apnoea. Your prescriber also looks at your full picture, BMI alone does not guarantee approval. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. For a clear overview of what affects that assessment, the treatment overview covers the main considerations.

One thing worth doing before you start: check that the pharmacy you are using appears on the GPhC pharmacy register. It takes under a minute, and it is the single most reliable way to confirm you are dealing with a legitimate, regulated service. Our registration number is 9012878.

Step 2, Choosing the right injectable: tirzepatide or semaglutide

There are two medicines on the UK injectable weight loss medication list: Mounjaro (tirzepatide) and Wegovy (semaglutide). They share a lot (weekly injection, gradual dose titration, GI-led side-effect profile) but they work differently.

Tirzepatide is a dual GIP and GLP-1 receptor agonist, the only one of its kind licensed for weight management in the UK. Semaglutide (Wegovy) acts on the GLP-1 receptor alone. In the SURMOUNT-1 clinical trial, tirzepatide at the highest dose produced an average body-weight reduction of around 20–21% over 72 weeks; in the STEP 1 trial, semaglutide 2.4mg averaged roughly 15% over 68 weeks. A direct head-to-head trial (SURMOUNT-5, published in the New England Journal of Medicine, 2025) found greater average weight loss with tirzepatide than with semaglutide 2.4mg over 72 weeks. Results vary between individuals.

For context on how the two medicines compare across other practical factors, this side-by-side guide covers mechanism, titration and eligibility differences. If you are also weighing up whether an injectable is the right format for you at all, our guide to the best non-injection weight loss medication sets out the alternatives worth considering. Which medicine suits you is a clinical decision made with your prescriber, not a consumer choice.

Wegovy also comes in a 7.2mg dose, approved by the MHRA in April 2026, though specific dose availability is confirmed at consultation. If you are based locally and want to understand what accessing a weight loss injection service in Stoke-on-Trent involves, that page sets out the options available to you.

Step 3, Your first injection: what actually happens

Both medicines come in a pre-filled pen designed for self-injection. You do not need to draw up a syringe or calculate doses. The pen does the measuring; you just need to choose your injection site, prepare the skin, and press.

Approved injection sites are the abdomen, thigh, or upper arm. Rotating sites each week reduces the chance of irritation in one spot. Before each injection, clean the skin with an alcohol swab, let it dry, then follow the steps in your Patient Information Leaflet. Correct injection technique matters more than most people expect, a detailed walkthrough is available on our injection technique page.

Used needles must go straight into a proper sharps container, never loose in a bin bag. A 1-litre sharps container is the standard size for home use. Storage between injections is straightforward: both medicines require refrigeration at 2–8°C, with a limited room-temperature window, the exact window is in the leaflet, and your prescriber's instructions take priority over anything you read elsewhere.

Tirzepatide starts at 2.5mg. That dose exists to settle your system, not to produce significant weight loss; the therapeutic effect builds as your prescriber moves you up through the schedule, typically every four weeks. If you want a broader look at how each weight loss medication injection works in practice, that page covers what patients commonly notice at each stage.

Step 4, Managing side effects and knowing when to call for help

Most people using injectable weight loss medication experience some gastrointestinal side effects, particularly in the early weeks. Nausea is the most common, followed by constipation, diarrhoea, indigestion, and occasional vomiting. These tend to be at their worst after a dose increase and ease off as your body adjusts. Eating smaller portions, avoiding fatty or spicy food during the adjustment period, and staying well hydrated all help.

The NHS patient guidance for tirzepatide on the NHS website lists which side effects are expected and which need prompt attention. Some need medical help quickly. Severe, persistent stomach pain that spreads to the back (with or without vomiting) should be assessed urgently because it can indicate pancreatitis, which the MHRA highlighted as a known, though infrequent, serious risk in a Drug Safety Update issued in January 2026. Gallbladder symptoms, signs of dehydration, or any allergic reaction also warrant same-day contact with a clinician or 999 in an emergency. If you are still getting to grips with which products are available, our page on weight loss injections medication names explains what each branded and generic option is called and how they relate to one another.

If you suspect a side effect, you can report it directly at the MHRA's Yellow Card scheme. Our aftercare team is available seven days a week for questions that fall short of emergency level. Repeat orders at nume are only dispensed after a fresh clinical review, so dose increases are not automatic, they happen when your prescriber is satisfied the previous dose is tolerated.

If you are ready to find out whether injectable weight loss medication is clinically appropriate for you, speak to our prescribers through a free consultation today.