Weight Loss Medication Injection: What Happens From First Dose to Real Results

Step 1, Understanding what the injection actually does

Both Mounjaro and Wegovy work by attaching to receptors on cells in the gut and brain that normally respond to hormones released after eating. This slows how quickly food leaves your stomach, increases the sense of fullness after smaller meals, and dials down the background appetite signals that make staying in a calorie deficit difficult. Tirzepatide does this via two receptor pathways (GIP and GLP-1); semaglutide via one (GLP-1 only). The practical effect for most people is that they feel satisfied sooner and think about food less between meals.

Neither injection burns fat directly. They change the hormonal environment so that eating less feels natural rather than punishing. Lifestyle changes (eating patterns, activity, sleep) still matter alongside treatment. The NHS medicines page for tirzepatide explains this mechanism in plain terms and is worth reading before you start.

If you are exploring whether a weight loss injectable medication is the right fit for you, familiarising yourself with the names of the weight loss injections available tends to make it easier to follow prescriber guidance and set more realistic expectations about the pace of progress.

Step 2, Getting the prescription and starting safely

Injectable weight loss medicines are not available over the counter. The licensed starting point for tirzepatide is 2.5 mg; for semaglutide (Wegovy) it is 0.25 mg. These opening doses exist to let your digestive system adjust, not to produce dramatic weight loss straight away. Dose steps happen roughly every four weeks, guided by the prescriber based on how you are tolerating the current pen.

To be eligible under the medicines' UK licences, adults generally need a BMI of 30 or above, or 27 or above alongside a weight-related health condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. A prescriber looks at the full picture (current medicines, medical history, any contraindications) not just the BMI number alone.

At nume, every consultation is read personally by a GPhC-registered Independent Prescriber. A real clinician reviews your answers the same day. If suitable, treatment is dispatched the same day (for orders placed by 12pm, Monday to Friday) and arrives free the next working day by DPD. Details on how weight loss injections are prescribed and supplied through a regulated UK pharmacy are worth checking before you commit to any provider.

Step 3, Giving the injection correctly

Both Mounjaro and Wegovy come as pre-filled pens, so there is no measuring or drawing up of liquid. The injection goes just under the skin (subcutaneous) in one of three sites: the abdomen (at least a few centimetres away from the navel), the outer thigh, or the back of the upper arm. Rotating between sites each week is important, using the same spot repeatedly can cause localised lumps or skin changes.

Cleaning the site beforehand is straightforward; alcohol swabs are a simple addition to your routine. The needle is short (typically 4–5 mm) and most people find the sensation minimal. The pen is pressed firmly against the skin and the injection button held until the dose is complete, the mechanism varies slightly between manufacturers, so following the specific pen's instructions matters. Used pens contain a retracted or covered needle and should be disposed of in an approved sharps container rather than household waste.

For a more detailed walkthrough of the technique, our injection technique guide covers each step with the specifics for both pen types.

Step 4, Side effects and what the first few weeks usually look like

The most commonly reported side effects are gastrointestinal: nausea, loose stools, constipation, indigestion and bloating. For most people these are most noticeable in the first week or two after starting or after a dose increase, then calm down as the body adjusts. Eating smaller portions, avoiding high-fat meals and staying well hydrated all help. Fatigue and mild headaches are also reported, particularly early on.

The full side-effect profile for injectable weight loss medicines covers both common and less common reactions in more detail. Serious but infrequent effects include pancreatitis, in January 2026 the MHRA issued a Drug Safety Update specifically highlighting severe, persistent stomach pain (sometimes radiating to the back) as a reason to seek urgent medical help. Any suspected side effects can be reported via the MHRA Yellow Card scheme.

People sometimes ask about weight loss medication that does not involve injections, the Wegovy tablet (oral semaglutide) received MHRA approval for weight management in June 2026 and is an alternative worth discussing with a prescriber. If injection-based treatment is right for you, check your eligibility with our clinical team.