Once a Week Injection for Weight Loss: What to Expect, Step by Step

Step 1: Clinical assessment, before anything is prescribed

The process begins not with an injection but with a consultation. Because Mounjaro and Wegovy are prescription-only medicines, a weight-loss injection cannot legally be supplied without a prescriber reviewing your eligibility first. That review covers your BMI, any weight-related health conditions, your current medicines, and your medical history. The licensed threshold is a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol or obstructive sleep apnoea. BMI alone does not guarantee a prescription, the prescriber weighs the whole picture.

At nume, that assessment is carried out by a GPhC-registered Independent Prescriber on the day you submit your consultation. Identity is verified by photo ID, and weight is confirmed on a live video call. A question our prescribers hear most weeks is whether the pen can just be sent out based on a short questionnaire. It cannot; the clinical review is the safeguard that makes the treatment safe and legal.

If you are 17 or younger, neither medicine is licensed for your age group. The position on injections for younger people is set by the product licences, not by individual providers.

Step 2: Starting at a low dose and increasing gradually

Once approved, treatment begins at a starter dose, 2.5 mg for Mounjaro, 0.25 mg for Wegovy. The first pen's job is to let your body adjust, not to produce weight loss immediately. Most people notice GI effects most in these early weeks: nausea is the most commonly reported, followed by loose stools, constipation, indigestion and occasionally fatigue. These effects are real but, for most, manageable, and they typically settle as your system adapts.

Dose increases happen on a schedule set by your prescriber, usually every four weeks, stepping through the available strengths until you reach a maintenance dose that balances effectiveness with tolerability. How the weekly injection schedule works in full is worth reading before your first pen arrives, so you know what a normal first month looks like. The titration process is not something to rush; the evidence for both medicines was built at the maintenance doses, not the starter ones.

For Mounjaro, the SURMOUNT-1 trial (involving around 2,500 adults with obesity) found an average body-weight reduction of roughly 20–21% at the 15 mg maintenance dose over 72 weeks, as published in the New England Journal of Medicine. Wegovy's STEP 1 trial showed approximately 15% average reduction at 2.4 mg. These are clinical trial figures, individual results vary.

Step 3: Giving the injection, sites, technique and rotation

Both pens are pre-filled and pre-set to the correct dose. You do not measure or draw up anything. The injection is subcutaneous (into the layer of fat just below the skin) and the approved sites are the abdomen, the front of the thigh, or the upper arm. Rotating between sites each week reduces the chance of localised skin reactions.

Many people find injecting into the abdomen most straightforward to begin with, and the technique for stomach injections is covered in more detail separately. Cleaning the skin beforehand is good practice; alcohol swabs are inexpensive and easy to keep alongside the pen. Used needles must be disposed of safely in an approved sharps bin, never in household waste.

The pen is stored in the fridge (2–8°C) until use. There is a limited window during which it can be kept at room temperature, but the exact period differs between Mounjaro and Wegovy, and between dose strengths, always follow the Patient Information Leaflet that comes with your specific pen rather than a general rule quoted online. For a broader sense of what both licensed options look like side by side, the weight-loss injection overview covers the key differences.

Safety, when to seek help, and what to report

Most side effects with once-weekly injections are mild to moderate and gastrointestinal in origin. Nausea, stomach discomfort, burping and changes in bowel habit are the most common. They are most noticeable in the first few weeks and after each dose increase, then usually improve.

Some symptoms need prompt attention. Severe, persistent abdominal pain that spreads to the back (with or without vomiting) should be assessed urgently, as it may indicate pancreatitis. The MHRA highlighted acute pancreatitis as a known risk with GLP-1 medicines in a Drug Safety Update published in January 2026, and that guidance is worth reading. Signs of a serious allergic reaction, gallbladder symptoms, or significant dehydration from repeated vomiting also warrant medical help rather than waiting.

If you experience any side effect you want to report, the MHRA's Yellow Card scheme accepts reports directly from patients. This feedback genuinely contributes to post-marketing safety monitoring for medicines that, like both Mounjaro and Wegovy, still carry a Black Triangle (▼) status.

Questions about what to do with a missed dose, how to handle a dose change, or how the injection fits with other medicines you take should go to your prescriber or to the Patient Information Leaflet, not to general guidance. Cost context for weekly injections is a separate question many people have once they understand how treatment works. If you are ready to find out whether you are eligible, start a free consultation with our prescribers and receive a same-day clinical review.