Weight loss injections given in the stomach once a week: a plain-English guide

The first injection: what you're actually doing and why the stomach is a popular choice

Picture the scene: the pen arrives in a plain, unbranded box with a DPD tracking notification already on your phone. Inside is a pre-filled KwikPen (Mounjaro) or pre-filled pen (Wegovy), ready to use with no mixing or measuring. You choose a spot on your abdomen, clean the area, pinch the skin gently, press the pen against it and click. The needle (finer than most people expect) is hidden inside the device. You'll barely see it.

The stomach is the most popular injection site for most people simply because it's easy to reach, the subcutaneous fat layer is consistent, and you can see what you're doing. The other licensed sites, the front of the thigh and the back of the upper arm, are equally valid; the choice is yours each week. What matters is that you rotate — returning to the exact same spot repeatedly can cause lipohypertrophy, a hardening of fatty tissue that affects absorption. A 2–3 cm gap from the navel and any previous mark is the practical rule.

The injection itself takes only a few seconds. Most people report little to no pain; a brief sting or mild redness at the site is normal and usually clears within an hour. How these pens are designed and what the injection experience involves is covered in more detail if you want a fuller picture before you start.

What the medicine is doing between one injection and the next

Once the medicine enters the subcutaneous tissue it's absorbed gradually into the bloodstream over days, which is exactly why once a week is enough rather than a once a day weight loss injection. Mounjaro activates both GIP and GLP-1 receptors (two gut-hormone pathways involved in appetite signalling and blood-sugar regulation) making it the only dual-agonist weight-loss medicine licensed in the UK. Wegovy targets the GLP-1 receptor alone. Both slow gastric emptying (food leaves the stomach more slowly, so fullness arrives sooner and lasts longer) and act on appetite centres in the brain.

The practical result, across a week, is that hunger between meals tends to quieten and portion sizes naturally reduce. Clinical trial data gives a sense of scale: in the SURMOUNT-1 trial, participants taking tirzepatide 15mg lost around 20–21% of body weight on average over 72 weeks alongside lifestyle changes, as published in the New England Journal of Medicine. These are trial averages, individual results vary, and a prescriber is the right person to discuss what's realistic for you.

Treatment begins at a low starter dose (2.5mg for Mounjaro; 0.25mg for Wegovy). The first pen's job is to let your system adjust rather than to drive significant weight loss straight away; the prescriber increases the dose in steps over several weeks. This titration schedule is set by your prescriber, not something to self-manage.

Side effects tied to the injection site and the GI effects that follow

Injection-site reactions (mild redness, a small lump, brief itching) are among the more common early experiences and usually settle within a day or two. Rotating sites each week reduces the chance of them becoming persistent. Good technique helps too: alcohol swabs to clean the site beforehand are simple, inexpensive and worth the habit.

The side effects most people ask about aren't at the injection site at all, they're gastrointestinal. Nausea is the most reported, particularly in the first few weeks or after a dose increase. Vomiting, loose stools, constipation, and indigestion can also occur. A fuller breakdown of what to expect and when to seek help covers the GI profile in detail, including what the NHS patient information describes as warning signs requiring urgent attention.

One worth flagging directly: severe, persistent stomach pain that spreads to the back (with or without vomiting) should be assessed urgently. The MHRA's January 2026 Drug Safety Update highlighted acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. It's rare, but knowing the symptom pattern matters. You can report any suspected side effect at the MHRA's Yellow Card scheme. And yes, constipation is a real and common complaint, there are practical ways to manage it that don't require stopping treatment.

Who can use weekly stomach injections for weight loss, and what the route looks like

The licensed eligibility for both Mounjaro and Wegovy covers adults with a BMI of 30 or above, or 27 and above where at least one weight-related condition is present, conditions such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone doesn't determine suitability: the prescriber reviews your full health picture, including any medicines you already take.

These medicines aren't suitable during pregnancy, while breastfeeding, or when actively trying to conceive. They're also not licensed for under-18s. Anyone with a personal or family history of medullary thyroid carcinoma or certain pancreatic conditions needs a careful prescriber conversation before starting. The NHS patient information for tirzepatide lists contraindications in full.

If you're comparing the cost of different routes, what weight loss injections typically cost in the UK sets out the landscape honestly, including what a legitimate private price should include. The NHS route exists too (Mounjaro is being rolled out through primary care under NICE's phased eligibility criteria) though private treatment through a regulated online pharmacy is the current realistic option for many people who don't yet meet the NHS thresholds or prefer not to wait. An overview of the different weekly injection options can help you see how they compare before committing to anything.

If this sounds like the right direction, the first step is a clinical consultation. At nume, a GPhC-registered Independent Prescriber reads every submission personally and the same day. Check your eligibility and start your free consultation.