How to Inject GLP-1 in the Stomach: UK Guide (October 2026)

01 · The core answer

Where to inject in the stomach area

The licensed stomach site for Mounjaro is the abdomen, kept well clear of the navel. Licensed injection sites are the stomach area (at least 5cm from the belly button), the front of the thigh, or the back of the upper arm if someone else gives the injection.¹ That is why the stomach is the most convenient choice: you can reach it yourself and see what you are doing.

In practice, choose a soft area of the lower or side abdomen, away from the belly button, away from the waistband line where clothing rubs, and away from anywhere that is bruised, scarred, hard or tender. If you want a closer comparison of every site, our guide to where to inject GLP-1 pens sets out the options, and the page on the Mounjaro stomach site goes deeper on the abdomen specifically.

Rotation matters because the same patch of skin repeatedly injected is more likely to become irritated. Mild redness, itching, a small bruise or a tender lump is common and normally settles within a few days, and rotating sites each week helps.¹

02 · What to do next

What to do next

If you already have a pen, read the leaflet that came with it and follow the technique it shows. A GPhC-registered prescriber reviews every order, and needles, a sharps bin and injection guidance are included. Our wider weight loss clinic page explains the full service.

03 · Step-by-step technique

Step-by-step: injecting into the stomach

The exact mechanics of the pen, including how to unlock, place and release it, are set out in the patient information leaflet, so follow it.

• Choose your day. Mounjaro is injected under the skin (subcutaneously) once a week, on the same day each week.¹
• Let the pen warm briefly. Letting the pen warm briefly out of the fridge reduces stinging.¹ Keep the cap on while it warms, because the cap protects the pen from light.
• Pick the spot. Use the stomach area, at least 5cm from the belly button, and away from bruised, scarred or tender skin.
• Wash your hands and prepare the skin. Clean hands and clean skin are sensible habits, and a pharmacy alcohol swab is a common way to do it.
• Inject as the pen instructs. Follow the leaflet for how to position the pen, how to start the injection and how long to keep it in place.
• Dispose safely. Put the used pen or needle into a sharps bin, never in the household bin.
• Note it down. Record the date and where you injected so that next week you move to a different spot. The free GLPTrackr app is built for tracking a GLP-1 journey.

Each KwikPen contains four weekly doses, so the same pen is used across a month, and the same routine repeats each week.¹ Our dedicated guide on using the GLP-1 pen covers the pen itself in more detail.

04 · Pinching and mistakes

Pinching, mistakes and injecting it wrong

Whether to pinch a fold of skin depends on the pen and its instructions, so check your leaflet rather than copying a video made for a different device. If the leaflet for your pen shows a pinch, do it. If it does not, you do not need to add one.

Can you inject GLP-1 wrong? Yes, in minor ways, and most are harmless but worth avoiding. Common slips include injecting too close to the belly button, injecting repeatedly in the same spot, and injecting into skin that is bruised or tender. Most of these show up as a sore spot, a bruise or a lump at the site. Mild reactions are common and normally settle within a few days.¹

Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹ So some reaction is a known, expected part of injecting weekly, not necessarily a sign you did anything wrong.

Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹ If you think you gave a dose incorrectly, for example the pen did not seem to deliver, do not simply inject again. Speak to your prescriber or pharmacist first, because the timing rules between doses matter.

On timing, the missed-dose rule is firm: If more than 4 days have passed, skip the missed dose entirely and take the next one on your normal day — never take two doses within 3 days of each other.¹

05 · Timing, food and sleep

Best time of day, food and sleep

There is no best clock time for Mounjaro. It can be injected at any time of day, with or without food, so choose a time you will remember every week.¹ Some people prefer the evening so that any early tiredness falls at home; others like a morning routine. Both are within the licence.

What to eat beforehand is likewise a comfort question, not a rule. Because side effects are most likely when starting or moving up a dose, plain, lower-fat foods and smaller meals help while you adjust.¹

Is it normal to feel sleepy or tired? Fatigue is a recognised common side effect, most noticeable in the first weeks and after dose increases. It usually reflects a mix of the body adjusting and simply eating less, so keep meals balanced (protein especially), stay hydrated, and keep gentle activity going.¹ For most people energy returns to normal within a few weeks. Tiredness is a listed side effect, so if it is new and persistent, mention it to your prescriber and consider reporting it through the Yellow Card scheme.¹⁹

If you want to move your injection day, you can change your weekly injection day if needed, as long as at least 3 days (72 hours) have passed since your last dose.¹

06 · What it does to your stomach

What GLP-1 does to your stomach

GLP-1 medicines slow how quickly the stomach empties and make you feel full sooner. For Mounjaro, tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹ In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.

Does your stomach shrink on GLP-1? Mounjaro slows how quickly the stomach empties; it does not physically make the stomach smaller. What changes is signalling and speed: food leaves more slowly and fullness arrives earlier, so smaller portions feel like enough. That is why the advice is to stop at the first feeling of fullness.¹

Is it stomach paralysis? Slowed emptying is how the medicine works, and slower stomach emptying is listed as an uncommon side effect (up to 1 in 100) in the UK patient leaflet. If you cannot keep fluids down, contact your clinical team — dehydration needs medical help straight away. If you have severe, persistent stomach pain, get medical help straight away. You can also report it via Yellow Card.¹⁹

Slower emptying also explains why the medicine has a gradual dose ladder. Everyone starts on 2.5mg once a week, a dose that is about letting your body adjust, not about weight loss itself.¹ Your digestion gets time to settle before each step up.

07 · Bloating, cramping, upset stomach

Bloated stomach, cramping and an upset stomach

Digestive symptoms are the most common effects of the medicine, and they are the ones people notice around injection day. The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹ So bloating, cramping and stomach pains fall largely into the expected group of abdominal discomfort, particularly in the early days and after a dose rise.

The trial numbers show the pattern. In the trials supporting Mounjaro's weight-management licence, nausea was reported by 24.6% of people on 5mg, 29.0% on 10mg and 28.0% on 15mg, compared with 8.5% on placebo.¹ Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ Individual results vary.

Why does the belly feel bloated? Food moving through the gut more slowly can leave you feeling full, tight or gassy. Constipation can add to it, because food moves through the gut more slowly and many people eat and drink less.¹ For more on this, see our pages on whether Mounjaro causes a bloated stomach and how to handle a Mounjaro bloated stomach.

Practical steps that help most people: eat smaller meals slowly, stop at the first feeling of fullness, favour plain lower-fat foods while adjusting, stay well hydrated, and avoid lying down straight after eating.¹ For constipation, fluid comes first, then fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative.¹

Most side effects appear in the first days after starting or increasing a dose, peak early and settle as your body adapts. If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up, so speak to your clinical team rather than pushing through severe symptoms.¹

08 · Stomach acid and reflux

Stomach acid, reflux and burping

Reflux, indigestion and burping are recognised common effects. Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

The same measures help: smaller meals eaten slowly, plain lower-fat foods, and avoiding lying down straight after eating.¹ A pharmacist can talk through suitable over-the-counter options alongside your prescription.

09 · When to get urgent help

When stomach pain needs urgent help

Most stomach discomfort is manageable, but some symptoms should not be waited out. Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹

If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹ If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion), because your prescriber may need to adjust those medicines.¹

You can report any suspected side effect to the MHRA through the Yellow Card scheme.¹⁹

10 · Storing and carrying your pen

Storing and carrying your pen

Keep Mounjaro pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days, useful for short trips, after which it should be discarded.¹ Keep the pen cap on to protect it from light, and never use a pen that has been frozen.

For unopened pens, store them in the fridge until first use; they keep until the printed expiry date. For any question about an unopened pen left out of the fridge, check the patient information leaflet and ask your pharmacy rather than guessing.¹³ Our orders arrive in discreet, temperature-controlled packaging, so open the box, check the pen and put it into the fridge promptly.

11 · Other injection sites

If the stomach is not for you

The stomach is common, not compulsory. If the abdomen is sore, scarred or simply uncomfortable, the front of the thigh is a licensed alternative, and the back of the upper arm works when another person gives the injection.¹ Our guide on injecting GLP-1 into the arm explains why the arm is realistic only with help.

Whichever site you use, the weekly routine is the same: same day, rotating site, clean technique and a sharps bin. If you ever feel unsure, our prescribing team offers side-effect support and ongoing monthly reviews, and you can contact support by email, account portal chat or phone during opening hours.