Weight Loss Injections UK: The Complete Patient Guide (2026)

Before your first injection: what to check when your medication arrives

When your first delivery arrives, there are six checks to run through before your first injection. This takes about five minutes and prevents the most common issues patients run into. The checklist below applies to both Mounjaro and Wegovy.

• Check the packaging and solution. The liquid should be clear and colourless to slightly yellow. Don't use the pen if the solution looks cloudy, discoloured, or contains visible particles. Contact your pharmacy for a replacement.
• Check the expiry date. This appears on both the outer carton and the pen itself. An expired pen should not be used.
• Refrigerate immediately. Your pens arrive in cool-chain packaging. Move them into the fridge (between 2°C and 8°C) as soon as possible. Store on the middle shelf in the original carton, away from the freezer compartment and the back wall where temperatures can drop sharply. Never freeze.
• Remove from the fridge 30 minutes before injecting. Injecting cold medication increases discomfort and can worsen injection-site reactions. Letting the pen reach room temperature also helps the medication flow properly through the pen mechanism.
• Check what's included. Mounjaro KwikPens need disposable needles attached before each injection, and these should arrive with your order. Wegovy auto-injectors are needle-free and come ready to use. If anything is missing or damaged, contact your pharmacy before your first injection.
• Pick your injection day. Both medications are taken once a week. Choose a day that fits your routine. You can change the day later if needed, but doses must be at least 72 hours apart.

How to inject: step by step

The first few injections feel more daunting than they are. Once you've done it two or three times, it becomes routine, and most patients find the whole process takes under five minutes from start to finish. The technique differs between Mounjaro (which uses a KwikPen with a disposable needle) and Wegovy (which uses a needle-free auto-injector), so the steps are split below.

How to inject Mounjaro (KwikPen)

A new needle must be attached before every injection. The pen itself is used for four weekly doses; the needle is single-use.

1. Wash your hands thoroughly with soap and water.
2. Remove the pen from the fridge 30 minutes before injecting.
3. Check the solution through the viewing window. It should be clear and colourless to slightly yellow. If cloudy or discoloured, do not use.
4. Attach a new needle. Remove the paper tab from the needle, push the needle straight onto the pen, and twist clockwise to secure. Remove the outer needle cap and keep it for safe disposal later. Remove the inner needle cap and discard.
5. Choose your injection site: the stomach (at least 5cm from the navel), the outer thigh, or the outer upper arm. Rotate sites with each injection. Avoid any area that is bruised, scarred, tattooed, or irritated.
6. Clean the site with an alcohol wipe and allow the skin to dry.
7. Pinch the skin gently. Insert the needle at a 90-degree angle (or 45 degrees if you have little subcutaneous fat). Press the dose button fully and slowly. Hold the needle in place for 10 seconds after the dose button stops moving, to make sure the full dose is delivered. Withdraw the needle.
8. Do not recap the needle. Place it directly into your sharps bin.
9. Record the injection: site used, date, time, and any notes about how it went. This helps you rotate sites properly and flag issues to your prescriber.

How to inject Wegovy (auto-injector)

Wegovy comes as a single-use, needle-free auto-injector. There is no needle to attach and no syringe to handle. The needle is hidden inside the device and retracts automatically after use.

1. Wash your hands thoroughly with soap and water.
2. Remove the pen from the fridge 30 minutes before injecting.
3. Check the solution through the viewing window. It should be clear and colourless to slightly yellow. If cloudy or discoloured, do not use.
4. Remove the pen cap by pulling it straight off. Do not twist it, and don't remove the cap until you're ready to inject.
5. Choose your injection site: the stomach (at least 5cm from the navel), the outer thigh, or the outer upper arm. Rotate sites with each injection.
6. Clean the site with an alcohol wipe and allow the skin to dry.
7. Place the pen flat against your skin at a 90-degree angle. Press firmly until you hear the first click, which means the injection has started. Keep the pen pressed against the skin until you hear the second click, which means the injection is complete. This can take up to 10 seconds.
8. Lift the pen away from the skin. The needle guard will extend automatically to cover the needle.
9. Dispose of the whole used pen in your sharps bin.
10. Record the injection: site used, date, and time.

Sharps disposal. Used Mounjaro needles and used Wegovy pens are classed as clinical waste and must go in a sharps bin. Never put them in household recycling or general waste. Your pharmacy should provide a sharps bin, or you can request one from your local council. Many councils collect full sharps bins for free; check with your local authority for the correct return route.

Storing your medication correctly

Storage is where most avoidable problems happen. Get this right and the rest of treatment gets much easier. The table below summarises the key storage rules for both medications, followed by the practical detail behind each one.

Never freeze your pens. Even brief freezing, including during transit in cold weather, causes irreversible damage to the active ingredient. If you suspect a pen has frozen, discard it and contact your pharmacy for a replacement. Visual inspection alone cannot rule out freeze damage.

Fridge placement matters. Store pens in the middle of the fridge, not in the door (where temperatures fluctuate every time the door opens) and not against the back wall (where temperatures can drop close to freezing). Keep them in the original carton to protect from light.

The room-temperature window is a fixed clock. Once a pen is removed from the fridge, the room-temperature window starts. For Mounjaro, that's 21 days. For Wegovy, it's 6 weeks. If the pen is unused at the end of that window, it must be discarded. It cannot be returned to the fridge to reset the clock.

Inspect before every injection. Look through the viewing window at the solution. Clear and colourless to slightly yellow is normal. If the solution looks cloudy, has visible particles, or has changed colour, do not inject. Contact your pharmacy.

Managing side effects: practical advice for the most common issues

Side effects are most common during the first four to eight weeks of treatment and at each dose escalation. Most settle within one to two weeks at a new dose as the body adjusts. The guidance below covers what is normal, what you can do at home, and when to contact your prescriber.

Nausea

Nausea is the most common side effect, particularly during the first four to eight weeks and at each dose escalation. It usually reduces within one to two weeks of reaching a new dose.

What helps:

• Eat smaller, more frequent meals rather than three large ones.
• Eat slowly, and stop as soon as you feel full.
• Avoid fatty, greasy, or heavily spiced food, especially on injection day and the day after.
• Bland foods (toast, crackers, plain rice, plain pasta) are usually well tolerated.
• Ginger tea, ginger biscuits, or peppermint tea can take the edge off.
• Stay upright for 30 to 60 minutes after eating.
• Cold or room-temperature food often sits better than hot food.

Contact your prescriber if nausea is severe, lasts more than two weeks at a stable dose, or stops you eating and drinking.

Vomiting

Vomiting is less common than nausea and is usually linked to dose escalation. The same dietary adjustments that help with nausea help here too. Take small, frequent sips of water or an electrolyte drink to stay hydrated.

Contact your prescriber if vomiting persists beyond 24 hours, if you see signs of dehydration (dark urine, dizziness, dry mouth), or if you cannot keep fluids down.

Constipation

Constipation is more common than diarrhoea in weight management patients, partly because reduced appetite often means reduced fibre and fluid intake.

What helps:

• Drink 2 to 2.5 litres of water a day.
• Increase fibre gradually (fruit, vegetables, wholegrains, legumes). Sudden increases can cause bloating.
• Light daily movement, such as a 20-minute walk, supports bowel function.
• Speak to your pharmacist about a gentle laxative if constipation persists.

Diarrhoea

Diarrhoea is usually mild and transient. Stay hydrated, temporarily reduce very high-fibre foods, and avoid high-fat foods until it settles. Contact your prescriber if it persists beyond a few days or if you become dehydrated.

Abdominal pain

Mild abdominal discomfort, especially during the first weeks, is common.

Red flag. Severe abdominal pain that does not go away, particularly pain that radiates through to your back, is a possible sign of pancreatitis and requires immediate medical attention. Do not wait. Stop the medication and seek urgent care.

When appetite is too low

For a small number of patients, appetite reduction goes too far. Signs include eating fewer than roughly 800 calories a day consistently, no appetite at all, or significant fatigue and weakness.

If this sounds like you, speak to your prescriber. A dose adjustment may be appropriate. In the meantime, aim for at least 1.2g of protein per kg of body weight each day to preserve muscle mass, even if your total calorie intake is low. Protein shakes and Greek yoghurt are useful when food doesn't appeal.

Injection site reactions

Mild redness, bruising, or swelling at the injection site is common and usually resolves within a few days.

What helps:

• Rotate injection sites with every dose.
• Let the pen reach room temperature before injecting.
• Don't inject into the same site twice in a row.
• Apply gentle pressure with clean cotton wool after injecting, but do not rub.

Contact your prescriber if a reaction spreads, feels warm to the touch, or gets worse rather than better over 48 hours.

What to eat on weight loss injections

The risk on GLP-1 medications is not eating too much. It is eating too little, and eating the wrong things when you do eat. Prioritising protein and nutrient density is the single biggest factor in getting a good outcome from treatment.

Protein first. Aim for 1.2 to 1.5g of protein per kg of body weight per day. Protein preserves muscle mass, which matters because GLP-1 weight loss includes both fat and muscle when protein intake is insufficient. Good sources: eggs, chicken, fish, Greek yoghurt, cottage cheese, legumes, tofu, and lean red meat.

Foods most patients tolerate well. Plain proteins, plain carbohydrates in moderate portions (rice, pasta, bread, potatoes), cooked vegetables, soups and broths, smoothies, and protein shakes.

Foods to limit, especially at dose escalation. Fatty or greasy food (significantly worsens nausea), heavily spiced or rich food, alcohol (many patients find they are much more sensitive on GLP-1 medications, and alcohol can worsen nausea), fizzy drinks (worsens bloating), and very large portions. The medication amplifies the feeling of fullness, so eating past the point of fullness can be genuinely uncomfortable.

Hydration. Aim for 2 to 2.5 litres of water a day. An electrolyte drink can help on days when you are struggling to eat or drink enough.

When appetite is very low. Eat small amounts of high-protein, nutrient-dense food even when you do not feel hungry. Skipping meals consistently leads to muscle loss, fatigue, and nutritional deficiencies. If low appetite is persistent, speak to your prescriber.

Exercise and activity

Resistance training is the single most useful exercise change you can make on weight loss injections. GLP-1 weight loss includes both fat and muscle, and resistance training (bodyweight exercises, resistance bands, or weights) is the most effective way to preserve muscle mass and improve body composition outcomes during treatment.

If you are starting from a low base of activity, don't overcomplicate it. A 20 to 30-minute walk most days is clinically meaningful. Two or three short sessions of bodyweight exercises a week (squats, press-ups, planks, lunges) are enough to preserve muscle alongside walking. You do not need a gym, and you do not need to train to exhaustion.

Managing exercise around side effects is mostly about timing and intensity. During the first few weeks at a new dose, when nausea is more common, reduce intensity or skip workouts if you feel unwell. Gentle walking is almost always tolerable, even on rougher days. Make sure you have had enough protein and fluid before exercising, particularly on days when appetite has been low.

What to do if you miss a dose

The rules are slightly different for Mounjaro and Wegovy. Check the right one.

Mounjaro (tirzepatide)

If you remember within 4 days (96 hours) of your missed dose, take it as soon as you remember and return to your usual weekly schedule. If more than 4 days have passed, skip the missed dose and take the next one on your usual scheduled day. Never take two doses within 3 days (72 hours) of each other.

Wegovy (semaglutide)

If you remember within 5 days of your missed dose, take it as soon as you remember and return to your usual weekly schedule. If more than 5 days have passed, skip the missed dose and take the next one on your usual scheduled day. Never take two doses within 48 hours of each other.

Multiple missed doses

If you have missed several doses in a row, contact your prescriber before restarting. Re-titrating from a lower dose is sometimes recommended to reduce the risk of side effects returning.

Travelling with your medication

Travel is manageable with a bit of planning. The main things to get right are cool storage, a travel letter for airport security, and enough supply to cover your trip plus a buffer.

Before you travel:

• Request a travel letter from your prescriber at least two to three weeks before departure. The letter should be on headed paper and include your name, the medication name and dose, a note that it is a prescription-only medication requiring refrigeration, and your prescriber's contact details.
• Work out how many pens you will need for the trip, and take at least one extra week of supply.
• Check expiry dates on all pens you are travelling with.
• If travelling internationally, check the destination country's rules on importing prescription medication. Some countries require advance notification or additional paperwork.

Storage on the go:

Airport security. Travel with your medication in its original packaging with the pharmacy label visible. Keep your travel letter with the medication, not in a separate bag. Declare the medication and any needles at the security checkpoint. Cooling packs over 100ml may need additional documentation, so keep the travel letter accessible.

Managing your injection schedule. Both medications are weekly, which gives flexibility. You can shift your injection day by up to 72 hours if needed, but doses must remain at least 72 hours apart. Where possible, inject before boarding or after landing rather than mid-flight.

Sharps disposal abroad. Research your destination before you travel. Carry a small travel sharps container for used needles (Mounjaro) or used pens (Wegovy). Do not dispose of sharps in hotel bins or local general waste.

What to do if your results plateau

A plateau is when weight loss slows or stops while you are still on treatment. Plateaus are normal, not a sign that the medication has stopped working, and there are several things to try before escalating dose.

Why plateaus happen. The body adapts to sustained calorie reduction by slowing metabolic rate, a process called adaptive thermogenesis. This is biologically normal and happens in every form of weight loss, not just with medication. It is not a failure, and it is not permanent.

What to review first. Before considering a dose change, work through these:

• Is your protein intake still at 1.2 to 1.5g per kg of body weight? Low protein slows progress and causes muscle loss.
• Is your hydration consistent at 2 to 2.5 litres a day?
• Have you added or increased resistance training? This has a direct effect on body composition even when scale weight stalls.
• Has food intake crept up as side effects have settled? A two-week food diary can be revealing, and most patients find at least one blind spot.

When dose escalation may be appropriate. If you have been on your current dose for at least 8 to 12 weeks, are tolerating it well, and have genuinely plateaued despite reviewing the points above, speak to your prescriber about a dose increase.

For Wegovy patients who have reached the 2.4mg maintenance dose, the 7.2mg single-dose pen (approved in the UK in April 2026) offers clinically meaningful additional weight loss for patients who are tolerating treatment well and have plateaued. In the STEP UP trial, the 7.2mg dose produced mean weight loss of 20.7% at 72 weeks, compared with 17.5% on 2.4mg and 2.4% on placebo.

When to consider switching. If you are on the maximum dose of your current medication and have genuinely plateaued despite everything above, your prescriber may discuss switching between Mounjaro and Wegovy. This is a clinical decision to make with your prescriber. Do not self-switch or combine medications.

Long-term treatment: what to expect beyond month 6

Weight loss slows after you reach your maintenance dose, and this is normal. The fastest rate of loss typically happens between months 2 and 6, during dose escalation. Most patients reach their lowest weight between months 9 and 18, at which point the clinical goal shifts from losing weight to maintaining it.

Weight regain after stopping treatment is well established in the evidence. In STEP-4, patients who stopped semaglutide regained an average of 6.9% of body weight in the 48 weeks that followed, while those who continued lost a further 7.9%. SURMOUNT-4 showed the same pattern for tirzepatide. The implication is clinical, not motivational: obesity is a chronic condition, and most patients who achieve sustained weight loss need ongoing treatment, in the same way that someone with high blood pressure needs ongoing medication. If you are thinking about stopping, discuss it with your prescriber first. Tapering and maintenance plans exist.

Ongoing clinical oversight matters. At a minimum, expect a prescriber check-in at each dose escalation and every 3 to 6 months once you are on a stable dose. If you are not receiving this level of oversight from your current provider, contact your prescriber to arrange a review. Foundry patients receive ongoing check-ins built into their treatment, and your prescriber is available through the platform for questions between reviews.

When to seek urgent medical attention

Most side effects are manageable at home. A small number require urgent medical attention. If in doubt, err on the side of getting help.

Call 999 or go to A&E immediately if you have:

• Severe abdominal pain that does not go away, especially pain that radiates through to your back (possible pancreatitis).
• A severe allergic reaction: difficulty breathing, rapid swelling of the lips, tongue, or throat, a fast or irregular heartbeat.
• Severe vomiting or diarrhoea causing dehydration: extreme dizziness, confusion, very dark urine, or an inability to keep any fluids down.

Contact your prescriber the same day or next working day if you have:

• Vomiting that persists beyond 24 hours.
• Diarrhoea that persists beyond a few days.
• Pain in the upper right side of your abdomen after eating fatty food (possible gallstones).
• Changes in your vision, particularly if you have a history of diabetic retinopathy.
• Injection site reactions that spread, feel warm to the touch, or worsen over 48 hours.
• Any side effect that is significantly affecting your daily life.

Suspected side effects can also be reported to the MHRA via the Yellow Card scheme.

Frequently asked questions