Taking GLP-1 Medication: What to Do and What to Expect (October 2026)

01 · The core answer

What does taking a GLP-1 medicine actually do?

It changes how hungry you feel and how quickly your stomach empties. GLP-1 is a natural gut hormone, and these medicines act on its receptors. With Mounjaro (tirzepatide), the medicine activates 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.¹ The medicine is a support, not a shortcut. You still need to change what and how much you eat, and move more.

In the SURMOUNT-1 trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages. Individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

02 · Your next steps

What should you do when you start?

Start by reading the patient information leaflet that comes in the box, then follow your prescriber's dosing plan exactly. The leaflet is the authoritative guide to how to use the pen, who should not use it and which symptoms need attention.¹³ Mounjaro is not suitable for everyone. You should not take it if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive. Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems. Always share your full medical history and medicine list during consultation.

A sensible checklist for the early weeks:

• Pick a day of the week that suits you and keep your injection on that same day.
• Check the pen label and strength before every injection.
• Eat smaller, slower meals and stop when you feel full.
• Keep a simple log of your dose, injection day, appetite and any symptoms. Our free GLPTrackr app can do this for you on iOS and Android.
• Contact your prescriber about anything new or worrying rather than waiting it out.

For a realistic picture of the early period, read what to expect in the first weeks. If your main question is whether treatment is appropriate for you, see our page on how safe GLP-1 treatment is.

03 · Getting treatment

Where do you go if you want to start?

If you are an adult considering Mounjaro, you can begin with a free online consultation, reviewed by a GPhC-registered prescriber. Our weight loss clinic explains the options. The Mounjaro pen starts with the 2.5mg strength; the standard price is £169.99 per month and all six doses are in stock. A new-customer discount applies to first orders and a reorder discount to every future order, with the current values shown live on the page.

04 · How GLP-1 drugs work

How do GLP-1 drugs work?

They act on natural gut-hormone receptors that help regulate appetite and slow how quickly the stomach empties. Mounjaro is a once-weekly injectable prescription medicine made by Eli Lilly, licensed in the UK for weight management in adults and for type 2 diabetes.¹

Tirzepatide activates both GIP and GLP-1 receptors. This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy/Ozempic).¹ Wegovy is another licensed weight-management option, and you can look at the Wegovy pen if you are comparing.

The practical effects are the ones described in the product information: appetite is regulated, the stomach empties more slowly, and the body responds better to insulin. That is why many people feel fuller after smaller portions and snack less between meals. It also explains why some digestive symptoms can appear, because the gut is working differently.

These medicines do not remove the need for food choices. They make a reduced-calorie diet easier to follow for many people, but they cannot do the work alone.

05 · How to take it

How do you take Mounjaro day to day?

You inject it once weekly, on the same day each week, using the pre-filled multi-dose KwikPen. It comes in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ Treatment normally begins at 2.5mg and steps up under your prescriber's direction.

For the injection itself, follow the pen's instructions and the patient information leaflet. The device instructions are the safe reference for timings and technique.¹³ Every order includes needles, a sharps bin and injection guidance.

If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time, then carry on with your usual day. 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. Never use a pen differently from how it is licensed, and never use the medicine in ways your prescriber has not agreed.

Eating after an injection is a common question; we cover it in how long after an injection you can eat.

06 · Storing your pen

How should you store a GLP-1 pen?

Keep unused pens in the fridge at 2–8°C until first use. Unopened pens keep until the printed expiry date.¹³

Once in use, a pen can be kept out of the fridge at a temperature not above 30°C for up to 30 days after first use, and must then be thrown away. Do not use a pen that has been above 30°C or has been frozen.¹ Write the start date on the pen or log it in the GLPTrackr app so you know when it is time to stop using it.

If a pen has been left somewhere warm or you are unsure whether it is still usable, check the leaflet and contact the pharmacy rather than injecting. Sterility and potency are no longer assured outside the tested window. When your order arrives, it is sent in temperature-controlled packaging, so put the pens in the fridge promptly.

Always keep pens, needles and the sharps bin out of children's reach.

07 · Side effects

How soon do side effects start, and what should you do?

Like all medicines, Mounjaro can cause side effects, and most are digestive. The most common, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹³ Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts.

What to do about them:

• Keep your prescriber informed about anything new, especially if it is persistent or affects eating and drinking.
• Follow the leaflet's advice on which symptoms need urgent medical help.
• Report suspected side effects through the Yellow Card scheme, which anyone can use.¹⁹

If you notice something new, tell your prescriber so they can assess it properly.

Our prescribing team offers side-effect support as part of ongoing monthly reviews.

08 · Food and diet

What is the GLP-1 diet, and what should you avoid?

There is no special named GLP-1 diet in the product information. The licensed approach is a reduced-calorie diet and increased physical activity, with the medicine supporting both.²

Because the medicine makes you feel full sooner, a few habits tend to help in practice:

• Build meals around protein and vegetables so smaller portions still feel satisfying.
• Eat slowly and stop at the first signs of fullness.
• Drink water regularly through the day.
• Go easy on very rich, fried or heavily sugary foods if they leave you feeling unwell.
• Keep alcohol modest and see how your body responds.

These are everyday nutrition tips, not clinical rules. If you have a medical condition or take other medicines, check with your prescriber before changing your diet substantially.

For more detail, we have guides on what to eat on treatment and what to put on your plate each day. Eating enough is part of the plan: the aim is steady, sustainable change, not eating as little as possible.

09 · Interactions

What medication interactions should you know about?

Because Mounjaro slows stomach emptying, it can affect how quickly some oral medicines are absorbed, particularly when you start treatment or increase the dose. The most important ones to check are contraception and diabetes treatments.

Contraception: Mounjaro can reduce absorption of oral contraceptives because stomach emptying slows. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ The MHRA has issued guidance on contraception with these medicines.⁷

Insulin and sulfonylureas: if you use these for diabetes, doses may need adjusting because of the risk of low blood sugar. This is not a ban on combining them, but it needs your prescriber or diabetes team to plan the changes.¹³

Always tell the prescriber about every prescription medicine, over-the-counter product and supplement you take. Supplements marketed as natural alternatives are not licensed equivalents and have no clinical oversight, so do not treat them as a substitute or add them without asking.

Our free consultation asks about your medicines and health history, and a GPhC-registered prescriber reviews your answers before anything is supplied.

10 · Not losing weight

Why might you not be losing weight, and how fast should it happen?

Response varies, and trial averages do not predict your own result. In SURMOUNT-1, results were reported over 72 weeks, and the reported averages rose with the dose.²

If progress seems slow, work through these points with your prescriber:

• Are you still on a starting or lower strength while your dose is being stepped up?
• Has your eating pattern changed, or are liquid calories and snacks quietly filling the gap?
• Has your activity level dropped?
• Are side effects making you eat in ways that cut against your goals?
• Are you using the pen correctly and on schedule?

Dose adjustments are a prescriber decision. The monthly reviews we provide are there to look at exactly this.

Staying on treatment matters too. A trial of continued treatment, SURMOUNT-4, looked at maintaining weight reduction.¹⁴ See whether you have to keep going for the practical side.

11 · Serious side effects

Serious side effects: when to get help straight away

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) — stop using Mounjaro and get medical help straight away; pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems) — get medical help straight away; or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing — get medical help straight away. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹

12 · Stopping and access

What happens when you stop, and how do you get access?

Talk to your prescriber before you stop. Mounjaro manages weight; it does not cure the biology behind it. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average.¹⁴ If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. Tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system. There is no way to speed this up, and nothing needs doing while it clears.¹

On access: Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

If you choose private treatment, check that the pharmacy is on the GPhC register; ours is GPhC 9012878. Be wary of anyone offering to supply without a consultation or prescription, because counterfeit medicines are a real safety risk.⁷

Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber. It is used alongside a reduced-calorie diet and increased physical activity. Individual results vary.