What Is a Known Effect of GLP-1? (October 2026)

01 · The core answer

The known effects of GLP-1 medicines in plain English

A GLP-1 medicine acts on the receptor for a natural gut hormone, and the effects you notice come from that action. Mounjaro (tirzepatide) is a good example. It 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 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 Those three effects are the ones the medicine is built to produce.

02 · Your next step

Is this relevant to you, and what should you do next?

If you are already taking a GLP-1 medicine and a symptom worries you, speak to your prescriber or pharmacist.

If you are considering treatment, the next step is a consultation, not a purchase. 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, and individual results vary.¹ Our weight loss clinic explains how that works from start to finish.

If you want to see the cost before deciding, the Mounjaro pen starting dose is £169.99 per month at the standard price. A new customer's first 2.5mg pen costs £129.99 with code FIRST40. Prices checked 10 October 2026.

If you are still at the "what even is this?" stage, our guide to what Mounjaro is also known as clears up the names.

03 · What GLP-1 does

What does GLP-1 actually do to your body?

GLP-1 is a natural gut hormone involved in appetite, digestion and the response to insulin. Medicines in this class are designed to act on the same receptor, and the product information describes three linked effects: they 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 does not remove the need to eat sensibly. It makes a sensible, reduced-calorie pattern easier to hold to.

Mounjaro differs from single-action GLP-1 medicines such as semaglutide (Wegovy and Ozempic) because it acts on both GIP and GLP-1 receptors.¹ Single-action medicines act on the GLP-1 receptor alone. Both approaches share the appetite and stomach-emptying effects, which is why "GLP-1 effects" is a useful umbrella phrase even though the medicines are not identical.

You may also come across the term incretin in this context. Our page on the GLP-1 incretin effect goes into the hormone side in more depth, and if you are curious about how researchers measure the hormone itself, there is a short page on the GLP-1 assay.

One caution on language. You will sometimes see "GLP-1" used loosely for anything that sounds metabolic. Not every diabetes or weight product is a GLP-1 medicine. If you are unsure whether a particular product belongs in this class, our pages on whether Semglee is a GLP-1 and whether pramlintide is a GLP-1 take the question head on.

04 · Weight loss as an effect

Weight loss: the effect most people come for

Weight reduction is a known and measured effect of tirzepatide when it is used with lifestyle changes. 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 A trial average does not predict your own result, and it would be wrong to treat any single figure as a promise.

The trial also reported how many people reached particular thresholds. At 72 weeks, 85% of participants on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight. Half of participants on 10mg and 57% on 15mg lost at least 20% of their body weight.²

For a worked example, for someone starting at 100 kg, 20.9% is about 21 kg. That is simple arithmetic on the trial average, not a forecast of what you will lose or how quickly.

If you are curious about food as well as medicine, our light-hearted page on GLP-1 smoothies looks at what people mean when they pair the two. Treat it as a food idea, not as a treatment.

05 · Side effects and their lists

What are the most common GLP-1 side effects?

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. They 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.¹³

Because these medicines slow how quickly the stomach empties, the digestive system is where your body is adjusting to the treatment.¹ If something about eating, digestion or how you feel after a dose changes, that is worth noting down and raising at your review rather than quietly putting up with it.

Practical points that do not need a statistic:

• Read the patient information leaflet that comes with your pen before the first injection.
• Tell your prescriber about any new or worsening symptom, especially one that is severe or does not settle.
• Use the ongoing monthly reviews to talk through how each dose step felt before you move up.
• Report suspected side effects through the Yellow Card scheme.¹⁹

Our team offers side-effect support as part of aftercare, alongside monthly reviews, dose-titration and follow-up assessments by the prescribing team.

06 · Who should not take it

Who should not take GLP-1 medicines?

Whether a GLP-1 medicine is right for you is a prescriber's decision, based on your health history, current medicines and the licence. Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber.¹ 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.

In the UK the medicine is licensed for weight management in adults and for type 2 diabetes.¹ NICE recommends it on the NHS for adults with a BMI of at least 35 plus at least one weight-related condition.⁴

This is why the consultation matters. Our process is a short health questionnaire, then a review by a GPhC-registered prescriber, not a scoring script. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.

If you have any medical condition, are taking other medicines, or are unsure, say so in the consultation. A candid answer protects you far better than a short one.

07 · Medicine interactions

Which medicines need care alongside GLP-1 treatment?

There is no single list of "medications that cannot be taken together" that applies to everyone, and you should be wary of any page that offers one. What matters is your own medicine list, and the leaflet and your prescriber are where that is checked.¹³ Two interactions are worth knowing about in advance.

Oral contraceptives. 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 medicine can reduce absorption of oral contraceptives because stomach emptying slows.⁷ Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly.

Insulin and sulfonylureas. Doses of these medicines may need adjusting because of the risk of low blood sugar.¹ That does not mean they are forbidden alongside Mounjaro. It means the prescriber who manages your diabetes should know, so that doses can be reviewed.

Beyond these, tell the prescriber about every prescription, over-the-counter medicine and supplement you take. Supplements marketed as "natural" alternatives are not a substitute for a licensed medicine, and they carry no clinical oversight.

08 · Black box warnings

What is the black box warning for GLP-1?

In the UK, safety information sits in the SmPC and the patient information leaflet.

If you have seen a headline about a warning on these medicines, take the headline to your prescriber or pharmacist and ask how it applies to you.

Safety monitoring continues after a medicine is licensed. Regulators review signals, and people who take medicines can contribute by reporting suspected side effects. If you notice something you think might be linked to your treatment, report it through the MHRA Yellow Card scheme.¹⁹

A symptom that is not on the leaflet list is not a listed side effect. Tell your prescriber about any new or lasting symptom, and you can report suspected side effects through the MHRA Yellow Card scheme.

09 · Before you start

What to know before starting a GLP-1 medicine

The most useful things to know before starting are practical rather than dramatic. Start with the fact that this is a long-term medical treatment, not a short course. Treatment is used alongside a reduced-calorie diet and increased physical activity, and the trial results are described in those terms.²

Dosing builds in steps. Mounjaro comes as a pre-filled multi-dose pen (KwikPen) in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹ Your prescriber decides when, and whether, to step up, based on how you are doing.

If you are comparing it with Wegovy, the Wegovy pens are 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg once weekly, stepping up after 4 weeks on each of 0.25mg, 0.5mg, 1mg and 1.7mg, so the 2.4mg maintenance dose starts from week 17. You can see more on the Wegovy pen page.

Practical points to settle early:

• Know your pen. The needles, sharps bin and injection guidance are included with every order, and the pen's own instructions are the thing to follow.
• Keep pens in the fridge at 2–8°C and never freeze them; unopened pens keep until the printed expiry date. 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 frozen.
• Plan your review dates. Repeat orders re-check suitability, and there is no auto-shipping.
• Decide who to tell. Your GP, and anyone managing diabetes or contraception, should know what you are taking.

On the NHS, access is being phased in over several years. NICE recommends Mounjaro for adults with a BMI of at least 35 plus at least one weight-related condition, and the 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.

10 · Stopping and clearance

What happens when you stop?

After a final dose, the medicine 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 The effects on appetite do not vanish overnight for that reason, and they fade as the medicine leaves your body.

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. Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load.

11 · Choosing a supplier safely

Getting a GLP-1 medicine safely

A genuine GLP-1 medicine comes from a registered pharmacy after a proper consultation, and any seller offering to supply without a consultation or prescription should be avoided. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷

You can check a pharmacy yourself. EverydayMeds is registered with the GPhC under number 9012878, and the register is public.¹² We supply Mounjaro through official LillyDirect supply, direct from Eli Lilly's UK distribution, as genuine KwikPens.

Our consultation is free, and the price shown is the price paid, with no hidden fees. Delivery is free, next-day and tracked by DPD in temperature-controlled, discreet packaging, and orders placed by 3pm are dispatched the same day. Saturday delivery is £14.99.

12 · 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); 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. 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 If you cannot keep fluids down, contact your clinical team — dehydration needs medical help straight away.