GLP-1 Bad Effects: An Honest UK Guide (October 2026)

01 · The short answer

Are there bad effects of GLP-1 medicines?

Yes. Every effective medicine has unwanted effects, and GLP-1 medicines are no exception. Tirzepatide, the active ingredient in Mounjaro, activates the GIP and GLP-1 receptors, which regulate appetite, slow how quickly the stomach empties and change how the body responds to insulin.¹ Those same actions explain why the digestive system is where many people notice the change first.

Frequencies differ between medicines, doses and trials, and a number copied from a different drug's trial can mislead you about the one you are considering. The patient information leaflet for your exact product is the right place for frequencies and for the signs that need prompt medical attention.¹³

If you want a more detailed look at the symptom list, our guide to GLP-1 side effects goes through it, and our piece on whether GLP-1 is bad for you covers the wider safety question. 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.

02 · Your decision

Is this relevant to me, and what should I do next?

It is relevant if you are considering treatment, have just started, or are worried about something you have read. Each situation has a sensible next step.

03 · Why effects happen

Why GLP-1 bad effects happen

Bad effects usually trace back to what the medicine is designed to do. Tirzepatide helps regulate appetite and slows how quickly the stomach empties, so most people feel fuller sooner and less hungry between meals.¹ A slower-emptying stomach is helpful for appetite, but it is also the likely reason digestive symptoms feature so heavily in the leaflet.

This also explains the timing. Effects are most likely to be noticed when the body is adjusting, which is why treatment starts on the lowest strength and steps up gradually. Each step is a moment to review how you are feeling with your prescriber.

Wegovy follows the same logic with a different ladder: 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg, stepping up after at least four weeks and reaching the 2.4mg maintenance dose from week 17.¹⁸ Gradual stepping is part of how the licensed treatments are meant to be used. It is not an optional extra.

Importantly, none of this means every person has every effect. Many people experience mild symptoms, some experience few, and a minority find the medicine does not suit them. A prescriber can adjust the plan, or recommend stopping, if the balance tips the wrong way.

04 · What first weeks feel like

What happens when you first start taking GLP-1

In the first few weeks you are most likely to notice appetite changes, and possibly some digestive symptoms, as your body adapts. Reduced hunger between meals is the intended effect.¹ 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.

Practical habits make the early weeks easier to judge. Keep a simple note of the day you inject, what you ate, and any symptoms. Our free companion app, GLPTrackr, is built for tracking a GLP-1 journey on iOS and Android, and a written record makes a conversation with a prescriber far more useful than a vague recollection.

Equally useful is knowing when to speak up. A symptom that is new, severe, getting worse or simply worrying deserves a call to your prescribing team or pharmacist rather than waiting for the next review.

Different symptoms have different meanings for different people, and that is exactly what the prescriber review and the leaflet are for. If you have already begun and want more on a particular symptom, try our guide to GLP-1 bad breath, or our overview of new side effects of Mounjaro.

05 · Dangers and who should not

Dangers, and who cannot take GLP-1 medicines

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.¹³ 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.

Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber, not a scoring script.

Two interaction points are worth knowing in plain terms. Tirzepatide can reduce absorption of oral contraceptives, because stomach emptying slows, so contraception should be discussed.⁷ 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 And if you take insulin or a sulfonylurea, doses may need adjusting because of the risk of low blood sugar. That is an adjustment conversation, not a blanket ban.

Disclosing everything honestly is the single most protective thing you can do: other medicines, supplements, past conditions, pregnancy plans and any history the leaflet asks about. If something about your health changes after you start, say so at the next review or sooner.

If you believe you have experienced a side effect, you can also report it yourself through the MHRA Yellow Card scheme.¹⁹ Reports help regulators spot patterns across the whole population, and you do not need proof that the medicine caused the problem to submit one.

06 · Is it worth the risk?

Is GLP-1 worth the risk?

That depends on what you stand to gain, what you stand to lose, and your own health, so it is a decision rather than a fact. The benefit side is well documented. 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.

On the other side sit the unwanted effects, the cost, the commitment to injecting weekly, and uncertainty that comes with any long-term medicine. A fair test is to ask yourself these questions:

• Is my weight affecting my health now, or likely to in future?
• Have I already tried diet and activity changes seriously?
• Would I be able to attend reviews and report symptoms promptly?
• Am I comfortable with a medicine that needs ongoing use to keep its effect?

NICE recommends tirzepatide on the NHS for adults with a BMI of at least 35 plus at least one weight-related condition. NHS primary-care rollout began in June 2025 with the highest-need groups first, and it will take years.⁴⁶ If you do not yet qualify or cannot wait, regulated private treatment is an option, and our weight loss clinic explains how it works.

07 · Long term and stopping

Is long-term use bad, and do I have to stay on it forever?

Long-term use is a question for your prescriber and your own review.

Nobody is locked in. Stopping should always be planned with your prescriber, and nothing needs doing while the medicine leaves your body. 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.¹

Be honest with yourself about what stopping means. 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. The medicine works while you take it, alongside a reduced-calorie diet and increased physical activity. Habits built during treatment are the part that stays with you, so use the time to build routines you could keep.

A good review conversation covers whether the current dose is still right, whether effects are manageable, and what your plan is for maintenance or stopping. Raise it early rather than waiting until you feel stuck.

08 · Which has the worst effects?

Which GLP-1 has the worst side effects?

No GLP-1 medicine can be ranked as worst for everyone. Mounjaro (tirzepatide), Wegovy (semaglutide) and the newer oral options each have their own product information, and comparing unwanted-effect percentages from separate trials is unreliable because the populations, doses and designs differ.

The fair method is to read each product's own document: the Mounjaro SmPC, the Wegovy SmPC and, for tablets, the SmPCs for Wegovy tablets and Foundayo.^1182021 Dose also matters within a single medicine, which is why gradual titration exists.

If you are weighing options, a prescriber can talk through which suits your history. The right choice depends on your health and tolerance, not on a league table of side effects.

09 · Spotting weak claims

The hard truth: how to read scary and glossy claims

Both extremes online are unreliable, so judge claims by their source. Frightening posts often present a single story as typical. Glossy sellers often present dramatic weight-loss claims with no mention of unwanted effects. Neither helps you decide.

A few checks help:

• Is the symptom in the SmPC or leaflet? If not, it is not a listed side effect, though you should still tell your prescriber about any new symptom.
• Does the claim give a number without a source? Treat unsourced percentages with caution.
• Is the seller offering to supply without a consultation or prescription? That is a warning sign for counterfeit or unsafe supply.
• Does it promise specific results? Trial averages do not predict your own result.

Genuine supply matters for safety. Counterfeit medicine alerts and the Yellow Card scheme are covered by the MHRA.⁷

Finally, the liver is a common worry. Our guide on whether GLP-1 is bad for your liver deals with that specific question.

10 · 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