GLP-1 Negative Side Effects Explained (October 2026)

01 · The core answer

What are the downsides of GLP-1 drugs?

The main downside is a run of digestive side effects, mostly in the early weeks and after each dose increase. Like all medicines, Mounjaro can cause side effects, and most are digestive.¹ Other downsides are less frequent: hair shedding, tiredness, injection-site reactions, and rare serious problems that need urgent care.

The pattern matters as much as the list. Most side effects appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts.¹³ That is one reason Mounjaro uses a gradual dose ladder. 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.¹³

For a broader view across the medicine class, see our overview of the negative side effects of weight loss drugs.

02 · Your next step

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

It is relevant if you are considering a GLP-1 medicine, or have just started one and want to know what is normal. The sensible next step is to match these side effects against your own health history, then talk it through with a prescriber who can see your full picture.

Related guides: Bad side effects of mounjaro and Mounjaro ocular side effects.

03 · Common digestive effects

Common digestive side effects

Digestive effects are the headline negative of GLP-1 treatment, and they follow directly from how the medicine works. Tirzepatide activates the GIP and GLP-1 receptors, which help regulate appetite and slow how quickly the stomach empties.¹

The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹ Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

Practical steps that help most people are simple. 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.¹³ If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹³

Constipation deserves its own mention because it is so common. Constipation is very common on Mounjaro for weight management (it may affect more than 1 in 10 people), because food moves through the gut more slowly and many people eat and drink less.¹³ Fluids come first, then fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.¹³

04 · What the trials found

How often do side effects happen in trials?

Trial figures show that stomach effects are frequent but that few people stop because of them. 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 Diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹

Timing is the encouraging part. Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ That matches what the dose ladder is designed to do: give your body time to adapt at each step.

On stopping, 1.9% of people on 5mg, 3.3% on 10mg and 4.3% on 15mg stopped treatment permanently because of gastrointestinal side effects, compared with 0.5% on placebo.¹ Across all side effects, in SURMOUNT-1, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo.²

Read these as trial averages, not predictions for you. These are trial averages and individual results vary. For a closer look at how people describe the experience, see our page on reported Mounjaro side effects.

05 · Serious side effects

Serious side effects and when to seek help

Serious side effects are uncommon but need urgent attention, so it pays to know the warning signs before you start. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis).¹³

Seek immediate medical help for pain in the upper right abdomen, fever or yellowing of skin or eyes (possible gallbladder problems).¹³ And get medical help straight away for signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing.¹³

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.¹³ This is an adjustment, not an automatic ban on using the medicines together.

If you think a medicine has caused a side effect, you can report it through the MHRA Yellow Card scheme.¹⁹ Reports help regulators spot patterns, and you do not need to be certain of the cause to make one.

06 · Hair, face and skin

Hair shedding, face and loose skin

Hair shedding, a thinner-looking face and loose skin are real concerns, but they largely reflect fast weight loss rather than a direct harm from the drug. Temporary hair shedding is a recognised side effect, reported as common in trials and linked to rapid weight loss itself rather than a direct drug effect.¹³ Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹

For most people it settles as weight stabilises. Protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.¹³

"Ozempic face" is not a drug effect. It is what losing a significant amount of weight quickly can do to facial volume, on any method including diet or surgery.¹³ The same applies to loose skin. The counter-measures are the ones that protect lean mass: adequate protein, resistance exercise, and a loss pace guided by your prescriber rather than maximum speed. Skin commonly adapts gradually after weight stabilises.¹³

If you want to slow things down, tell your prescriber. Holding a dose for longer is a normal clinical option, not a failure.

07 · Fatigue and injection site

Fatigue and injection-site reactions

Tiredness 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. Keep meals balanced (protein especially), stay hydrated, and keep gentle activity going.¹³ For most people energy returns to normal within a few weeks; tell your clinical team if tiredness is severe or not settling.¹³

Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹ Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days.¹³

Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging.¹³ Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹³ Needles, a sharps bin and injection guidance come with every order we dispatch.

08 · Mood and mental health

Mood and mental health

Regulators have looked at this directly and have not found a causal link. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link between GLP-1 medicines and low mood or suicidal thoughts, and the MHRA keeps this under continuous review.¹⁷

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

That does not make the topic irrelevant. Any big change in weight, appetite and routine can affect how you feel. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷

Food is social, emotional and habitual. Eating far less can feel strange, even when it is the intended effect, and some people find that adjustment harder than the physical side effects. Mentioning it at a monthly review is exactly what the review is for.

09 · Medicines and who should not use

Who should not use GLP-1, and which medicines to watch

This is a decision for a prescriber, not a checklist you can complete alone. Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber, and it is used alongside a reduced-calorie diet and increased physical activity.¹ In the UK it is licensed for weight management in adults and for type 2 diabetes.¹

You should not take Mounjaro if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive; stop it at least one month before 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. Tell your prescriber about every condition and medicine so the review is complete.

On interactions, two are worth knowing. Insulin and sulfonylureas can cause low blood sugar alongside Mounjaro, so doses may need adjusting rather than avoiding the combination.¹³ And the medicine slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed.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 Do not add other weight-loss products on your own; check with a pharmacist first.

For NHS access, treatment is being phased in over several years, and NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition.⁴ The NHS primary-care rollout began in June 2025 with the highest-need groups first.⁶

10 · Stopping treatment

What happens when you stop?

Stopping is allowed at any time, and the medicine leaves your system gradually. Tirzepatide takes roughly four to five weeks to clear the system after a final dose; there is no way to speed this up, and nothing needs doing while it clears.¹ 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 Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load.

Because it clears slowly, appetite and side effects may not change overnight. Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

People stop for many reasons: tolerability, cost, reaching a goal, or personal circumstances. Public stories about individual celebrities rarely tell you why someone stopped, and they are no guide to your own experience. The trial data show that a small minority stop because of side effects, and if a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up.

11 · Myths and internet claims

Ozempic smell, breasts and teeth

"Ozempic breasts", "Ozempic teeth" and a supposed medicine smell are not listed side effects in the Mounjaro patient leaflet; tell your prescriber about any new or lasting symptom.¹³

Changes in body shape during major weight loss are a general feature of losing weight quickly by any method, as with facial volume and loose skin.¹³ If you notice a new symptom of any kind, discuss it with your prescriber, and your dentist if it concerns your mouth, and consider reporting it via the Yellow Card scheme.¹⁹

12 · What I wish I knew

What to know before starting

A few practical points make the first months easier. First, the early weeks are usually the hardest, and the dose ladder exists so you adapt gradually. Second, how you eat matters: small, slow, plainer meals reduce stomach symptoms.¹³

Third, protein and fluids matter more than most people expect, both for energy and for protecting lean mass. Fourth, you can ask to hold a dose if a step up is rough, and your clinical team would rather hear from you than have you push through severe symptoms.¹³

Finally, treat it as a medicine with trade-offs. The trial results are substantial, but so are the digestive effects for some people. A good prescriber helps you weigh both. You can also track doses and symptoms in the free GLPTrackr companion app, which helps when you talk to your prescriber at monthly reviews.