GLP-1 Side Effects: What to Expect and When to Act (October 2026)

01 · The core answer

What are the main GLP-1 side effects?

The main GLP-1 side effects are gut-related, and they are usually temporary.

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. Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

How you react depends on your dose, your diet and how quickly the dose goes up. Our side-effect comparison guide sets these effects against other options.

02 · Your decision

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

If you are thinking about starting a GLP-1 medicine, the side-effect picture is one factor in a wider decision. It sits alongside your health history, your goals and whether you meet the licensed criteria. 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. Individual results vary.

If you already take a GLP-1 medicine and have a new symptom, work out which group it falls into. A mild, early digestive symptom after a dose change is the expected pattern. Most side effects follow a pattern: they appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts.¹ A severe or unusual symptom is different: do not wait it out. Contact your clinical team, and get medical help straight away for any of the serious symptoms listed under serious side effects.

Practical next steps are simple. Keep a short daily note of symptoms, meals and dose day; our guide on how to track side effects shows how, and the free GLPTrackr companion app is built for exactly this. If a symptom is bothering you, our guide to managing side effects day to day goes step by step.

Ongoing monthly reviews, dose-titration and side-effect support from the prescribing team come with the service.

03 · Weighing it up

Is taking a GLP-1 worth the risk?

Whether it is worth it depends on your starting point, and only you and a prescriber can judge that. Side effects are common but mostly mild and short-lived. Serious ones are uncommon. Against that, in SURMOUNT-1, people on 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. These are trial averages, and your own result may differ.²

A sensible way to decide is to ask three questions. Do you meet the licensed criteria? Are you prepared for a few unpleasant weeks around each dose increase? Do you have conditions, medicines or a history that your prescriber needs to know about? Answer honestly in the consultation. A prescriber who knows your full history can spot problems before they start, which is the point of a clinical review rather than a scoring script.

04 · Nausea, vomiting, diarrhoea

Nausea, vomiting and diarrhoea: the biggest side effects

Nausea is the side effect people ask about most, and in the weight-management trials it was also the most reported. The biggest side effect of GLP-1 treatment is therefore usually nausea, with vomiting and diarrhoea close behind. These effects are linked to these medicines slowing how quickly the stomach empties.¹

The trial figures put numbers on it. 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. 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 matters as much as frequency. Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ That is why the gradual dose ladder exists, and why the first weeks after each step up are the ones to plan around. Many people find it easier to schedule the injection for a day when the following day is quiet.

Most people cope without stopping. Even so, a small minority do stop. 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 If you are struggling, tell your clinical team early. Options exist before stopping, including holding your current dose for longer.

What helps most people

Practical steps that help most people: 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.¹ These sound basic, but they target the cause. A slower-emptying stomach copes badly with large, rich meals, so a plate that once felt normal can now trigger nausea.

When it is more than a nuisance

If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹

Our wider guide to weight loss side effects covers how these symptoms fit into the whole treatment journey, and the GLP-1 side effects blog guide offers a plain-English companion read.

05 · Constipation, reflux, appetite

Constipation, reflux and reduced appetite

Constipation is the digestive effect that catches people out, because it sits alongside diarrhoea on the list of common problems. It 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.¹ Eating less means less bulk, and drinking less makes stools harder, so the two effects compound each other.

What helps: fluid intake first (aim for regular water through the day), fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative.¹ Contact your clinical team for pain, bloating or more than a few days without a bowel movement.¹

Indigestion, reflux and burping

Indigestion, reflux and burping are listed as common effects. The same habits that help nausea help here. Eat smaller portions, stop at the first feeling of fullness, favour plain lower-fat food while your body adjusts, and avoid lying down straight after eating.¹ If you already have acid reflux, mention it in your consultation so your prescriber can factor it in.

Reduced appetite: effect or purpose?

Reduced appetite appears on the common side-effect list, but it is also the way the medicine is designed to work. Tirzepatide activates the GIP and GLP-1 receptors, which help regulate appetite and slow how quickly the stomach empties. In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹

The risk is eating too little. When appetite drops sharply, it is easy to skip protein, fibre and fluids, and that feeds directly into constipation, fatigue, hair shedding and muscle loss. Think of a reduced appetite as a prompt to make each meal count rather than as permission to skip meals. If you cannot manage regular, balanced meals, tell your prescriber. A slower dose increase may suit you better.

For other forms of treatment, see our page on oral GLP-1 side effects.

Drinking enough is the single habit that ties most of these effects together. Water supports bowel movements, steadies dizziness, softens constipation and helps energy. Keeping a bottle with you and sipping through the day works better than trying to catch up in the evening, when a slow-emptying stomach is least comfortable.

06 · Fatigue, dizziness, hair loss

Fatigue, dizziness and hair shedding

Fatigue 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.¹ If you feel flat when you start, that is not a sign the treatment is failing, and it is not a sign that something is wrong with you. Your body is running on less food while it learns the new appetite signals.

What helps is keeping meals balanced (protein especially), staying hydrated, and keeping gentle activity going. For most people energy returns to normal within a few weeks. Tell your clinical team if tiredness is severe or not settling.¹ Short walks often lift energy better than resting, even when resting feels more tempting.

Dizziness

Dizziness is listed among the common effects. It often links to eating and drinking less, so check fluids first. Stand up slowly, and do not drive or use machinery if you feel lightheaded. If you take insulin or a sulfonylurea, dizziness, shakiness, sweating or confusion can signal low blood sugar. Your prescriber may need to adjust those medicines, so report it promptly.¹

Hair shedding

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. For most people it settles as weight stabilises.¹ Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹

Protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.¹ Crash dieting on top of a GLP-1 medicine is the combination most likely to make shedding more noticeable. If shedding is distressing or does not ease as your weight steadies, raise it at your monthly review.

Putting the energy picture together

These three effects share a theme: the body is adjusting to a large change in how much it eats. Three habits cover most of it. Eat protein at each meal, drink regularly, and keep moving. They will not remove every symptom, but they remove the avoidable ones. If you notice a pattern, such as tiredness always peaking two days after your injection, that is useful information for your prescriber and is easy to see in a symptom log.

Some people look to supplements to fill the gap. Our page on side effects of GLP-1 supplements explains why low-cost supplements with no clinical oversight are not a like-for-like alternative to a licensed, prescribed medicine, and why dosing is not standardised.

07 · Injection site and face

Injection-site reactions, "Ozempic face" and loose skin

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

Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹ Follow the instructions that come with the pen, and use the needles and sharps bin supplied with every order. Unopened pens stay in the fridge until first use; in-use pens are licensed for up to 30 days (not above 30°C) out of the fridge.

What does GLP-1 do to your face?

Nothing directly. "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.¹

Loose skin and muscle

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.¹

In practice that means three habits. Include a protein source at each meal, even when your appetite is small. Add resistance exercise, such as bodyweight work or weights, rather than relying on walking alone. And be wary of chasing the fastest possible loss. If your prescriber suggests holding a dose for longer, that is often to protect you, not to slow you down.

New or unlisted symptoms

If you notice a new or lasting symptom that is not on the patient leaflet list, tell your prescriber, and you can report suspected side effects through the MHRA Yellow Card scheme.¹⁹

For sexual and hormonal changes, see our guide to sexual side effects.

08 · Serious side effects

Serious side effects and when to seek help

Serious side effects are uncommon but need urgent attention, so 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. This could be pancreatitis. Seek immediate medical help also for pain in the upper right abdomen, fever or yellowing of skin or eyes, which could mean gallbladder problems. And seek immediate medical help for signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing.¹

Telling serious from ordinary

Ordinary digestive discomfort tends to come in waves, links to meals and eases over days. Warning-sign pain is different. It is severe, it does not ease, and it may spread to the back — get medical help straight away. If you are unsure, get medical help straight away; in an emergency call 999.

Low blood sugar with other medicines

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 This is not the same as saying the medicines cannot be combined. It means the doses need review, so tell your prescriber about everything you take, including over-the-counter products.

Is there anyone who shouldn't take a GLP-1?

Yes. 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. 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. 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.¹³ A prescriber looks at your health history, your current medicines and whether you meet the licensed criteria. Always share your full medical history and medicine list during consultation. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.

Reporting a side effect

You can report any suspected side effect through the MHRA Yellow Card scheme, whether or not it is already listed.¹⁹ Reports help regulators spot patterns across the whole country, and you can also tell your clinical team, who can support you with the next steps.

09 · Mood and mental health

Mood, mental health and stopping because of side effects

Regulators have looked specifically at whether GLP-1 medicines are linked to low mood or suicidal thoughts: the EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link, 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 mean mood is irrelevant. Any big change in weight, appetite and routine can affect how you feel. Eating socially may feel different, food may become less enjoyable, and body changes can bring mixed emotions. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷

How often do people stop because of side effects?

Most people continue. 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.¹ So a small minority stop, and most carry on once they adjust.

If you decide to stop

Talk to your clinical team first. Tirzepatide takes a while to leave your body, so symptoms do not vanish the day you stop. 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 Side effects may therefore linger for a while after your last injection.

10 · Timeline and dose ladder

When side effects start, how long they last and the dose ladder

GLP-1 side effects usually start within the first days after starting treatment or increasing a dose. They peak early and settle as your body adapts.¹

This pattern is one reason Mounjaro uses a gradual dose ladder. Treatment begins at 2.5mg; after at least 4 weeks the dose increases to 5mg, then can step up by 2.5mg at a minimum of 4-week intervals (7.5mg, 10mg, 12.5mg) to a maximum of 15mg weekly, depending on how you respond and tolerate it. The ladder exists so that your body meets each increase in turn rather than the full dose at once.

What if a dose is hard to tolerate?

If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up. Speak to your clinical team rather than pushing through severe symptoms.¹ Holding a dose is a normal, supervised decision, not a failure. Do not change your dose, skip steps or adjust pens yourself, because dose changes belong with your prescriber.

How much weight did people lose in SURMOUNT-1?

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 and individual results vary.

NHS or private?

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 with the highest-need groups first.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. Whichever route you take, the side-effect profile is the same medicine and the same advice applies.

If you buy privately, choose a registered pharmacy. Ours is GPhC-registered (9012878), supplies genuine KwikPens through official LillyDirect supply, and includes free next-day tracked delivery, same-day dispatch when ordered by 3pm, and ongoing monthly clinical reviews including side-effect support. For more on the medicines side by side, see our guide to side effects across GLP-1 medications.