Mounjaro and Wegovy have overlapping side effects because both act on incretin pathways involved in appetite, fullness, blood sugar and gastric emptying. Mounjaro contains tirzepatide, a GIP and GLP-1 receptor agonist. Wegovy contains semaglutide, a GLP-1 receptor agonist. The UK patient leaflets for both medicines list digestive symptoms as the main patient-facing side-effect group. [1][3]
That overlap does not make the medicines interchangeable. Mounjaro and Wegovy have different active ingredients, product information, dose schedules and safety considerations. A comparison article should therefore do two jobs: answer which side effects are common, and separate normal adjustment symptoms from signs that need medical advice or urgent care.
Note: If a symptom is severe, new, persistent, or stops you eating and drinking normally, do not treat it as something to simply push through.
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Side-effect area
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Mounjaro (tirzepatide)
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Wegovy (semaglutide)
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Very common digestive effects
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Nausea, diarrhoea, vomiting, abdominal pain and constipation are listed as very common in UK patient information.
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Headache, nausea, vomiting, diarrhoea, constipation, stomach pain and feeling weak or tired are listed as very common in UK patient information.
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Other digestive effects
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Indigestion, bloating, burping, gas, reflux or heartburn, gallstones and delayed gastric emptying can occur.
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Indigestion, burping, gas, bloating, gastritis, reflux or heartburn, gallstones and delayed gastric emptying can occur.
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Dizziness and blood pressure
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Dizziness and low blood pressure are listed in weight-management patients.
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Dizziness is common; low blood pressure and lightheadedness on standing are uncommon.
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Heart rate
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Fast pulse is listed; it is common when used for type 2 diabetes and uncommon in weight management.
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Fast heartbeat is listed as an uncommon side effect; the SmPC also describes increased heart rate in clinical trials.
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Gallbladder and pancreas
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Gallstones and cholecystitis are listed; acute pancreatitis is uncommon but potentially serious.
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Gallstones are listed; acute pancreatitis is uncommon but potentially serious.
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Low blood sugar
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Most relevant in people using insulin or sulphonylureas, especially for diabetes treatment.
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Most relevant in people with diabetes using insulin or sulphonylureas.
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Vision
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Diabetic retinopathy or macular oedema should be raised before use; eye problems need medical discussion.
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Diabetic retinopathy warning applies in diabetes; semaglutide also has a very rare NAION eyesight warning.
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Mental health
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Not established by MHRA as a causal GLP-1 effect, but mood changes or self-harm thoughts still need prompt support.
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Not established by MHRA as a causal GLP-1 effect, but mood changes or self-harm thoughts still need prompt support.
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Pregnancy and contraception
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Do not use during pregnancy. The pill may be less reliable around starting and dose increases in people with obesity or overweight.
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Do not use during pregnancy. Stop at least 2 months before trying to become pregnant, according to UK patient information.
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Seek urgent medical help if you develop severe, persistent stomach or upper abdominal pain, especially if it spreads to your back or comes with vomiting. This can be a sign of acute pancreatitis. Also get urgent advice for yellowing of the skin or whites of the eyes, dark urine, pale stools, fever or chills with upper abdominal pain, sudden loss of vision or rapidly worsening eyesight, severe allergic symptoms, or thoughts that you may harm yourself.
This is a specific Mounjaro warning, not a minor footnote. If you use an oral contraceptive and have obesity or overweight, UK product information advises using a barrier method, such as condoms, or switching to a non-oral contraceptive method for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.
Mental health and mood changes: The MHRA has concluded that current evidence does not support a causal association between GLP-1 medicines and depression, suicidal ideation or suicide. It also says it will continue to monitor severe psychiatric reactions. That should not be read as a reason to ignore mood changes. If you notice new or worsening depression, anxiety, unusual behaviour, or thoughts of self-harm, contact your prescriber promptly. If you cannot keep yourself safe, call 999, go to A&E, or use NHS 111 mental health crisis support. [5][9]
Why side effects often happen when starting or increasing dose
Both medicines are introduced gradually because gastrointestinal side effects are more likely while the body adjusts. The Mounjaro patient leaflet says nausea, diarrhoea and vomiting are most common when first starting tirzepatide and usually decrease over time in most patients. The Wegovy SmPC notes that several gastrointestinal effects are mainly seen during dose escalation. [1][4]
This is why people often search for side effects after their first injection or after moving up a dose, such as Mounjaro 7.5 mg. A flare after a dose increase can happen, but that does not mean the right answer is to adjust the medicine yourself. Dose changes, pauses, slower escalation, switching or stopping should be handled by the prescriber.
Common digestive side effects
The practical day-to-day side effects that patients most often notice are usually digestive: nausea, diarrhoea, constipation, vomiting, stomach pain, indigestion, reflux, heartburn, burping, bloating and gas. These are not trivial if they affect hydration, nutrition, work, sleep or quality of life.
- What may help with nausea, indigestion, heartburn and burping
- Eat smaller meals and stop before you feel uncomfortably full.
- Avoid very rich, greasy, spicy or large meals if they trigger nausea, reflux, burping or diarrhoea.
- Sip fluids across the day, especially if you have vomiting or diarrhoea.
- Avoid eating too little. Very low intake can worsen nausea, dizziness, tiredness and constipation.
- Stay upright after meals if reflux or heartburn is a problem.
- Ask a pharmacist or prescriber before adding medicines for constipation, diarrhoea, indigestion or reflux, especially if symptoms are persistent.
- Contact your prescriber if symptoms keep returning after each dose, are getting worse, or stop you eating or drinking normally.
For common digestive symptoms, the aim is not to suppress every mild symptom at all costs. It is to stay hydrated, maintain enough nutrition, reduce obvious triggers and recognise when symptoms have moved beyond normal adjustment.
Side effects that need clearer action
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Warning sign
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Why it matters
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Action
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Severe, persistent abdominal pain, especially if it spreads to the back or comes with nausea or vomiting
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Can indicate acute pancreatitis, which is uncommon but potentially life-threatening.
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Seek urgent medical help. Do not wait for the next routine review.
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Upper abdominal pain, pain under the right ribs, fever, chills, jaundice, dark urine, pale or clay-coloured stools, itching or feeling very unwell
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Can suggest gallbladder or bile-duct complications. Gallstones are listed for both medicines, and rapid weight loss can also increase gallstone risk.
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Get same-day medical advice or urgent care depending on severity.
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Repeated vomiting or diarrhoea, inability to keep fluids down, very little urine, marked weakness, or dizziness on standing
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Digestive side effects can cause dehydration and may worsen kidney problems.
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Get same-day medical advice; urgent care if severe.
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Swelling of the face, lips, tongue or throat; breathing problems; wheezing; rash; feeling faint; rapid swelling
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May indicate a severe allergic reaction.
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Seek emergency medical help.
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Fast pulse, palpitations, chest pain, fainting, or a heart rate change that feels new or persistent
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Heart rate changes are listed separately for both medicines and should not be treated only as an allergy symptom.
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Contact a clinician promptly; seek urgent help for chest pain, fainting or severe symptoms.
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Sudden vision loss or rapidly worsening eyesight, especially with Wegovy/semaglutide
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The MHRA has issued a semaglutide warning for very rare NAION, a serious eye condition.
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Urgently attend eye casualty or A&E if advised/needed.
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New or worsening depression, unusual behaviour, thoughts of self-harm, or feeling unable to stay safe
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A causal link has not been established by MHRA, but these symptoms are always safety-critical.
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Contact your prescriber or urgent mental health support. Call 999 or go to A&E if you cannot stay safe.
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Cold sweat, shaking, confusion, weakness, drowsiness, hunger, fast heartbeat or difficulty concentrating in someone using insulin or sulphonylureas
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May indicate low blood sugar, especially in people with diabetes using certain medicines.
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Follow your diabetes plan and seek urgent help if severe or not improving.
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Mounjaro side effects in more detail
Mounjaro's UK patient leaflet lists nausea, diarrhoea, vomiting, abdominal pain and constipation as very common. It also lists indigestion, bloating, burping, gas, reflux or heartburn, dizziness in weight-management patients, low blood pressure in weight-management patients, fatigue, injection-site reactions, hair loss in weight-management patients, fast pulse and gallstones. [1]
The same leaflet includes serious warnings for acute pancreatitis and severe allergic reactions. The SmPC also lists delayed gastric emptying, cholecystitis and heart rate increased in its adverse-reaction table. For people using oral contraceptives, the contraception warning is one of the most important Mounjaro-specific differences to raise before treatment starts and whenever the dose changes. [1][2]
Wegovy side effects in more detail
Wegovy's UK patient leaflet lists headache, nausea, vomiting, diarrhoea, constipation, stomach pain and feeling weak or tired as very common. It also lists dizziness, indigestion, burping, gas, bloating, gastritis, reflux or heartburn, gallstones, hair loss, injection-site reactions, taste changes and low blood sugar in patients with diabetes as common. [3]
Wegovy also needs clear signposting for less common but more serious issues: acute pancreatitis, complications of diabetic eye disease, kidney or bladder stones, severe allergic reactions, bowel obstruction, increased bilirubin with jaundice, fast heartbeat and the very rare semaglutide NAION warning for sudden or rapidly worsening eyesight. [3][4][7]
Is Mounjaro safer than Wegovy?
It is not accurate to give a simple public ranking of Mounjaro as safer than Wegovy, or Wegovy as safer than Mounjaro. They are different medicines with overlapping but not identical side-effect profiles. The right comparison depends on the patient's medical history, current medicines, diabetes status, contraception needs, previous pancreatitis, gallbladder history, eye disease, kidney problems, mental health history, and how they respond during dose escalation.
For example, someone using the pill needs the Mounjaro contraceptive warning. Someone with diabetes and eye disease needs careful discussion around blood sugar changes and eye symptoms. Someone with persistent upper abdominal pain, jaundice or repeated vomiting needs clinical assessment, not a generic side-effect comparison.
Can you take Mounjaro and Wegovy together?
No. Mounjaro and Wegovy should not be combined by the patient. Wegovy's UK patient leaflet says it should not be used in combination with other medicines that contain GLP-1 receptor agonists. The MHRA also advises people to consult a healthcare professional before switching between GLP-1 medicines because they vary in strength, use and suitability. [3][5]
Should you stop, switch or reduce the dose if side effects happen?
Do not change the dose, delay escalation, restart after a break, stop treatment or switch medicines without prescriber advice. The exception is when symptoms require urgent care, such as severe persistent abdominal pain, severe allergic symptoms, sudden vision loss, serious dehydration, or a mental health emergency. In those situations, the priority is urgent assessment, not waiting for a routine message thread.
