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Serious Side Effects

Learn about serious side effects of weight loss injections, including warning signs to watch for and when to seek medical advice.

Whether you are considering starting weight-loss injections or are already on treatment, this guide covers the serious side effects of Mounjaro and Wegovy that need urgent attention or specialist input — what they are, how to recognise them, and what to do. LetsLoseWeight is an independent comparison site; we do not prescribe medication. The information below is for context, not personal medical advice — if you suspect you are experiencing any of the symptoms below, contact a clinician promptly.

What "serious" means here

In medicines regulation, a "serious" side effect is one that is life-threatening, requires hospitalisation, causes lasting harm, or is otherwise medically significant. Most serious side effects of weight-loss injections are uncommon or rare by frequency — meaning they affect fewer than 1 in 100 patients in clinical trials — but they matter precisely because they are potentially severe.

For everyday side effects (nausea, diarrhoea, headache and so on), see our common side effects guide.

Acute pancreatitis

Inflammation of the pancreas is the serious side effect most associated with the GLP-1 medicine class. It is uncommon in clinical trials of Mounjaro and Wegovy — affecting up to 1 in 100 people — but warrants prompt attention because untreated pancreatitis can be life-threatening.

Symptoms

  • Severe upper-abdominal pain that comes on suddenly, often in the centre of the abdomen, frequently radiating to the back
  • Persistent vomiting, often without clear relief from eating or drinking
  • A bent-forward posture as patients often find this slightly more comfortable
  • Fever, fast heartbeat or sweating in more severe cases

What to do

If you have these symptoms while on Mounjaro or Wegovy, stop taking the medicine and seek urgent medical attention — call NHS 111 for advice or go to A&E if symptoms are severe. Mention that you take a weight-loss injection so the clinical team can prioritise the right blood tests (lipase and amylase).

Risk factors

Pancreatitis risk is higher in people with a previous history of pancreatitis, gallstone disease, heavy alcohol use, or very high triglyceride levels. Anyone with these factors should discuss them carefully with their prescriber before starting.

Gallbladder problems (gallstones and cholecystitis)

Both Mounjaro and Wegovy can cause gallbladder problems, partly through rapid weight loss and partly through direct drug effects. Gallstones (cholelithiasis) are listed as uncommon for Mounjaro and common for Wegovy. Inflammation of the gallbladder (cholecystitis) is uncommon for both.

Symptoms

  • Pain in the upper right abdomen — often coming in waves and lasting hours
  • Pain after fatty meals, sometimes radiating to the right shoulder or shoulder blade
  • Nausea or vomiting accompanying the pain
  • Yellowing of the skin or whites of the eyes (jaundice) — a more serious sign suggesting bile duct obstruction
  • Fever or chills with the pain — may indicate gallbladder infection

What to do

For ongoing right-upper-abdominal pain, see your GP. For severe pain with fever, jaundice or persistent vomiting, go to A&E — gallbladder infection or a stuck gallstone needs prompt treatment.

Severe allergic reactions

Severe allergic reactions to either medicine — anaphylaxis or angioedema — are rare (up to 1 in 10,000 people).

Symptoms

  • Rapid swelling of the face, lips, tongue or throat
  • Difficulty breathing or wheezing
  • Sudden severe rash or hives, especially with breathing or swallowing problems
  • Fast heartbeat, light-headedness, collapse

What to do

Call 999 immediately or go straight to A&E. Do not delay — anaphylaxis is a medical emergency. If a reaction has been confirmed, the medicine should not be restarted.

Medullary thyroid carcinoma and MEN 2

Both Mounjaro and Wegovy carry a boxed-style warning about thyroid C-cell tumours, including medullary thyroid carcinoma (MTC). In animal studies (rats and mice), tirzepatide and semaglutide both caused thyroid C-cell tumours. It is not known whether the same applies to humans, but the warning means:

  • The medicines must not be used in anyone with a personal or family history of MTC.
  • The medicines must not be used in anyone with multiple endocrine neoplasia syndrome type 2 (MEN 2) — a rare inherited condition.
  • Patients should report new neck symptoms promptly.

Symptoms to report

  • A lump or swelling in the neck that is new or growing
  • Persistent hoarseness
  • Difficulty swallowing
  • Shortness of breath without an obvious cause

What to do

If any of these develop, see your GP without waiting. Most lumps in the neck are not cancer, but ruling MTC out is a clinical decision, not a self-assessment one.

Severe vomiting, diarrhoea and dehydration

Severe gastrointestinal side effects are more common but can become serious when they cause dehydration and acute kidney injury. The Mounjaro and Wegovy SmPCs both list acute kidney injury as an uncommon side effect, almost always in the context of severe vomiting or diarrhoea.

Signs of dehydration

  • Very dark or scant urine, or no urine for several hours
  • Severe thirst, dry mouth
  • Dizziness on standing, light-headedness
  • Confusion or unusual sleepiness in older adults

What to do

If you cannot keep fluids down, contact NHS 111 or your GP urgently. Severe dehydration may need IV fluids in hospital.

Diabetic retinopathy worsening

In people with type 2 diabetes, rapid improvements in blood sugar — which both Mounjaro and Wegovy can cause — can sometimes worsen pre-existing diabetic eye disease in the short term. This was first observed in the SUSTAIN-6 trial of semaglutide in diabetes (Marso et al., NEJM 2016) and remains a recognised risk.

Symptoms

  • New or worsening blurred vision
  • Floaters, dark spots in vision, or sudden vision loss
  • Anything affecting central vision

What to do

Anyone with diabetic eye disease should arrange more frequent eye monitoring after starting weight-loss injections. Sudden vision changes warrant urgent attention — contact your GP, NHS 111 or an eye casualty service the same day.

Mental health concerns

The MHRA, EMA and FDA have all reviewed reports of suicidal thoughts and self-harm in people taking GLP-1 receptor agonists. Reviews so far have not found a causal link, but a small risk cannot be definitively ruled out and the regulators have asked patients and clinicians to remain alert.

What to do

Anyone experiencing low mood, suicidal thoughts or self-harm urges while on weight-loss injections should contact their GP, NHS 111 (option 2 for mental health), or the Samaritans on 116 123 (24/7, free) and discuss with their prescriber. If anyone is in immediate danger, call 999 or go to A&E.

Hypoglycaemia (low blood sugar)

For people with type 2 diabetes taking Mounjaro or Wegovy alongside insulin or a sulfonylurea, hypoglycaemia is common and can become serious. Symptoms include sweating, shaking, hunger, fast heartbeat, confusion, slurred speech and — in severe cases — loss of consciousness. Anyone in this group should have a clear hypo plan from their diabetes team and may need their other diabetes medicines adjusted.

Reporting serious side effects

In the UK, all suspected side effects to any medicine — including weight-loss injections — should be reported through the MHRA Yellow Card scheme. Reports of serious side effects are particularly valuable: they help identify safety signals that emerge only after a medicine is in widespread use.

Both Mounjaro and Wegovy carry a black triangle (▼) in the BNF and patient information leaflet, indicating additional safety monitoring is in place.

Frequently asked questions

How will I know if abdominal pain is "serious"?
Mild, settling abdominal discomfort is common and usually nothing to worry about. Serious pain tends to be: severe rather than mild, persistent (hours rather than minutes), often associated with vomiting that does not settle, and sometimes radiating from the centre of the abdomen to the back (pancreatitis) or to the right shoulder (gallbladder). Anything that worries you is worth a call to NHS 111.

Should I stop the medicine if I'm worried?
For severe symptoms — pancreatitis, allergic reaction, severe vomiting — yes, stop and seek urgent help. For more uncertain symptoms, contact your prescriber or NHS 111 first; they may advise pausing the medicine while you are assessed.

Are serious side effects more likely at higher doses?
The relationship is not as simple as "higher dose = higher risk for serious events." Some risks (gastrointestinal severity, dose-related kidney injury secondary to dehydration) do increase with dose. Others (pancreatitis, allergic reactions) are not strongly dose-dependent. The slow titration schedule reduces gastrointestinal severity but does not eliminate the rarer serious risks at any dose.

Can I restart the medicine after a serious side effect?
For some side effects (mild gallbladder symptoms that resolve without intervention), restarting may be possible after specialist advice. For others (anaphylaxis, MTC concerns, confirmed pancreatitis) the medicine should not be restarted. This is a clinical decision, not a patient one.

Next steps

Sources

This guide is for general information only and is not a substitute for professional medical advice. Symptoms suggesting a serious side effect need prompt clinical assessment — call NHS 111, contact your GP, or in emergencies call 999.

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Serious Side Effects of Weight Loss Injections UK | LetsLoseWeight