Safety & Side Effects

Safety & Side Effects

Learn about the safety of weight loss injections, including side effects, risks, and how to manage them when using Mounjaro or Wegovy.

Whether you are doing your homework before starting a weight-loss injection or already on treatment and looking for guidance you can trust, this is the safety and side-effects hub for Mounjaro, Wegovy and the wider GLP-1 medicine class in the UK. Each section below summarises an aspect of safety and links to a deeper guide. LetsLoseWeight is an independent comparison site; we do not prescribe medication, and the information here is for context, not personal medical advice.

Are these medicines safe?

Mounjaro and Wegovy are both prescription-only medicines licensed in the UK by the Medicines and Healthcare products Regulatory Agency (MHRA). They have been studied in large randomised controlled trials (the SURMOUNT and STEP programmes) and are recommended by NICE for use within the NHS under TA1026 (tirzepatide) and TA875 (semaglutide).

Safety, in medical terms, is a balance: do the expected benefits outweigh the expected risks for the person taking the medicine? For someone meeting the eligibility criteria, the trial evidence and regulatory reviews suggest that they do — provided the medicine is used through a regulated UK prescriber and supply chain, and provided patients are aware of the side effects to watch for.

For a fuller answer including risk in context, see our are weight loss injections safe? guide.

Common side effects

Most side effects of Mounjaro and Wegovy are gastrointestinal — nausea, diarrhoea, constipation, indigestion, bloating, reflux — and most settle within a few weeks as the body adjusts. Headache, fatigue, dizziness, hair loss, and injection-site reactions are also common.

For the full list, frequencies, and what to do, see our common side effects guide.

Serious side effects

Serious side effects are uncommon or rare, but warrant urgent attention when they happen:

  • Acute pancreatitis — severe upper-abdominal pain radiating to the back, with vomiting
  • Gallbladder problems — gallstones, cholecystitis
  • Severe allergic reactions — face/throat swelling, breathing difficulty (call 999)
  • Diabetic retinopathy worsening in people with type 2 diabetes
  • Acute kidney injury secondary to severe dehydration

Both medicines also carry a boxed-style warning about thyroid C-cell tumours and medullary thyroid carcinoma (MTC) — they must not be used by anyone with a personal or family history of MTC, or with multiple endocrine neoplasia syndrome type 2 (MEN 2).

For symptoms, what to do, and when to seek help, see our serious side effects guide.

Practical strategies for managing side effects

Most common side effects can be reduced with simple practical changes — smaller, more frequent meals, careful food choices, hydration, slow titration, and good injection technique. Hair loss and fatigue often respond to attention to protein, iron and weight-loss pace.

For diet, timing and injection-technique tips, see our reduce side effects guide.

Per-medication side-effect profiles

The two licensed medicines are similar but not identical. Some side effects are more common with one than the other:

  • Mounjaro (tirzepatide) — slightly higher rate of skin hypersensitivity reactions; specific SmPC warning about reduced absorption of oral contraceptives. See our Mounjaro side effects guide.
  • Wegovy (semaglutide) — higher rate of headache (very common rather than common); higher rate of gallstones (common rather than uncommon). See our Wegovy side effects guide.

For an investigational medicine still in trials, see our retatrutide side effects guide.

Eligibility, contraindications and drug interactions

Some people should not take Mounjaro or Wegovy regardless of how the side-effect profile looks. Contraindications include personal or family history of medullary thyroid carcinoma, MEN 2, hypersensitivity to the active ingredient, and pregnancy. Cautions apply for type 1 diabetes, severe gastrointestinal disease, history of pancreatitis, diabetic retinopathy, and significant kidney or liver impairment.

For the full list and what UK prescribers usually check, see our eligibility guide.

Pregnancy and breastfeeding

Neither medicine is licensed in pregnancy or while breastfeeding. The Mounjaro SmPC advises stopping at least one month before a planned pregnancy; the Wegovy SmPC advises at least two months because semaglutide has a longer half-life. Anyone who becomes pregnant while taking either medicine should stop and contact their prescriber.

Mental health

The MHRA, EMA and FDA have all reviewed reports of suicidal thoughts and self-harm in people taking GLP-1 receptor agonists. The most recent reviews have not found a causal link, but a small risk cannot be definitively ruled out. 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 (free, 24/7), and discuss with their prescriber.

Counterfeit and unregulated supply

Most safety incidents in the UK media around weight-loss injections relate not to side effects of the licensed medicines but to counterfeit or unregulated supply: products bought from social media sellers, foreign websites, or beauty clinics outside the regulated supply chain. The MHRA has issued public warnings about these.

A regulated UK pharmacy is registered with the General Pharmaceutical Council (GPhC), uses a registered prescriber, and supplies medicines through licensed wholesalers. Anyone buying outside this chain is taking on counterfeit risk, no clinical safety net, and no insurance protection.

Reporting side effects (Yellow Card)

In the UK, suspected side effects to any medicine — common or serious — can be reported through the MHRA Yellow Card scheme. Anyone can report: patients, parents, carers, and healthcare professionals. Reports help the MHRA identify new safety signals and update prescribing guidance. Both Mounjaro and Wegovy carry a black triangle (▼) in the BNF and patient information leaflet, indicating they are under additional safety monitoring.

When to seek help

Call 999 or go to A&E for:

  • Severe upper-abdominal pain radiating to the back, with persistent vomiting (signs of pancreatitis)
  • Rapid swelling of the face, lips, tongue or throat, or breathing difficulty
  • Severe allergic skin reactions
  • Suicidal thoughts with intent

Contact your GP or NHS 111 for:

  • Severe or persistent vomiting or diarrhoea preventing fluid intake
  • Severe abdominal pain that does not settle
  • Yellow skin or whites of the eyes (jaundice)
  • New neck lump, hoarseness, or trouble swallowing
  • Significant changes in vision (especially in type 2 diabetes)
  • Persistent low mood, anxiety or thoughts of self-harm

For uncertain symptoms, NHS 111 is a fast route to a clinical opinion.

Frequently asked questions

Are weight-loss injections safe long-term?
The longest published trials run to several years and have not surfaced unexpected long-term safety concerns. Post-marketing surveillance through the Yellow Card scheme and regulatory reviews continues. As a class, GLP-1 medicines have been used since the late 2000s in diabetes; the weight-management indication is newer.

Are private weight-loss injections as safe as NHS ones?
The medicine itself is identical — same manufacturer, same regulatory licence, same supply chain. What differs is the wraparound: NHS access through specialist services typically includes structured monitoring; private services vary in the depth of follow-up. Choose a regulated provider with proper aftercare regardless of route.

What's the safest way to start?
Through a clinical assessment with a regulated UK prescriber, with the lowest starter dose, slow titration, and a clear plan for what to do if side effects develop. Buying outside the regulated supply chain is the single biggest avoidable safety risk in this category.

Next steps

Use the links above to dive into any specific area, or:

Sources

This guide is for general information only and is not a substitute for professional medical advice. Speak to a qualified prescriber about whether weight-loss injections are suitable for you.

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Safety and Side Effects of Weight Loss Injections UK | LetsLoseWeight