Safety & Side Effects

Common Side Effects

Learn about the most common side effects of weight loss injections, including nausea, fatigue, and digestive symptoms, and what they mean.

Whether you are starting weight-loss injections for the first time or trying to make sense of symptoms a few weeks in, this guide covers the common side effects of GLP-1 and GLP-1/GIP weight-loss medicines used in the UK — Mounjaro and Wegovy — what they feel like, when they tend to peak, and how the patterns differ between medicines. LetsLoseWeight is an independent comparison site; we do not prescribe medication. The information below is general context, not personal medical advice.

What "common" means on a UK medicines licence

Side effects on UK and European licences are grouped into standardised frequency bands. The terms "very common" and "common" have specific meanings:

  • Very common — affects more than 1 in 10 people
  • Common — affects up to 1 in 10 people

Most side effects of weight-loss injections fall into one of these two categories, which is why the patient information leaflet and SmPC for both Mounjaro and Wegovy lead with a long list of digestive symptoms.

For dedicated lists by medicine, see our Mounjaro side effects guide and Wegovy side effects guide. This page covers what they have in common.

The dominant theme: gastrointestinal effects

Both Mounjaro and Wegovy slow gastric emptying and act on appetite signalling in the brain. The same mechanisms that make them effective for weight management are also why their most common side effects are digestive.

Nausea

Nausea is the most frequently reported side effect of both medicines. It tends to be:

  • Most pronounced in the first 1–2 weeks after starting or stepping up the dose
  • Worse after large or fatty meals — slowed gastric emptying means food sits longer in the stomach
  • Variable in intensity — many people describe a low-level queasiness rather than vomiting

Nausea typically settles as the body adjusts. Practical strategies are covered in our reduce side effects guide.

Diarrhoea, constipation and other GI symptoms

Both ends of the gastrointestinal spectrum are common:

  • Diarrhoea — often most noticeable in the first couple of weeks after a dose increase.
  • Constipation — sometimes more persistent than diarrhoea, particularly if appetite reduction has cut fluid and fibre intake.
  • Indigestion (dyspepsia), heartburn, bloating, burping — all listed as common on both Mounjaro and Wegovy SmPCs.
  • Reduced appetite — listed as a side effect even though it is partly the medicine's intended effect.
  • Abdominal pain — usually mild and short-lived, but persistent or severe pain should be reported to a clinician (it can be an early sign of pancreatitis or gallbladder problems).

Tiredness, dizziness and headache

Beyond the digestive system, the most commonly reported non-GI side effects are:

  • Fatigue — particularly in the first few weeks. Often worsened by reduced calorie intake.
  • Dizziness — usually mild. May relate partly to reduced food intake.
  • Headache — listed as very common in the Wegovy SmPC and common in the Mounjaro SmPC.

These tend to settle as the body adjusts and as eating patterns stabilise on the new medicine.

Hair loss

Both medicines list alopecia (hair loss) as a common side effect. It is usually:

  • Temporary — resolves once weight stabilises.
  • More likely with rapid weight loss — rather than a direct drug effect on hair follicles.
  • Helped by adequate protein and iron intake, and by managing the pace of weight loss.

If hair shedding is significant or distressing, mention it to your prescriber.

Injection-site reactions

Most people will have at least one mild injection-site reaction over the course of treatment:

  • Redness or mild irritation at the injection site
  • Itching, light bruising
  • Small lumps that resolve over a few days

The licensed injection sites are the abdomen, the front of the thigh, and the back of the upper arm. Rotating between sites — and within each area — significantly reduces local reactions. See our how to inject guide for technique.

Differences between Mounjaro and Wegovy

The medicines have broadly similar common side-effect profiles, but a few differences are worth noting:

Mounjaro Wegovy
Headache Common Very common
Hair loss Common Common
Gallstones Uncommon Common
Hypersensitivity (skin) reactions Common Uncommon
Oral contraceptive interaction Specific warning in SmPC Not specifically warned

Source: Mounjaro SmPC and Wegovy SmPC on the EMC.

The SURMOUNT-5 head-to-head trial in 2025 found broadly comparable rates of gastrointestinal side effects between the two medicines, with some differences in severity at the highest doses (Aronne et al., NEJM 2025). For most patients, tolerability between the two is closer than the headline weight-loss differences would suggest.

What "common" doesn't mean

Common does not mean inevitable. The frequency categories indicate the proportion of people who reported a given side effect in clinical trials — not that every patient will experience every common symptom.

It also does not mean trivial. A symptom listed as "common" can still be severe enough in an individual case to require dose adjustment or stopping treatment. Listen to your body and talk to your prescriber if a side effect is more than mildly inconvenient.

How long do common side effects last?

For most people, gastrointestinal side effects:

  • Peak in the first 1–2 weeks after starting or after each dose increase
  • Settle within 2–4 weeks at a stable dose
  • Recur briefly after a dose step-up, then settle again

If side effects are still significant after 3–4 weeks at a stable dose, it is worth a conversation with your prescriber. Options include staying at the current dose for longer before next step-up, or going back down to the previous dose level.

When common side effects need attention

Most common side effects can be managed with practical adjustments. Contact your GP or NHS 111 if:

  • Vomiting or diarrhoea is severe or prevents you keeping fluids down
  • Abdominal pain is severe or does not settle
  • You have signs of dehydration: dry mouth, dizziness on standing, very dark or scant urine

Call 999 or go to A&E for severe upper-abdominal pain that radiates to the back (signs of pancreatitis), rapid swelling of the face or throat, or breathing difficulty.

For the rarer but more serious side effects, see our serious side effects guide.

Reporting side effects

In the UK, suspected side effects to any medicine — including weight-loss injections — can be reported through the MHRA Yellow Card scheme. Anyone can report. Both Mounjaro and Wegovy carry a black triangle (▼) in the BNF and patient information leaflet, indicating they are under additional safety monitoring; patients are encouraged to report any suspected side effect, however minor, while monitoring is in place.

Frequently asked questions

Will I definitely get nausea?
No. While nausea is the most commonly reported side effect, many people experience it only mildly or not at all. Slow titration and dietary adjustments reduce both the likelihood and severity.

Are common side effects worse at higher doses?
For most people, yes — each dose step-up tends to bring a brief return of mild gastrointestinal symptoms. The 4-week interval between dose increases (longer if needed) is specifically designed to give the body time to adjust before facing more medication.

Can I take anti-nausea medicine alongside?
Standard anti-emetics (such as cyclizine, prochlorperazine or ondansetron) can sometimes be used short-term, but only on prescriber advice. Some interact with other medicines or are inappropriate for certain groups. Don't self-medicate without checking with a clinician.

Are common side effects dangerous?
Most are uncomfortable but not dangerous in themselves. The main risks come indirectly — for example, severe vomiting or diarrhoea can cause dehydration, which can affect kidney function. That is why persistent severe symptoms warrant medical attention.

Next steps

Sources

This guide is for general information only and is not a substitute for professional medical advice. If you experience symptoms after taking weight-loss injections, talk to your prescriber, your GP, or call NHS 111.

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Common Side Effects of Weight Loss Injections UK | LetsLoseWeight