Key points
- Gastrointestinal side effects (nausea, vomiting, diarrhoea) are the most common and typically improve over time
- Gradual dose escalation is designed to minimise side effects
- Most side effects are mild to moderate and do not require stopping treatment
- Serious side effects are rare but include pancreatitis and gallbladder disease
- Always report persistent or concerning symptoms to your prescriber
Understanding GLP-1 side effects
GLP-1 receptor agonists, including semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro), work by mimicking gut hormones that regulate appetite and blood sugar. Because the GLP-1 receptor is expressed throughout the gastrointestinal tract and central nervous system, side effects most commonly involve the digestive system.
Clinical trial data and post-marketing surveillance reported to the MHRA provide a detailed picture of what side effects occur, how frequently they arise and how they can be managed. This guide covers all three major GLP-1 medications available in the UK.
Very common side effects (more than 1 in 10 people)
These are the most frequently reported adverse effects across all GLP-1 medications. They are especially common during the dose-escalation phase and typically become less severe over time.
Nausea
Nausea is the single most commonly reported side effect of GLP-1 medications. In clinical trials, it was reported by up to 44 per cent of participants taking Wegovy and approximately 30 per cent of those taking Mounjaro. It tends to peak during the first few weeks at each new dose level and gradually subsides.
Management tips for nausea:
Eat smaller, more frequent meals rather than large portions. Avoid fatty, greasy or heavily spiced foods. Eat slowly and stop eating when you feel full. Stay well hydrated with small sips of water throughout the day. Some patients find that ginger tea or peppermint helps. If nausea is severe, your prescriber may slow the dose escalation or prescribe an anti-emetic.
Diarrhoea
Diarrhoea is reported by approximately 15 to 30 per cent of patients. It is usually mild and self-limiting, resolving within the first few weeks of treatment. Staying well hydrated is important. If diarrhoea is persistent or severe, consult your prescriber, as dehydration can lead to other complications.
Common side effects (up to 1 in 10 people)
Vomiting
Vomiting occurs in approximately 10 to 25 per cent of patients, depending on the medication and dose. Like nausea, it is most common during dose escalation. If vomiting is frequent or severe, it is important to maintain hydration and contact your prescriber, as persistent vomiting can lead to dehydration and electrolyte imbalances.
Constipation
Constipation is reported by 10 to 15 per cent of patients. GLP-1 medications slow gastric emptying, which can also affect bowel transit time. Increasing fibre intake, drinking plenty of fluids and maintaining physical activity can help. Over-the-counter laxatives may be used if needed, after consulting with your pharmacist.
Abdominal pain and discomfort
Abdominal pain, bloating and dyspepsia are reported by up to 10 per cent of patients. These symptoms are usually mild and related to the slowing of gastric emptying. Eating smaller meals and avoiding foods that trigger symptoms can provide relief.
Headache
Headaches are reported by approximately 10 to 14 per cent of patients in clinical trials. They are usually mild and can be managed with standard over-the-counter analgesics such as paracetamol. Adequate hydration may also help.
Fatigue
Some patients experience tiredness or low energy, particularly during the initial weeks of treatment. This may be partly related to reduced caloric intake. Ensuring adequate nutrition and sleep, and maintaining a balanced diet despite reduced appetite, is advisable.
Injection site reactions
Mild reactions at the injection site, including redness, swelling, itching or bruising, occur in a small proportion of patients. Rotating injection sites between the abdomen, thigh and upper arm can help reduce this. Severe injection site reactions are rare.
Decreased appetite
Reduced appetite is a primary therapeutic effect of GLP-1 medications but is listed as a side effect because, in some patients, appetite suppression can be more pronounced than desired. This can lead to excessively rapid weight loss or nutritional deficiency if dietary intake falls too low. If you find yourself unable to eat adequate portions, discuss this with your prescriber.
Uncommon side effects (up to 1 in 100 people)
Dizziness
Dizziness may occur, particularly in patients also taking blood-pressure-lowering medications. As weight loss progresses, blood pressure often decreases, which may necessitate adjustment of antihypertensive medications.
Taste disturbances
Some patients report changes in taste perception (dysgeusia) while taking GLP-1 medications. This is usually temporary and resolves without specific treatment.
Hair thinning
Hair thinning (telogen effluvium) has been reported by some patients, particularly those experiencing rapid weight loss. This is not unique to GLP-1 medications and can occur with any significant caloric deficit. Ensuring adequate protein and nutrient intake may help minimise this effect. Hair growth typically recovers once weight stabilises.
Increased heart rate
A modest increase in resting heart rate (typically 2 to 4 beats per minute) has been observed in clinical trials of GLP-1 receptor agonists. The clinical significance of this finding is uncertain, and it has not been associated with increased cardiovascular risk. The SELECT trial for semaglutide actually demonstrated cardiovascular benefit.
Serious side effects (rare)
While rare, some side effects of GLP-1 medications are potentially serious and require prompt medical attention.
Seek immediate medical attention if you experience: severe, persistent abdominal pain (which may radiate to the back), signs of an allergic reaction (difficulty breathing, swelling of face/throat, severe rash), severe and persistent vomiting with inability to keep fluids down, or signs of low blood sugar (confusion, trembling, excessive sweating) if taking insulin or sulphonylureas.
Pancreatitis
Acute pancreatitis has been reported in patients taking GLP-1 medications, although the absolute risk is low (estimated at fewer than 1 in 1,000 patients). Symptoms include severe, persistent upper abdominal pain that may radiate to the back, often accompanied by nausea and vomiting. If you experience these symptoms, stop taking the medication and seek urgent medical care. Patients with a history of pancreatitis should discuss this risk with their prescriber before starting treatment.
Gallbladder disease
Gallstones (cholelithiasis) and gallbladder inflammation (cholecystitis) occur more frequently in patients taking GLP-1 medications than in those taking placebo. Rapid weight loss is a known risk factor for gallstone formation, which partly explains this association. Symptoms include right upper abdominal pain, particularly after eating, sometimes with nausea and fever. If you suspect gallbladder problems, contact your GP or attend A&E if symptoms are severe.
Acute kidney injury
Acute kidney injury has been reported, typically in the context of severe dehydration caused by persistent vomiting or diarrhoea. Patients with pre-existing kidney disease may be at higher risk. Maintaining adequate hydration is essential. If you are unable to keep fluids down due to vomiting, seek medical attention promptly.
Hypoglycaemia
When used alone, GLP-1 medications have a low risk of causing hypoglycaemia because their insulin-stimulating effect is glucose-dependent. However, the risk of hypoglycaemia is significantly increased when GLP-1 medications are used alongside insulin or sulphonylureas. Your prescriber may need to reduce the dose of these other medications when initiating a GLP-1 agonist.
Mental health effects
The MHRA and the European Medicines Agency (EMA) have reviewed reports of suicidal ideation and self-harm in patients taking GLP-1 receptor agonists. To date, a causal link has not been established, but pharmacovigilance monitoring continues. If you experience changes in mood, thoughts of self-harm or suicidal ideation while taking a GLP-1 medication, contact your prescriber or seek urgent help immediately.
Side effects comparison by medication
While all GLP-1 medications share a similar side-effect profile, there are some differences in the frequency and severity of specific adverse effects.
| Side effect | Ozempic | Wegovy | Mounjaro |
|---|---|---|---|
| Nausea | ~20% | ~44% | ~25–30% |
| Diarrhoea | ~10% | ~30% | ~15–20% |
| Vomiting | ~8% | ~25% | ~10–15% |
| Constipation | ~6% | ~10% | ~10–15% |
| Headache | ~7% | ~14% | ~7% |
| Fatigue | ~5% | ~10% | ~5% |
Approximate rates based on published clinical trial data. Individual experience may vary. Higher rates for Wegovy partly reflect the higher maintenance dose.
Managing side effects: practical tips
Most GLP-1 side effects can be managed effectively with straightforward adjustments to diet, hydration and lifestyle. Here are evidence-based strategies recommended by healthcare professionals.
Dietary adjustments
- Eat smaller, more frequent meals (5 to 6 small meals rather than 3 large ones)
- Choose bland, easily digestible foods when experiencing nausea
- Avoid high-fat, greasy and very rich foods
- Eat slowly and chew food thoroughly
- Stop eating as soon as you feel comfortably full
- Avoid lying down immediately after eating
Hydration
- Drink at least 1.5 to 2 litres of water daily
- Take small, frequent sips rather than large quantities at once
- If vomiting or diarrhoea occurs, consider oral rehydration solutions
- Limit carbonated drinks, which may worsen bloating
Timing and injection technique
- Some patients find that injecting in the evening reduces daytime nausea
- Rotate injection sites to minimise local reactions
- Ensure the pen is at room temperature before injecting for greater comfort
When to contact your prescriber
- Side effects that do not improve after 2 to 3 weeks at the same dose
- Inability to eat or drink adequate amounts
- Persistent vomiting (more than 24 hours)
- Severe or worsening abdominal pain
- Signs of dehydration (dark urine, dizziness, dry mouth)
- Any symptom you find concerning or distressing
Yellow Card Scheme: In the UK, you can report suspected side effects of any medicine directly to the MHRA through the Yellow Card Scheme at yellowcard.mhra.gov.uk. This helps monitor the ongoing safety of all medicines.
Long-term safety considerations
GLP-1 receptor agonists have been available for over 15 years (liraglutide since 2009, semaglutide since 2018). Long-term safety data from clinical trials extending to 2 years and real-world post-marketing surveillance are generally reassuring.
Areas of ongoing monitoring include:
- Thyroid safety: Animal studies showed an increased risk of thyroid C-cell tumours with GLP-1 agonists, leading to contraindications in patients with MTC or MEN 2. Long-term human data have not confirmed an increased thyroid cancer risk, but monitoring continues.
- Cardiovascular outcomes: The SELECT trial demonstrated that semaglutide 2.4 mg reduced major adverse cardiovascular events by 20 per cent. The cardiovascular outcomes trial for tirzepatide (SURPASS-CVOT) is ongoing.
- Mental health: Pharmacovigilance review of reports of suicidal ideation is ongoing. No causal link has been established to date.
- Gastrointestinal events: Rare reports of gastroparesis (severe delayed gastric emptying) and intestinal obstruction are being monitored.