GLP-1 Side Effects: What to Expect
Understanding the potential side effects of GLP-1 medications (Wegovy, Ozempic, Mounjaro) helps you know what to expect and when to seek help. This guide is based on official Summary of Product Characteristics (SmPC) data and clinical trial findings.
When to Seek Immediate Medical Help
- • Severe, persistent abdominal pain (possible pancreatitis)
- • Signs of allergic reaction (swelling, difficulty breathing)
- • Severe vomiting that does not stop
- • Signs of severe dehydration
Call NHS 111 for advice or 999 for emergencies
Understanding GLP-1 Side Effects
GLP-1 receptor agonists (semaglutide and tirzepatide) share similar side effect profiles because they work through similar mechanisms. The most common side effects are gastrointestinal and often improve as your body adjusts to the medication.
The gradual dose escalation schedule used with these medications is specifically designed to minimise side effects. Starting at a low dose and slowly increasing allows your body time to adapt.
Note: Side effect frequencies are based on clinical trial data reported in the official Summary of Product Characteristics (SmPC) for each medication. Individual experiences may vary. Always report side effects to your prescriber and via the MHRA Yellow Card scheme.
Common Side Effects
These side effects are experienced by a significant proportion of people taking GLP-1 medications. They are generally mild to moderate and often improve over time.
Nausea
Very commonUsually occurs during dose increases and often improves over time
Very common (affects more than 1 in 10 people)
Management Tips
Eat smaller meals, avoid fatty foods, stay hydrated
Diarrhoea
Very commonMay occur especially during the first few weeks of treatment
Very common (affects more than 1 in 10 people)
Management Tips
Stay hydrated, avoid triggering foods, contact prescriber if severe
Vomiting
Very commonMore common during dose escalation phase
Very common (affects more than 1 in 10 people)
Management Tips
Eat slowly, smaller portions, avoid lying down after eating
Constipation
Very commonDue to slowed gastric emptying
Very common (affects more than 1 in 10 people)
Management Tips
Increase fibre intake, stay well hydrated, gentle exercise
Abdominal pain
Very commonStomach discomfort or cramping
Very common (affects more than 1 in 10 people)
Management Tips
Usually mild; seek help if severe or persistent
Headache
CommonMay occur, particularly when starting treatment
Common (affects up to 1 in 10 people)
Management Tips
Stay hydrated, rest, standard pain relief if needed
Fatigue
CommonTiredness, especially during calorie restriction
Common (affects up to 1 in 10 people)
Management Tips
Ensure adequate nutrition and sleep, may improve over time
Decreased appetite
CommonThis is often the intended effect for weight loss
Common (affects up to 1 in 10 people)
Management Tips
Ensure adequate nutrition despite reduced appetite
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Serious Side Effects
While rare, serious side effects can occur. Being aware of the warning signs allows you to seek appropriate medical attention promptly.
Pancreatitis
Uncommon (may affect up to 1 in 100 people)Symptoms: Severe, persistent abdominal pain that may radiate to the back, with or without vomiting
Action: Stop medication and seek immediate medical attention
Gallbladder problems
Common (up to 1 in 10 people may develop gallstones)Symptoms: Pain in upper right abdomen, especially after eating fatty foods, nausea, vomiting
Action: Contact your doctor promptly
Hypoglycaemia
Risk increased when combined with insulin or sulfonylureasSymptoms: Shakiness, sweating, confusion, fast heartbeat, hunger
Action: Treat with fast-acting glucose; discuss medication adjustment with prescriber
Kidney problems
UncommonSymptoms: Reduced urination, swelling, fatigue
Action: Stay well hydrated; seek medical attention if symptoms occur
Allergic reactions
RareSymptoms: Rash, itching, swelling of face/tongue/throat, difficulty breathing
Action: Seek immediate medical attention (call 999 if severe)
Diabetic retinopathy complications
UncommonSymptoms: Vision changes, blurred vision
Action: Report to your doctor; may need eye examination
Thyroid Tumour Warning
Important Safety Information
In animal studies, GLP-1 medications caused thyroid C-cell tumours in rodents. It is not known whether this occurs in humans. GLP-1 medications should not be used by people with:
- • Personal or family history of medullary thyroid carcinoma (MTC)
- • Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)
Report any symptoms such as a lump in the neck, hoarseness, difficulty swallowing, or shortness of breath to your doctor.
Who Should Not Take GLP-1 Medications
GLP-1 medications are not suitable for everyone. The following are contraindications based on official prescribing information:
- Personal or family history of medullary thyroid cancer
- Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)
- Allergy to semaglutide, tirzepatide, or any ingredients
- Pregnancy or breastfeeding
- History of pancreatitis
- Diabetic retinopathy (eye disease)
- Severe gastrointestinal disease
- Kidney impairment (monitor hydration)
- Taking insulin or sulfonylureas (hypoglycaemia risk)
Tips for Managing Side Effects
Many common side effects can be managed with practical adjustments. Always discuss persistent or concerning symptoms with your prescriber.
For Nausea
- • Eat smaller, more frequent meals
- • Avoid fatty, fried, or spicy foods
- • Eat slowly and stop when you feel full
- • Avoid lying down right after eating
- • Try bland foods when symptoms are active
For Digestive Issues
- • Stay well hydrated (especially if vomiting/diarrhoea)
- • Increase fibre gradually for constipation
- • Gentle physical activity may help
- • Consider timing of medication
- • Talk to your pharmacist about over-the-counter options
Dose Timing
- • Take at the same time each week
- • Some find taking before bed helps with nausea
- • Can be taken with or without food
- • If you miss a dose, take within 5 days; otherwise skip
General Wellbeing
- • Ensure adequate protein intake
- • Consider vitamin supplementation if eating less
- • Maintain regular check-ins with prescriber
- • Report side effects via MHRA Yellow Card
Reporting Side Effects
Reporting side effects helps improve medication safety for everyone. You can report directly to the MHRA, even if you're not certain the medication caused the effect.
MHRA Yellow Card Scheme
Report suspected side effects of medicines, vaccines, e-cigarettes, or medical devices:
- • Online at yellowcard.mhra.gov.uk
- • Via the Yellow Card app (iOS and Android)
- • By phone: 0800 731 6789
Frequently Asked Questions
Related Guides
References & Sources
- Wegovy Summary of Product Characteristics (SmPC). Electronic Medicines Compendium (EMC).
- Ozempic Summary of Product Characteristics (SmPC). Electronic Medicines Compendium (EMC).
- Mounjaro Summary of Product Characteristics (SmPC). Electronic Medicines Compendium (EMC).
- MHRA Yellow Card Scheme. yellowcard.mhra.gov.uk
Medical Disclaimer
This information is provided for educational purposes only and is not intended as medical advice. Side effect information is based on official prescribing information but individual experiences may vary. Always consult your prescriber about any side effects you experience. Do not stop or change your medication without medical advice.
Last updated: December 2025
Stay Updated on GLP-1 Safety Information
Get notified about availability changes, price updates, and new research. Plus receive our free UK medication guide when it launches.