GLP-1 Side Effects: Complete Guide

What to Expect, What's Normal, and When to Worry

GLP-1 medications are highly effective for weight loss, but they do come with side effects. The good news: most are mild, temporary, and manageable.

This guide covers every known side effect of Mounjaro, Wegovy, and Ozempic - how common they are, how long they last, and what to do about them.

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The Most Common Side Effects

Gastrointestinal Effects (70-80% of people)

Nausea - The Big One

• Frequency: 44% (Wegovy), 18% (Mounjaro)
• When it starts: First 1-3 days
• Peak: Week 1-2
• Resolution: Week 3-4 for most

What it feels like:

• Queasiness
• Reduced appetite (this is the point)
• Possible vomiting if severe

Management:

• Eat smaller, more frequent meals
• Avoid fatty/greasy foods
• Stay hydrated
• Take with food if needed
• Consider anti-nausea medication (speak to clinician)

When to worry:

• Persistent vomiting (>24 hours)
• Unable to keep fluids down
• Severe abdominal pain with nausea

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Diarrhoea

• Frequency: 30% (Wegovy), 12% (Mounjaro)
• When it starts: First week
• Duration: 1-2 weeks typically

Management:

• Reduce fat intake temporarily
• Stay hydrated (electrolytes)
• BRAT diet (bananas, rice, applesauce, toast)
• Loperamide (Imodium) if needed

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Constipation

• Frequency: 24% (Wegovy), 17% (Mounjaro)
• Why it happens: Slowed gastric emptying
• Duration: Weeks to months (ongoing for some)

Management:

• Hydrate (2-3L water daily)
• Fibre (vegetables, psyllium husk)
• Magnesium supplement (citrate or oxide)
• Movement (walking stimulates bowels)
• Stool softeners if needed

When to worry:

• No bowel movement for >5 days
• Severe abdominal pain
• Blood in stool

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Vomiting

• Frequency: 24% (Wegovy), 5% (Mounjaro)
• Usually: Associated with overeating or high-fat meals

Management:

• Smaller meals
• Slower eating
• Avoid trigger foods
• Anti-emetics if prescribed

When to worry:

• Repeated vomiting
• Signs of dehydration
• Vomiting blood

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Abdominal Pain/Discomfort

• Frequency: 20% (Wegovy), 10% (Mounjaro)
• Usually: Mild, cramping, gas-related

Management:

• Simethicone (Wind-eze) for gas
• Heat pad for cramps
• Smaller meals
• Peppermint tea

When to worry:

• Severe pain
• Pain with fever
• Pain radiating to back (possible pancreatitis)

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Other Common Side Effects

Injection Site Reactions

• Frequency: 5-10%
• Symptoms: Redness, swelling, itching, bruising
• Duration: 1-3 days

Management:

• Rotate injection sites
• Ice if sore
• Avoid rubbing
• Usually resolves

When to worry:

• Spreading redness (infection)
• Severe swelling
• Pus

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Headache

• Frequency: 10-15%
• Usually: Mild, tension-type
• Duration: First 1-2 weeks

Management:

• Hydration (dehydration common cause)
• Paracetamol
• Rest
• Caffeine (if you normally use)

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Fatigue

• Frequency: 10-15%
• Usually: First 1-2 weeks
• Causes: Calorie reduction, adaptation

Management:

• Ensure adequate calories (not too low)
• Sleep 7-9 hours
• Light exercise (can boost energy)
• B vitamins if deficient

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Dizziness

• Frequency: 8-10%
• Usually: First week, related to reduced food intake

Management:

• Stand up slowly
• Hydrate
• Don't skip meals entirely
• Check blood pressure if persistent

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Burping/Reflux

• Frequency: 5-10%
• Cause: Slowed gastric emptying

Management:

• Smaller meals
• Don't lie down after eating
• Antacids if needed
• Avoid trigger foods (spicy, acidic)

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Less Common Side Effects

Hair Loss

• Frequency: 2-5%
• Cause: Usually rapid weight loss (telogen effluvium), not medication directly
• Duration: Temporary, regrows

Management:

• Ensure adequate protein
• Biotin supplement (limited evidence)
• Patience (regrows when weight stabilises)

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Gallbladder Issues

• Frequency: 2-4%
• Cause: Rapid weight loss increases gallstone risk
• Symptoms: Right upper abdominal pain, nausea, especially after fatty meals

Management:

• Gradual weight loss if possible
• Report pain to clinician
• Ultrasound if suspected

When to worry:

• Severe right upper quadrant pain
• Pain after eating
• Jaundice (yellowing skin/eyes)

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Pancreatitis (Rare but Serious)

• Frequency: 0.1-0.5%
• Symptoms: Severe abdominal pain (often radiating to back), nausea, vomiting

This is an emergency.

Seek immediate medical attention if:

• Severe abdominal pain
• Pain radiating to back
• Persistent vomiting
• Fever with abdominal pain

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Hypoglycaemia (If on Diabetes Medication)

• Risk if: Taking insulin or sulfonylureas
• Symptoms: Shakiness, sweating, confusion, rapid heartbeat

Management:

• Monitor blood glucose closely
• Adjust diabetes medication with doctor
• Keep glucose tablets handy

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Serious but Rare Side Effects

Thyroid C-Cell Tumours

Animal studies: Rodents developed thyroid C-cell tumours at high doses.

Human risk: Unclear. No confirmed cases in humans with GLP-1 medications.

Contraindications (do NOT use if):

• Personal/family history of medullary thyroid carcinoma (MTC)
• MEN2 (multiple endocrine neoplasia syndrome type 2)

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Severe Allergic Reactions

• Frequency: Very rare (under 0.1%)
• Symptoms: Difficulty breathing, facial swelling, hives

Emergency treatment required.

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Side Effect Timeline

Week 1-2: Peak

Week 3-4: Improving

Most GI side effects resolve or significantly reduce.

Month 2+: Settled

Most people have minimal ongoing side effects.

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Factors That Affect Side Effects

Dose

Higher dose = more side effects

This is why titration is important:

• Start low
• Increase gradually
• Body adapts

Injection Timing

Morning vs evening:

• Some find morning injection worse (nausea during day)
• Others prefer morning (side effects while at work, resolve by evening)
• Experiment to find what works

Diet

High-fat meals worsen GI side effects:

• Fried foods
• Creamy sauces
• Large portions

Better choices:

• Lean protein
• Vegetables
• Small, frequent meals

Individual Variation

Genetics: Some people metabolise differently Gut microbiome: May influence tolerance Previous GI issues: IBS, etc., may worsen initially

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Managing Side Effects: Practical Guide

Nausea Protocol

Day 1-3 (worst):

• Ginger tea
• Small, bland meals
• Avoid strong smells
• Fresh air
• Anti-nausea medication if prescribed

Day 4-7:

• Gradually reintroduce normal foods
• Still avoid greasy foods
• Should be improving

Week 2+:

• Usually minimal
• Occasional queasiness after large meals

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Constipation Protocol

Daily:

• 2-3L water
• Vegetables with every meal
• Walk 20+ minutes

If needed:

• Psyllium husk (fibre supplement)
• Magnesium citrate (200-400mg)
• Stool softener (docusate)

Avoid long-term:

• Stimulant laxatives (senna), dependency risk

Ginger is the gentlest first step for nausea, a soluble fibre such as ispaghula husk plus magnesium glycinate helps with the slowed-gut constipation, and a daily probiotic may ease overall digestive upset. Introduce one thing at a time so you can tell what is helping.

For a deeper look at every symptom and how to manage it, see our GLP-1 side effects: full UK guide.

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When to Contact Your Clinic

Call same day:

• Severe abdominal pain
• Persistent vomiting (>12 hours)
• Signs of dehydration
• Blood in vomit or stool

Call within 24-48 hours:

• Side effects not improving by week 3
• Severe constipation (>5 days)
• Significant weight loss (>10% in month 1)
• No appetite at all

Routine check-in:

• Discuss ongoing mild side effects
• Dose adjustment discussion

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Stopping the Medication

If side effects are intolerable:

1. Contact your clinician first
2. May lower dose
3. May switch medications (e.g., Mounjaro > Wegovy)
4. May add supportive medications

Don't just stop - discuss options.

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The Good News

Most side effects are:

• Mild (not severe)
• Temporary (resolve within weeks)
• Manageable (diet, lifestyle adjustments)
• Worth it (for weight loss benefits)

Clinical trial data:

• 5-10% of people discontinue due to side effects
• 90-95% tolerate well with proper management

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Side Effects by Medication

Mounjaro (Tirzepatide)

Advantage: Slightly fewer GI side effects than Wegovy at equivalent efficacy

Rates:

• Nausea: 18%
• Diarrhoea: 12%
• Vomiting: 5%
• Constipation: 17%

Wegovy (Semaglutide 2.4mg)

Rates:

• Nausea: 44%
• Diarrhoea: 30%
• Vomiting: 24%
• Constipation: 24%

Ozempic (Semaglutide 1mg)

Rates: Lower than Wegovy (lower dose)

• Nausea: 20%
• Diarrhoea: 15%
• Vomiting: 10%
• Constipation: 15%

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FAQ

Do side effects mean it's working? Not necessarily. Side effects and efficacy aren't directly linked. Some people have no side effects and excellent results.

Will side effects return when I increase dose? Often yes, but usually milder than initial start. Some people titrate up with minimal issues.

Can I take anti-nausea medication? Yes - speak to your prescribing clinician. Ondansetron or prochlorperazine can help.

What about long-term side effects? GLP-1 medications have been used for diabetes since 2005 (18+ years). Long-term safety profile is good. Unknowns exist but rare serious issues.

Do side effects mean I should stop? Only if severe or intolerable. Most side effects are temporary and manageable.

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Summary

Most common: Nausea, diarrhoea, constipation, vomiting (GI effects) Usually resolve: Week 3-4 Management: Diet adjustments, hydration, time When to worry: Severe pain, persistent vomiting, dehydration signs Serious but rare: Pancreatitis, severe allergic reaction

The bottom line: Side effects are real but temporary for most people. The weight loss benefits usually outweigh temporary discomfort.

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Related Articles

• First Month on GLP-1: What to Expect
• Mounjaro vs Wegovy vs Ozempic
• Best UK Clinics for GLP-1

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Last updated: April 2026 Medical disclaimer: This information is for educational purposes. Discuss side effects with your prescribing clinician.