Mounjaro Side Effects & Management UK 2025

Mounjaro is generally well-tolerated, but like all medications, it can cause side effects. Most are gastrointestinal (nausea, diarrhoea, constipation) and typically mild-moderate, occurring primarily during initial weeks or after dose increases. This guide covers all documented side effects, evidence-based management strategies, and when medical attention is required.

🔵 Common Side Effects (>10% of Patients)

These side effects occur frequently but are usually mild-moderate and temporary:

1. Nausea (20-30% of Patients)

Typical Pattern:

• Most common in first 4-8 weeks and for 2-3 days after dose increases
• Usually peaks 24-48 hours post-injection
• Severity decreases over time as body adjusts
• By week 12-16, most patients report minimal or no nausea

Management Strategies:

• Eat smaller, more frequent meals: 5-6 mini-meals vs. 3 large meals
• Avoid trigger foods: Fatty, spicy, very sweet, or strong-smelling foods
• Cold foods: Less nausea-inducing smell (e.g., smoothies, salads, sandwiches)
• Ginger: Ginger tea, ginger biscuits, or ginger supplements (1-2g daily)
• Peppermint: Peppermint tea or peppermint oil capsules
• Stay upright: Don’t lie down immediately after eating
• Slow eating: Chew thoroughly, take 20-30 minutes per meal
• Hydration: Sip water throughout day (avoid large gulps)
• Anti-nausea medication: If severe, GP can prescribe domperidone or cyclizine

2. Diarrhoea (15-20% of Patients)

Typical Pattern:

• Often occurs during first few weeks or after dose escalation
• Usually resolves within 1-2 weeks as GI tract adapts
• Frequency: Typically 3-5 loose stools daily (not continuous)

Management Strategies:

• Hydration crucial: 2.5-3 litres water daily (with electrolytes if severe)
• BRAT diet initially: Bananas, Rice, Applesauce, Toast
• Avoid: High-fat foods, caffeine, alcohol, dairy (if lactose intolerant)
• Probiotics: Live yoghurt or probiotic supplements
• Soluble fibre: Psyllium husk, oats (avoid insoluble fibre like bran)
• Anti-diarrhoeal: Loperamide (Imodium) if needed—check with GP first

3. Constipation (15-18% of Patients)

Typical Pattern:

• Mounjaro slows gastric emptying, contributing to constipation
• Can occur at any stage but often worsens with higher doses
• Defined as <3 bowel movements per week with straining

Management Strategies:

• Increase fibre: Target 25-35g daily (vegetables, fruits, whole grains)
• Hydration essential: 2-3 litres water daily
• Exercise: Physical activity stimulates bowel motility
• Warm beverages: Coffee, warm water with lemon in morning
• Prunes: Natural laxative—4-5 prunes or 150ml prune juice daily
• Over-the-counter laxatives: Senna, bisacodyl, or lactulose (short-term use)
• Avoid: Excessive cheese, red meat, processed foods

4. Decreased Appetite (10-15% Report as Side Effect)

Context: This is actually Mounjaro’s intended mechanism, but some find appetite reduction excessive.

Management (If Too Severe):

• Focus on nutrition density: Ensure adequate protein/nutrients even with smaller portions
• Meal timing: Eat at scheduled times even if not hungry (maintain metabolic rate)
• Calorie-dense foods: Nuts, nut butters, avocados (healthy fats)
• Protein shakes: If struggling with solid food
• Monitor intake: Minimum 1,200 calories daily (women), 1,500 (men) unless supervised
• Discuss dose: If appetite elimination complete, may need dose reduction

5. Vomiting (5-12% of Patients)

Typical Pattern:

• Less common than nausea; usually accompanies severe nausea
• Most likely in first 2-4 weeks or after dose escalation
• Typically 1-2 episodes within 48 hours post-injection

Management:

• Clear liquids initially: Water, ice chips, diluted fruit juice
• Gradual food reintroduction: Start with bland, room-temperature foods
• Anti-emetics: GP can prescribe ondansetron or prochlorperazine
• Seek help if: Unable to keep fluids down for 12+ hours (dehydration risk)

6. Abdominal Pain/Discomfort (5-10% of Patients)

Typical Pattern:

• Usually mild, described as “bloating,” “fullness,” or cramping
• Related to slowed gastric emptying
• Often accompanies other GI symptoms

Management:

• Small portions: Prevents over-filling stomach
• Avoid gas-producing foods: Beans, cruciferous vegetables, carbonated drinks
• Digestive enzymes: May help with bloating
• Peppermint tea: Relaxes GI smooth muscle
• Seek help if: Pain severe, persistent, or radiating to back (possible pancreatitis)

🟡 Uncommon Side Effects (1-10% of Patients)

Fatigue/Tiredness

• Often related to reduced caloric intake or rapid weight loss
• Management: Adequate protein (80-120g daily), iron supplementation if low, maintain exercise routine

Headache

• Usually mild; occurs primarily in first 2-4 weeks
• Management: Paracetamol, adequate hydration (dehydration worsens headaches), avoid skipping meals

Dizziness

• Often related to dehydration, low blood sugar (if diabetic), or rapid standing
• Management: Hydration, rise slowly from sitting/lying, monitor blood glucose if diabetic

Injection Site Reactions

• Mild redness, itching, or small lumps at injection site
• Management: Rotate sites, ensure room-temperature pen, clean site thoroughly, ice if swelling occurs

Hair Thinning/Loss

• Not directly caused by Mounjaro but by rapid weight loss (telogen effluvium)
• Temporary; hair regrowth begins 3-6 months after weight stabilisation
• Management: Adequate protein, biotin supplements (5,000-10,000 mcg daily), multivitamin

Gallstones

• Risk increases with rapid weight loss (any method, not specific to Mounjaro)
• Symptoms: Upper right abdominal pain, especially after fatty meals
• Management: Gradual weight loss (<1kg/week ideal), avoid very-low-calorie diets

🔴 Rare But Serious Side Effects (<1% of Patients)

1. Pancreatitis (Inflammation of Pancreas)

Incidence: <0.5% of patients

Symptoms:

• Severe, persistent abdominal pain (upper abdomen, radiating to back)
• Pain worsens when lying flat, improves when leaning forward
• Nausea and vomiting accompanying severe pain
• Fever

Action: Call 999 or go to A&E immediately. Do not take further Mounjaro doses.

2. Severe Allergic Reaction (Anaphylaxis)

Incidence: <0.1% of patients

Symptoms:

• Difficulty breathing, throat swelling
• Severe rash, hives, or facial swelling
• Rapid heartbeat, dizziness, fainting

Action: Call 999 immediately. Use EpiPen if available.

3. Severe Hypoglycaemia (Low Blood Sugar)

Risk Group: Primarily affects those also taking insulin or sulphonylureas (not typical for weight-loss-only patients)

Symptoms:

• Confusion, irritability, rapid heartbeat
• Sweating, trembling, extreme hunger
• Loss of consciousness, seizures (severe cases)

Action: Consume fast-acting carbs (glucose tablets, fruit juice). If diabetic, follow your hypoglycaemia protocol. Seek emergency care if severe.

4. Kidney Problems

Incidence: <0.5%; usually related to severe dehydration from vomiting/diarrhoea

Symptoms:

• Decreased urination, dark urine
• Swelling in feet/ankles
• Severe fatigue, confusion

Prevention: Adequate hydration; seek help for persistent vomiting/diarrhoea before dehydration worsens.

5. Vision Changes

Incidence: <0.3%; primarily in diabetic patients with existing diabetic retinopathy

Symptoms:

• Sudden vision changes, blurred vision
• Floaters or flashes of light

Action: Contact ophthalmologist or go to A&E same day.

📅 Side Effect Timeline: What to Expect

🎯 When to Contact Your Prescriber

Non-Urgent (Call Within 1-3 Days):

• Side effects not improving after 2 weeks at same dose
• Significant weight loss plateau (no loss for 4+ weeks)
• Injection site reactions persisting beyond 3 days
• Questions about dose adjustments or side effect management
• Hair thinning concerns

Urgent (Call Same Day):

• Persistent vomiting (unable to keep fluids down 12+ hours)
• Signs of dehydration (dark urine, dizziness, rapid heartbeat)
• Severe diarrhoea lasting 24+ hours
• Moderate-severe abdominal pain
• Injection site infection (spreading redness, warmth, pus)
• Blood glucose concerns if diabetic

Emergency (Call 999 or A&E):

• Severe abdominal pain radiating to back
• Difficulty breathing, throat swelling
• Severe allergic reaction symptoms
• Vision changes or eye pain
• Signs of severe hypoglycaemia (confusion, loss of consciousness)
• Chest pain or signs of blood clot

💊 Medications to Avoid/Use Cautiously with Mounjaro

Caution Required:

• Oral medications: Mounjaro slows gastric emptying—take oral meds 1 hour before Mounjaro injection or 4 hours after
• Insulin/Sulphonylureas: Increased hypoglycaemia risk; dose adjustments often needed
• Warfarin: May affect INR; monitor more frequently
• Birth control pills: Absorption may be affected during first 4 weeks; use backup contraception

Generally Safe:

• Paracetamol, ibuprofen (for pain relief)
• Anti-hypertensives (blood pressure medication)
• Statins (cholesterol medication)
• Antidepressants (SSRIs, SNRIs)
• Supplements (multivitamins, omega-3s)

🎯 Key Takeaways: Mounjaro Side Effects

• Most common: Nausea (20-30%), diarrhoea (15-20%), constipation (15-18%)—usually mild-moderate and temporary
• Timeline: Peak severity weeks 2-8, significant improvement by week 12, minimal by week 20+
• Management works: Small frequent meals, hydration, ginger/peppermint, slower titration dramatically reduce severity
• Rare serious effects: Pancreatitis (<0.5%), allergic reactions (<0.1%)—know warning signs
• Discontinuation rate: Only 5-11% stop due to side effects; vast majority complete treatment
• Dose increases: Expect temporary symptom worsening for 3-5 days after escalation
• Individual variation: Some experience no side effects; others need active management—both normal
• Communication crucial: Keep prescriber informed; dose adjustments can resolve most issues
• Long-term tolerance: Body adapts; maintenance phase typically side-effect free

📚 Related Guides

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