Mounjaro vs Wegovy Side Effects: A UK Patient Review & Safety Comparison Guide
For years, Wegovy (semaglutide) held the title of the “Gold Standard” for weight loss in the UK. That changed in 2024 and solidified in 2025. With the arrival of Mounjaro (tirzepatide), the benchmark shifted.
Recent head-to-head data from the SURMOUNT-5 trial confirms what many clinicians suspected: Mounjaro is statistically more effective for weight loss. But effectiveness isn’t the only metric that matters to a patient. If a medication makes you too ill to function, the percentage of weight lost becomes irrelevant.
Thousands of UK patients are currently facing a dilemma. Do you stick with the proven track record of Wegovy, or switch to the more potent Mounjaro?
This guide goes beyond the clinical leaflets. We analyse real Mounjaro vs Wegovy side effects reviews in the UK, examine the latest 2025 MHRA safety warnings, and break down the practical differences between the pens to help you make an informed choice.
Mounjaro vs Wegovy: The 2025 Head-to-Head (Clinical Efficacy)
Before we dissect the side effects, we must understand why the side effects differ. These two medications work on similar biological pathways, but they are not identical.
Mechanism of Action (The Science)
Wegovy acts on a single hormone receptor: GLP-1 (glucagon-like peptide-1). This hormone signals to your brain that you are full and slows down how fast your stomach empties (gastric emptying).
Mounjaro is the first of a new class called “twincretins.” It acts on GLP-1 plus a second hormone called GIP (glucose-dependent insulinotropic polypeptide). This dual action essentially hits the weight loss accelerator from two angles: it suppresses appetite and improves how your body breaks down sugar and fat.
The SURMOUNT-5 Results
For a long time, we had to compare separate trials (STEP-1 vs SURMOUNT-1). However, the SURMOUNT-5 trial (2025) finally gave us direct comparative data.
| Metric | Wegovy (Semaglutide 2.4mg) | Mounjaro (Tirzepatide 15mg) |
| Average Weight Loss | ~13.7% | ~20.2% |
| Participants Losing >25% Body Weight | ~14% | ~32% |
| Mechanism | Single Agonist (GLP-1) | Dual Agonist (GLP-1 + GIP) |
The Takeaway: Mounjaro provides significantly higher weight loss. However, pushing the body harder often comes with a different “texture” of side effects, which we review below.
Side Effects Comparison: Clinical Data vs. Real Patient Reviews
If you read the Patient Information Leaflet (PIL) for both drugs, the lists look nearly identical: nausea, vomiting, diarrhoea, constipation.
But “nausea” is a broad term. Does it mean a slight queasiness, or being unable to get out of bed? By analysing UK patient forums and reviews (such as those on Drugs.com and UK-specific communities like r/UKMounjaro), distinct patterns emerge that clinical trials often miss.
Gastrointestinal Issues: The “Big Four”
The most common reason people discontinue either drug is GI distress.
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Wegovy: Users frequently report a “heavy” nausea that lingers throughout the day. Constipation is also a dominant complaint, often requiring regular laxative use (e.g., Senna or CosmoCol).
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Mounjaro: While nausea exists, many reviews describe it as more acute but transient—often hitting hard 24-48 hours after injection and then fading. However, Mounjaro has a higher report rate for “digestive stagnation” (severe bloating).
The “Texture” of Side Effects (User Experience)
This is where the real difference lies. The side effect profile of Mounjaro feels different from Wegovy.
1. Sulphur Burps (The Mounjaro Signature)
One specific term dominates Mounjaro vs Wegovy side effects reviews in the UK: “Sulphur Burps.”
While possible on Wegovy, this is overwhelmingly reported by Mounjaro users. It is caused by the GIP component slowing digestion so significantly that food ferments in the stomach, producing hydrogen sulfide gas.
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User Sentiment: “It tastes like rotten eggs and nothing gets rid of it.”
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Management: Peppermint oil capsules and avoiding heavy carbs the day before injection.
2. The “Wegovy Fatigue” vs. “Mounjaro Energy”
Wegovy reviews often mention a deep, dragging exhaustion, sometimes called “The Wegovy Wall.” Patients report sleeping 10+ hours and still feeling drained.
Conversely, Mounjaro users often report “cleaner” energy levels once they adjust to the dose, likely due to better blood sugar regulation from the GIP hormone.
3. Allodynia (Skin Sensitivity)
A rare but specific side effect appearing in Mounjaro reviews is allodynia—a sensation where the skin feels sunburned or bruised even when there is no mark. This is less commonly reported in Wegovy reviews.
| Feature | Wegovy (What Users Say) | Mounjaro (What Users Say) |
| Nausea Style | Lingering, background queasiness. | Acute waves, often triggered by greasy food. |
| Energy Levels | High reports of fatigue/lethargy. | generally better energy; less “brain fog.” |
| Specific Symptom | Severe Constipation. | Sulphur Burps & Indigestion. |
| Skin/Body | “Ozempic Face” (rapid fat loss). | Skin sensitivity / tenderness. |
Pro-Tip: NHS Managing Common Side Effects suggests staying upright for 30 minutes after eating to reduce the risk of reflux and sulphur burps on either medication.
Critical Safety Update (January 2025): The MHRA Warning
We need to address a serious safety update that affects both medications. In January 2025, the MHRA (Medicines and Healthcare products Regulatory Agency) issued updated guidance regarding Pulmonary Aspiration Risk.
Because GLP-1 drugs slow down gastric emptying, food stays in your stomach much longer than normal. This poses a risk during surgery requiring general anaesthesia. If your stomach isn’t empty, you could vomit while unconscious and inhale it into your lungs (aspiration).
The New Protocol
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Old Advice: Standard fasting (6 hours for food, 2 hours for water).
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New Advice: If you are taking Mounjaro or Wegovy, you must inform your anaesthetist immediately.
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The Reality: Many UK hospitals now suggest stopping the medication 1 to 3 weeks before elective surgery to ensure the stomach is truly empty.
Other “Red Flag” Risks
While rare, you must be aware of:
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Pancreatitis: Severe stomach pain radiating to the back.
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Gallstones: Sudden pain in the upper right abdomen.
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Thyroid Tumours: Both drugs carry a boxed warning for thyroid C-cell tumours (observed in rodents).
[MHRA Drug Safety Update – GLP-1 Agonists]
The “Pen Experience”: KwikPen vs. FlexTouch
When comparing Mounjaro vs Wegovy side effects reviews UK, we cannot ignore the hardware. The injection device dictates your weekly experience.
Wegovy: The FlexTouch Pe
In the UK, Wegovy typically uses the FlexTouch pen.
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Pros: Very easy to use. The needle is often integrated or simple to attach.
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Cons: Plastic waste (depending on the specific pack dispensed).
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User Experience: “It’s foolproof. Click, hold, done.”
Mounjaro: The KwikPen (UK Version)
Mounjaro in the UK is dispensed in a KwikPen. This is a multi-dose pen containing 4 doses (a month’s supply).
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The Mechanism: You attach a new needle each week, dial up the dose, and inject.
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The “Fifth Dose” Issue: Users often notice a small amount of liquid left after the 4th dose. Do not attempt to extract this. It is a safety overfill.
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The Stiffness: A common complaint in reviews is that the KwikPen plunger can feel “stiff” or hard to depress, especially for users with arthritis or weak grip strength.
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Shelf Life: Once used for the first time, the KwikPen expires in 30 days at room temperature (below 30°C).
Common Mistake: Failing to remove the inner needle cover. We see many reviews where users “injected” but the liquid ran down their leg because the second cap was still on.
Switching from Wegovy to Mounjaro (UK Protocol)
A massive volume of UK searches relates to switching. Perhaps you hit a weight loss plateau on Wegovy, or the fatigue is unmanageable.
Can you switch? Yes.
Can you switch straight over? No.
The 7-Day Washout Rule
Most UK private prescribers (like Boots, Superdrug, or simple online pharmacies) follow a strict Washout Protocol.
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Stop Wegovy: Take your final shot.
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Wait 7 Days: Let the drug clear slightly from your system.
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Start Mounjaro: Administer your first dose on the day your next Wegovy shot would have been due.
The Dosage Reset
Danger: You cannot swap max-dose Wegovy (2.4mg) for max-dose Mounjaro (15mg). The potency is different.
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Most clinicians will start you on the lowest dose of Mounjaro (2.5mg) or arguably the second step (5mg) to assess tolerability.
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Jumping straight to a high dose guarantees severe side effects (vomiting, dehydration).
Mounjaro vs Wegovy Reviews: What UK Users Are Saying
To summarise the sentiment across the UK in late 2024 and 2025:
The Case for Wegovy:
“I stuck with Wegovy because I knew what to expect. The weight loss is slower (about 1lb a week), but I didn’t get the sulphur burps. It feels gentler, even if the fatigue is annoying.”, Sarah, Leeds (via Trustpilot)
The Case for Mounjaro:
“I switched after stalling on Wegovy for three months. The difference is night and day. The ‘food noise’ is completely gone. I lost 6lbs in my first month on Mounjaro. The first two days after the jab are rough (nausea), but then I feel great.”, David, London (via r/UKMounjaro)
Conclusion
When comparing Mounjaro vs Wegovy side effects reviews in the UK, the data paints a clear picture.
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Choose Mounjaro if: You want the highest statistical chance of significant weight loss (20%+) and can tolerate potentially sharper, acute gastric side effects like sulphur burps. It is generally better for those who want to avoid the “chronic fatigue” associated with Wegovy.
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Choose Wegovy if: You prefer a medication with a longer usage history, or if you find the Mounjaro KwikPen mechanism difficult to manage.
Final Thought: The “best” medication isn’t just the one that burns the most fat—it’s the one you can tolerate for 12 to 18 months.
Next Step: If you are considering switching, do not attempt to guess the dosage. Consult a registered prescriber to plan your “Washout Week” safely.
FAQs
Which has worse side effects, Mounjaro or Wegovy?
It depends on individual tolerance. Mounjaro users report more acute digestion issues (sulphur burps, reflux) and skin sensitivity. Wegovy users report higher rates of general fatigue, depression, and headaches.
Can I switch straight from Wegovy to Mounjaro?
No. You typically need a “washout period” of at least 7 days (the time between injections) to prevent overdose symptoms. You will also likely need to restart at a lower dose.
Does Mounjaro cause hair loss like Wegovy?
The drug itself doesn’t cause hair loss, but rapid weight loss does. This condition, Telogen Effluvium, is temporary. Because Mounjaro causes faster weight loss, the risk of temporary shedding is slightly higher.
Why does Mounjaro cause sulphur burps?
Mounjaro contains GIP, which significantly delays gastric emptying. Food sits in the stomach longer, fermenting and releasing hydrogen sulfide gas.
Is Mounjaro available on the NHS in 2025?
Yes, NICE has approved Mounjaro (tirzepatide) for NHS use, but the rollout is staged. It is prioritized for those with a BMI over 35 with specific comorbidities. Waiting lists are currently long.
Do I need to fast before surgery if I take Mounjaro?
Yes. You must inform your surgical team. Recent MHRA guidance suggests you may need to stop the medication for weeks, not just days, prior to surgery to prevent aspiration.
What is “Ozempic Face” and does Mounjaro cause it?
“Ozempic Face” is simply a slang term for facial volume loss due to rapid weight loss. Because Mounjaro causes even faster weight loss than Ozempic/Wegovy, the effect can actually be more pronounced on Mounjaro.
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