What Side Effects Does Mounjaro Have?

Mounjaro works as a dual GIP and GLP-1 receptor agonist — it acts on appetite-regulating pathways and slows gastric emptying (the rate at which the stomach empties after eating). This mechanism is behind many of the digestive side effects people commonly report, especially early in treatment or after a dose step-up.

Gastrointestinal effects

• Nausea — one of the most frequently reported effects; often most noticeable in the days following an injection and tends to become less prominent over time for many users.
• Diarrhoea — commonly reported, particularly in the first weeks. You can read more about why Mounjaro can cause diarrhoea and what patterns people notice.
• Constipation — slower gastric emptying can also affect bowel regularity in the opposite direction. Our page on Mounjaro and constipation explores this further.
• Burping and reflux — excess belching and indigestion are also commonly reported. If this is something you are experiencing, see why Mounjaro may cause burping.
• Vomiting and stomach discomfort — less universal but still frequently mentioned, especially around dose increases.

Other commonly reported effects

• Fatigue — feeling more tired than usual, particularly in the early period of treatment.
• Headache and dizziness — reported by some users, often linked to reduced food and fluid intake.
• Injection-site reactions — mild redness, itching, or discomfort at the injection site.

A few factual notes about the pen itself

Some people also ask whether Mounjaro contains semaglutide or insulin — it does not. Mounjaro's active substance is tirzepatide, which is distinct from semaglutide (the active substance in Wegovy) and is not insulin. People also sometimes ask about expiry dates and shelf life: Mounjaro pens do carry a use-by date, and storage conditions matter. Rather than relying on a recalled figure, always check the patient information leaflet that came with your pen or the electronic Medicines Compendium entry for current, accurate guidance on storage and expiry. Similarly, the question of whether a pen contains an extra dose is addressed in the official product information — your leaflet or pharmacist is the best source for the specifics of your pen format.

For a fuller overview of all commonly reported effects, visit our Mounjaro side effects hub.

Logging how you feel after each dose is one of the most practical things you can do on a Mounjaro journey — not to self-diagnose, but to build a clear picture you can share with your prescriber.

GLP Trackr's side-effect tracker lets you record symptoms — nausea, fatigue, digestive discomfort, or anything else you notice — and see how they change over time. Because entries are timestamped alongside your dose logs, patterns become visible: you might notice symptoms cluster in the 24–48 hours after an injection, or that they have gradually reduced over successive weeks. That kind of personalised data is far more useful to bring to an appointment than a vague recollection.

The app also generates a doctor report — a summary of your tracked doses, symptoms, weight, and wellbeing — so your prescriber has a concrete record to work from rather than relying on memory alone.

GLP Trackr is available free on the App Store and Google Play. Start a 7-day free trial and begin building your side-effect log from your very next dose.

Important: if any side effect is severe, persistent, or feels out of the ordinary — including significant abdominal pain, persistent vomiting, vision changes, or anything that concerns you — contact your healthcare professional promptly. GLP Trackr records your experience so you can discuss it with your prescriber; it is not a substitute for their advice. Any concerns about whether a side effect is serious should always go to a qualified healthcare professional, not an app. You should also report side effects through your national reporting scheme (such as the UK's Yellow Card scheme) as directed in your medication's patient information leaflet.