Weight Loss Jab Side Effects: What Most People Experience

Both Mounjaro and Wegovy work on appetite-regulating pathways in the body — Mounjaro as a dual GIP and GLP-1 receptor agonist, Wegovy as a GLP-1 receptor agonist. One effect of this is slower stomach emptying, which is closely linked to the gastrointestinal side effects many people report, particularly early in treatment or after a dose adjustment.

Commonly reported side effects

• Nausea — among the most frequently reported experiences; often most noticeable after the first few doses or a dose increase, and tends to ease over time for many people
• Vomiting and diarrhoea — reported by some users, typically mild and short-lived
• Constipation — slower gastric motility can contribute to this; staying well hydrated is often mentioned as helpful
• Indigestion, reflux, and burping — commonly noted, especially after eating larger or richer meals
• Fatigue and headache — some people report low energy or mild headaches, particularly in the early weeks
• Injection-site reactions — redness, itching, or mild bruising at the injection site are possible

It is worth noting that not everyone experiences side effects, and the pattern varies from person to person. According to NHS guidance, gastrointestinal effects are among the most commonly reported with this class of medicine.

For a closer look at side effects specific to Mounjaro, see our guide to what to expect with the Mounjaro injection, or explore Wegovy side effects if that is the medicine you have been prescribed. Some people also report urinary changes — worth knowing about if you notice anything in that area.

When do side effects ease?

For many people, gastrointestinal side effects are most pronounced in the first week or two after a new dose level and tend to lessen as the body adjusts. That said, everyone's experience is different, and there is no single timeline that applies to all. If side effects feel severe, persistent, or are affecting your ability to eat or drink, that is a reason to contact your prescriber or doctor — do not wait and hope they pass on their own.

One of the most practical things you can do when navigating side effects is keep a consistent record — not to self-diagnose, but to bring clear, detailed information to your prescriber or doctor when you speak to them.

GLP Trackr's side-effect tracker lets you log symptoms as they happen — nausea, fatigue, digestive discomfort, injection-site reactions, and more — and view them over time as patterns. Instead of trying to recall "was it worse last week or the week before?", you have a timeline you can actually refer back to. Many people find this especially useful around dose changes, where spotting whether symptoms follow a predictable pattern (and then ease) can be genuinely reassuring.

The app also includes a doctor report feature, which pulls your logged data — symptoms, doses, weight, wellbeing — into a summary you can share or bring to your next appointment. It is the kind of detail that helps your prescriber understand your experience, not just a verbal summary from memory.

For a broader overview of managing side effects on these medicines, visit our guide to Mounjaro side effects, which covers the full picture in more depth.

Important: GLP Trackr helps you record what you experience so you can have more informed conversations with your healthcare team — it is not a substitute for their advice. If you experience anything severe, persistent, or worrying — including severe abdominal pain, signs of an allergic reaction, or significant changes in mood — contact your prescriber or doctor promptly. Any concerns about your dose or treatment should always go through your healthcare professional, never be managed alone.

Track your symptoms, spot your patterns, and walk into every appointment prepared. Download GLP Trackr on the App Store or Google Play and start your free 7-day trial.