Are the New Weight Loss Injections Safe?

Mounjaro and Wegovy are once-weekly injectable medicines prescribed for weight management in adults who meet specific clinical criteria. Mounjaro's active substance is tirzepatide, a dual GIP and GLP-1 receptor agonist; Wegovy's active substance is semaglutide, a GLP-1 receptor agonist. Both work by acting on appetite-regulating pathways in the body, leading many people to experience reduced appetite and a greater sense of fullness.

Both medicines hold regulatory authorisation in multiple countries and carry a black triangle symbol (▼), which means they are subject to additional monitoring. This is standard for newer medicines and means that reporting any side effects is especially encouraged — through your healthcare professional and your national reporting scheme (such as the UK's Yellow Card scheme, or the equivalent in your country).

Commonly reported side effects

The most frequently reported side effects are gastrointestinal in nature: nausea, vomiting, diarrhoea, constipation, indigestion, and reflux. Fatigue, headache, dizziness, and injection-site reactions are also commonly reported. These effects are not universal — many people experience them mildly, and they often ease as the body adjusts, particularly after dose changes.

Some rarer but more serious concerns — including effects on the pancreas, gallbladder, heart rate, and thyroid — are outlined in the patient information leaflet that comes with your medicine. It is important to read this and discuss any personal risk factors with your prescriber before starting or continuing treatment.

If you experience severe, persistent, or worrying symptoms at any point, contact your healthcare professional promptly rather than waiting for your next scheduled appointment.

For a fuller look at what these medicines are and how they work, see our guide to weight loss injections or read more about what the new weight loss injections are called.

One of the most practical ways to stay on top of your safety and wellbeing while using these medicines is to keep a consistent log of how you feel over time. GLP Trackr's injection-site tracking and rotation feature lets you record every injection — including the site (abdomen, thigh, or upper arm) — so you can rotate correctly and spot any localised reactions linked to a particular area. The app suggests your next site automatically based on your history, taking one decision off your plate each week.

Alongside site tracking, the side-effect logging tool lets you record symptoms like nausea, fatigue, or dizziness as they happen, building a timeline you can actually bring to your next prescriber appointment. Patterns that might be easy to forget or underestimate become visible when they are logged consistently — which makes conversations with your healthcare professional more informed and more useful.

GLP Trackr is a tracking and journaling tool, not a medical service. It helps you observe and record your experience; it is not a substitute for the guidance of your prescriber or doctor, who should always be your first point of contact for any safety concerns.

For more detail on how these medicines are obtained and used, visit our overview of the new weight loss injections or read our dedicated page on whether the new weight loss injection is safe.

Log your injection sites, track how you feel, and build a picture of your journey you can share with your prescriber — download GLP Trackr on the App Store or Google Play and explore it free for 7 days.