Weight Loss Injections: Are They Safe?

Medicines like Mounjaro and Wegovy work by acting on appetite-regulating pathways in the body. Semaglutide (Wegovy) activates GLP-1 receptors, which are involved in signalling fullness and slowing the rate at which the stomach empties. Tirzepatide (Mounjaro) acts on both GLP-1 and GIP receptors, a dual mechanism that influences appetite and how the body processes energy. Many people experience a noticeable reduction in appetite and what is often described as reduced "food noise" — the constant background pull toward eating.

Commonly reported side effects

Because these medicines affect the digestive system, gastrointestinal side effects are the most frequently reported. These include nausea, vomiting, diarrhoea, constipation, indigestion, and reflux. Most commonly, these are most noticeable after a dose is started or increased, and many people find they ease over time. Other commonly reported effects include fatigue, headache, dizziness, and reactions at the injection site such as redness or mild discomfort.

When to seek medical attention

Some symptoms warrant prompt contact with a healthcare professional — including severe or persistent abdominal pain, signs of an allergic reaction, or any symptom that feels serious or unusual. If you experience anything that concerns you, contact your prescriber or doctor rather than waiting. Side effects can also be reported to your national pharmacovigilance scheme — for example, the UK's Yellow Card scheme — though the scheme in your country may differ; your prescriber can advise.

It is worth reading the patient information leaflet that came with your pen, as it contains the full list of side effects and guidance specific to your medicine. You can also find detailed product information through the electronic Medicines Compendium (eMC).

Who these medicines are not suitable for

These are not medicines for everyone. Prescribers assess suitability carefully, taking into account factors such as existing medical conditions (including thyroid history, pancreatitis, or gallbladder issues), other medicines being taken, pregnancy or plans to conceive, and overall health. If your circumstances change after you are prescribed, always discuss this with your prescriber or doctor promptly.

To understand more about the different types of injectable medicines used for weight management, see what types of weight loss injections are available. For a broader overview of this area, the weight loss injections guide covers the key topics in one place.

One of the most practical things you can do while taking a prescribed weight loss injection is keep a clear record of how you feel — particularly around injection days. Patterns that are hard to notice day-to-day often become obvious when you look back across several weeks of logged data.

GLP Trackr's side-effect tracking is built specifically for this. You can log symptoms — nausea, fatigue, injection-site reactions, digestive changes — and the app surfaces patterns over time, showing you how your experience shifts as your body adjusts. That logged history can be genuinely useful to bring to your next prescriber appointment, giving them a clearer picture than memory alone.

The app also lets you log each injection, including the date, dose strength, and injection site, and tracks your rotation history so you can vary sites systematically. Rotating sites — between the abdomen, thigh, and upper arm — is generally recommended to reduce the chance of localised skin reactions, and having a clear log makes that easier to manage.

If a dose ever feels different or a side effect seems new or more pronounced, log it in the app and raise it with your prescriber or doctor — they know your full health picture and are best placed to advise. GLP Trackr helps you record what you are experiencing; it is not a substitute for professional medical guidance.

Curious about the cost side of your treatment? Take a look at what affects the cost of weight loss injections for a neutral overview.

Ready to keep a clearer record of your journey? GLP Trackr is free to try for 7 days — available on the App Store and Google Play. Log your injections, track how you feel, and bring better information to every prescriber conversation.