Side Effects of Weight Loss Injections: What People Commonly Experience

Both tirzepatide (Mounjaro) and semaglutide (Wegovy) work on appetite-regulating pathways in the body. Tirzepatide acts on both GIP and GLP-1 receptors; semaglutide acts on GLP-1 receptors alone. One widely reported effect of both is slower gastric emptying — the rate at which the stomach moves food through — which is thought to contribute to many of the digestive side effects people notice.

Commonly reported gastrointestinal effects

• Nausea — often the most frequently mentioned, particularly after starting or after a dose adjustment
• Vomiting — less common than nausea but reported by some people, especially early on
• Diarrhoea or constipation — both can occur; some people experience one, some the other, some neither
• Indigestion, reflux, or bloating — a sensation of fullness or discomfort after eating, sometimes with burping

Other commonly reported effects

• Fatigue or low energy — particularly in the early weeks
• Headache or dizziness — often linked to reduced food and fluid intake
• Injection-site reactions — mild redness, itching, or swelling at the site of the injection

These effects are described as common in the official product information for both medicines. They are not universal — many people experience only mild symptoms, and some notice very little. According to NHS guidance, digestive side effects with this class of medicine are typically most pronounced early in treatment and often improve with time.

For a closer look at how these side effects present specifically with tirzepatide, see the full guide to Mounjaro side effects, and for a broader overview of this medication class, the side effects of GLP-1 medicines page covers the shared mechanisms in more detail.

It is worth noting that some effects reported during weight loss injection treatment may also relate to eating less, drinking less, or changes in routine rather than the medicine itself. Keeping a record of what you experience — and when — is one of the most useful things you can do.

One of the most practical things you can do when navigating side effects is to log them consistently. GLP Trackr's side-effect tracker lets you record exactly what you're experiencing — nausea, fatigue, headache, digestive discomfort, injection-site reactions, or anything else — along with the date, time, and severity. Over days and weeks, patterns often emerge: symptoms that cluster just after an injection, improve with food, or fade as a titration settles. That kind of data is genuinely useful to bring to your prescriber or doctor.

The app also includes a daily check-in where you can log energy levels, mood, sleep quality, and how much "food noise" you're experiencing — useful context that sits alongside your side-effect notes and gives a fuller picture of how you're feeling at each stage of your journey.

If you want a clear summary to share at your next appointment, GLP Trackr's doctor report pulls your logged data into a readable format — no trying to remember what happened three weeks ago.

For a comprehensive overview of what's known about this medicine's reported effects, the Mounjaro side effects hub and the related detailed Mounjaro side effects guide are good places to explore further.

A note on safety: most side effects of weight loss injections are mild and temporary, but some symptoms warrant prompt attention. If you experience anything severe, persistent, or that concerns you — including severe abdominal pain, persistent vomiting, signs of an allergic reaction, or significant mood changes — contact your healthcare professional promptly. GLP Trackr helps you record what you're going through so you can discuss it clearly with your prescriber; it is not a substitute for their guidance, and any decisions about your treatment should be made with them.

Side effects should also be reported to your national pharmacovigilance scheme where relevant — for example, the UK's Yellow Card scheme — in addition to telling your prescriber or doctor.

Ready to start building a clear picture of your journey? GLP Trackr is free to try for 7 days — download it on the App Store or Google Play and start logging how you feel from day one.