Commonly Reported Side Effects of Weight Loss Injections
Both Mounjaro and Wegovy are prescription-only injectable medicines that act on appetite-regulating pathways in the body. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist. Because they slow stomach emptying and affect digestive processes, gastrointestinal side effects are the most frequently reported — though not everyone experiences them, and they vary in intensity.
Gastrointestinal effects
- Nausea — commonly reported, particularly after starting or increasing a dose
- Vomiting — less common but reported by some people
- Diarrhoea or constipation — both can occur; some people experience one or the other
- Indigestion and reflux — a feeling of fullness, bloating, or acid discomfort
- Burping — especially around mealtimes
Other commonly reported effects
- Fatigue — a sense of tiredness or low energy, particularly early on
- Headache or dizziness — reported by some users, often in the early weeks
- Injection-site reactions — redness, itching, or mild swelling at the site of the injection
These effects are described as common across clinical information sources. According to NHS guidance, gastrointestinal symptoms are the most frequently observed class of side effect with this type of medicine. For a closer look at how these effects specifically present with tirzepatide, the page on side effects of Mounjaro goes into more detail, and if you are on semaglutide, the more severe side effects of Wegovy covers the less common but more serious effects worth knowing about.
It is worth noting that side effects often follow a pattern — they can flare briefly after a dose step-up and then settle. However, if any symptom feels severe, persistent, or worrying, contact your prescriber or doctor promptly rather than waiting it out. Certain symptoms — including severe or persistent abdominal pain, signs of an allergic reaction, or significant changes in mood — should always be discussed with a healthcare professional without delay.
average weekly weight change in the first 16 weeks of Mounjaro & Wegovy use*
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Tracking Side Effects on Your Weight Loss Injection Journey
One of the most useful things you can do when experiencing side effects is to keep a clear record of them — when they occur, how intense they are, and how long they last. This kind of log gives your prescriber or doctor genuinely useful information at your next appointment, rather than relying on memory.
GLP Trackr's side-effect tracking feature is built exactly for this. You can log symptoms as they happen — nausea, fatigue, digestive discomfort, or anything else — and the app builds a timeline so you can start to see patterns: whether symptoms cluster after your weekly injection, ease off mid-week, or seem connected to what you have eaten. That insight is something you can bring directly to your prescriber, rather than describing things vaguely from memory.
Alongside side-effect logging, the daily check-in lets you record how your energy and mood feel each day — helpful context when you are trying to work out whether tiredness is linked to a recent dose or something else entirely. The app also keeps a record of each injection, including the date and dose, so there is always a clear picture of your treatment history to refer back to.
Our wider guide to managing side effects on Mounjaro and similar weight loss injections covers the full picture, and for a broader overview of what people commonly experience, this overview of weight loss injection side effects is a useful companion read.
GLP Trackr is not a substitute for your prescriber's advice — any decision about your treatment, or any concern about a symptom, should always go to your healthcare professional. The app helps you record what is happening so those conversations are clearer and more informed.
Start logging your side effects and spotting your own patterns — GLP Trackr is free to try for 7 days on the App Store and Google Play.
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Start every day with a 60-second wellness check-in. Log how you're feeling, track nausea, appetite, energy, and mood — giving you and your care team a complete picture of how your medication is working.
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Log every dose with a single tap. Track your dose history, phase in your cycle, estimated medication levels, and upcoming injection dates — with reminders so you never miss a scheduled shot.
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Smart Meal Planning
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Beautiful, easy-to-read charts show your weight loss trajectory, weekly nutrition trends, medication responses, and wellbeing scores over time — turning raw data into genuine insights.
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Frequently asked questions.
The most commonly reported side effects are gastrointestinal — nausea, constipation, diarrhoea, indigestion, and burping. Fatigue, headache, dizziness, and mild injection-site reactions are also frequently reported. These effects are not universal and vary between individuals.
For many people, side effects such as nausea are most noticeable in the first few days after starting or increasing a dose, and often ease within one to two weeks as the body adjusts. Individual experience varies; always discuss persistent symptoms with your prescriber or doctor.
Both medicines share a similar gastrointestinal side-effect profile, as both act on GLP-1 pathways. Mounjaro also acts on GIP receptors. The nature and intensity of side effects can differ between individuals and between medicines — your prescriber is best placed to advise on your specific experience.
Contact your prescriber or doctor promptly if any side effect is severe, persistent, or worrying — particularly severe abdominal pain, signs of an allergic reaction, significant mood changes, or vomiting that prevents you from eating or drinking. Do not wait for a scheduled appointment if symptoms concern you.
Logging symptoms as they happen — noting severity, timing, and duration — gives your prescriber useful information. GLP Trackr's side-effect tracker lets you record symptoms and spot patterns over time, so you can have a clearer, more informed conversation at your next appointment.