What Side Effects Are Commonly Reported with Slimming Injections?
The two types of slimming injections most widely prescribed for weight management are GLP-1 receptor agonists. Wegovy contains semaglutide, which acts on GLP-1 pathways to reduce appetite and slow gastric emptying. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist. Because both medicines affect the digestive system, gut-related side effects are the most frequently reported across both.
Commonly reported side effects
- Nausea — often the most talked-about effect, particularly after a dose increase; many people find it settles within a week or two
- Constipation or diarrhoea — changes in gut motility are common, especially early on
- Indigestion and reflux — slower stomach emptying can contribute to a feeling of fullness or heartburn
- Burping and bloating — also linked to the slower digestive pace these medicines can cause
- Fatigue and headache — reported by some people, often in the days following an injection
- Injection-site reactions — mild redness or discomfort at the site are not unusual
These effects are not universal — individual experience varies considerably, and not everyone will notice all or any of them. According to NHS guidance, most side effects associated with GLP-1 medicines are mild to moderate and often improve over time. You can find factual information about specific medicines on the NHS website.
It is worth knowing that both Mounjaro and Wegovy carry a black triangle (▼) status, meaning they are subject to additional safety monitoring. Any side effects you experience should be reported to your healthcare professional and, where relevant, to your national reporting scheme (such as the UK's Yellow Card scheme or the equivalent in your country).
For a broader look at how GLP-1 medicines affect the body, the page on side effects of GLP-1 medicines covers the wider picture, and you can find detail specific to each medicine on the pages covering side effects of Mounjaro and side effects of weight loss injections.
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Tracking Side Effects with GLP Trackr
One of the most practical things you can do when you notice a side effect is write it down — when it started, how severe it felt, and whether it coincided with a dose or a particular food. That kind of record becomes genuinely useful when you speak with your prescriber.
GLP Trackr's side-effect tracker lets you log symptoms as they happen and review them as a timeline, so patterns that are hard to spot day-to-day become visible over weeks. If nausea tends to peak on the day after your injection, or fatigue clusters around a particular dose stage, the data shows it. You can also use the daily check-in to note energy, mood, and sleep alongside any symptoms — giving you a fuller picture to bring to appointments.
The app also includes a doctor report feature, which pulls your logged data into a clear summary — the kind of record that makes a five-minute appointment far more informative for both you and your prescriber.
If you want to stay on top of the full picture, the Mounjaro side effects hub brings together guidance on the most commonly discussed symptoms and how to track them.
Important: if any side effect is severe, persistent, or causing you concern — or if you experience symptoms such as intense abdominal pain, significant vomiting, or anything that feels serious — contact your healthcare professional promptly. GLP Trackr helps you record what you are experiencing so you can discuss it with your prescriber; it is not a substitute for their advice, and any decisions about your treatment should always be made with them.
Log your symptoms, spot your patterns, and go into every appointment prepared — download GLP Trackr on the App Store or Google Play and start a free 7-day trial.
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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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Frequently asked questions.
The most commonly reported side effects of GLP-1 slimming injections such as Mounjaro and Wegovy are gastrointestinal: nausea, constipation, diarrhoea, indigestion, and bloating. Fatigue, headache, and mild injection-site reactions are also frequently noted. Effects vary between individuals and often ease after the initial adjustment period.
For many people, side effects like nausea are most noticeable in the first days after a dose increase and ease within one to two weeks as the body adjusts. Some people experience little disruption throughout; others find effects persist longer. Your prescriber or doctor is best placed to advise based on your individual experience.
There is significant overlap — both medicines commonly cause gastrointestinal effects such as nausea, constipation, and indigestion, as both affect gut motility. Because tirzepatide (Mounjaro) acts on an additional pathway (GIP), the precise side-effect profile differs slightly. Your prescriber can explain what to expect from the specific medicine you have been prescribed.
Contact your healthcare professional promptly if a side effect is severe, persistent, or worrying — particularly intense abdominal pain, persistent vomiting, signs of an allergic reaction, or anything that feels serious. Do not wait for a scheduled appointment if symptoms concern you.
Logging symptoms as they happen — noting timing, severity, and any patterns — helps you and your prescriber understand how the medicine is affecting you. GLP Trackr's side-effect tracker lets you record symptoms over time and review trends, and produces a summary you can bring to appointments.