Common Side Effects of Weight Loss Injections
Both Mounjaro and Wegovy work on appetite-regulating pathways in the body — Mounjaro as a dual GIP and GLP-1 receptor agonist, Wegovy as a GLP-1 receptor agonist. One effect they share is slowing gastric emptying, which is thought to contribute to several of the gastrointestinal side effects people commonly report.
Digestive side effects
The most frequently reported side effects of slimming injections are gastrointestinal in nature. These include:
- Nausea — often most noticeable in the first day or two after an injection, or following a dose increase
- Vomiting — less common than nausea, but reported by some users
- Diarrhoea or constipation — both can occur, sometimes alternating
- Indigestion, reflux, or bloating — linked to slower stomach emptying
- Burping — a commonly noted and sometimes unexpected effect
Other commonly reported effects
Beyond the digestive system, people on weight loss injections also report:
- Fatigue or low energy — especially in the early weeks
- Headache or dizziness
- Injection-site reactions — mild redness, itching, or tenderness where the pen was used
It is worth noting that these are commonly reported effects, not universal ones — many people experience only mild symptoms or none at all. According to NHS guidance, most side effects associated with GLP-1 medicines are mild to moderate and tend to settle with time.
For a closer look at how Mounjaro's side effects are documented, the full Mounjaro side effects overview covers the picture in more detail. If you are taking Wegovy, the Wegovy side effects page walks through what semaglutide users commonly report.
Some people also notice changes related to the urinary tract; if that is something you have experienced, urinary side effects on Mounjaro explores what is known.
average weekly weight change in the first 16 weeks of Mounjaro & Wegovy use*
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Tracking Your Side Effects — and Why It Matters
Keeping a record of the side effects you notice — when they happen, how severe they feel, and how long they last — turns vague discomfort into useful information you can bring to your prescriber. That conversation is far easier when you have a clear log rather than a rough memory.
GLP Trackr's side-effect tracker is built specifically for this. Each time you notice a symptom, you can log it in seconds: the type of effect, its severity, and the date and time. Over days and weeks, the app surfaces patterns — for instance, whether nausea reliably peaks in the 24 hours after your injection, or whether fatigue correlates with lower protein intake on a particular day. The insights view brings these correlations together so you arrive at your next prescriber appointment with something concrete to discuss.
The app also lets you log your injection date, dose strength, and injection site alongside your side-effect entries — so if you notice that a particular site produces more localised reactions, that pattern becomes visible too.
If you experience any side effect that is severe, persistent, or feels worrying, contact your healthcare professional promptly rather than waiting for your next scheduled appointment. GLP Trackr is a tracking tool — it supports your journey and helps you have better-informed conversations with your prescriber, but it is not a substitute for clinical advice. Any concerns about whether a symptom is serious should always go to your doctor or prescriber first.
It is also worth knowing that both Mounjaro and Wegovy carry additional monitoring requirements — if you notice a new or unexpected side effect, reporting it through your national pharmacovigilance scheme (such as the UK's Yellow Card scheme) as well as telling your healthcare professional is encouraged.
Ready to start logging how you feel? GLP Trackr is free to try for 7 days — available on the App Store and Google Play. Your side-effect history, dose log, and insights are all in one place from day one.
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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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Frequently asked questions.
Nausea, diarrhoea, constipation, indigestion, fatigue, and injection-site reactions are among the most commonly reported side effects of GLP-1 weight loss injections such as Mounjaro and Wegovy. Most are mild to moderate and tend to improve as the body adjusts.
Many people find that side effects like nausea are most noticeable in the first few days after starting or after a dose adjustment, and often ease within one to two weeks. Individual experience varies — if symptoms persist or feel severe, speak with your prescriber or doctor.
Yes, nausea is one of the most commonly reported effects of both Mounjaro (tirzepatide) and Wegovy (semaglutide). It is thought to be related to slower gastric emptying. It is commonly reported as most noticeable shortly after an injection and tends to lessen over time for many people.
Contact your prescriber or doctor promptly if a side effect is severe, does not ease, or feels worrying. Symptoms such as persistent vomiting, severe abdominal pain, or any effect that concerns you should always be assessed by a healthcare professional rather than managed alone.
Logging each symptom — its type, severity, and timing — lets you spot patterns and share a clear record with your prescriber. GLP Trackr's side-effect tracker lets you do this alongside your dose and injection-site log, making it easier to have informed conversations at appointments.