What side effects are commonly reported with weight loss injections?
The most frequently reported side effects with GLP-1 receptor agonist injections — the class that includes both tirzepatide and semaglutide — are gastrointestinal in nature. These include nausea, vomiting, diarrhoea, constipation, bloating, and indigestion. Other commonly reported effects include fatigue, headache, dizziness, and reactions at the injection site.
Why these effects happen
These medicines act on appetite-regulating pathways in the body and slow the rate at which the stomach empties. This slowing effect is understood to be a key reason why nausea and digestive discomfort are so commonly reported, particularly in the weeks after starting or after a dose is adjusted upward. For many people, these effects settle over time as the body adapts.
Do the two medicines differ in their side-effect profiles?
Mounjaro (tirzepatide) acts on both GIP and GLP-1 receptors, while Wegovy (semaglutide) acts on GLP-1 receptors alone. Both share a broadly similar gastrointestinal side-effect profile. Individual experience varies, and there is no universal answer about which causes fewer side effects for any given person — that judgement belongs with your prescriber. For a more detailed look at how the two compare, see our guide to which GLP-1 medicine has the least side effects.
Timing and patterns
Side effects tend to cluster around the first few days after a dose, especially following an increase in strength. Many people report that the overall burden reduces once they have been on a stable dose for several weeks. You can read more about what to expect in our overview of side effects of weight loss injections and our dedicated page on commonly reported Wegovy side effects.
If any side effect is severe, persistent, or causing concern, contact your healthcare professional promptly rather than waiting for your next scheduled appointment.
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Tracking side effects to understand your own pattern
One of the most practical things you can do when experiencing side effects is log them consistently — noting what you felt, when it happened, and how intense it was. Over time, this kind of record reveals patterns that are genuinely useful to discuss with your prescriber: whether symptoms cluster after each dose, whether they are improving, or whether something feels different from your usual experience.
GLP Trackr's side-effect tracker is built for exactly this. Each time you notice a symptom — nausea, fatigue, digestive discomfort — you can log it in a few taps, with a severity rating and timestamp. The app then shows you trends across your journey, so you can see at a glance whether things are easing week by week. When you head into an appointment, the doctor report feature gives your prescriber a clear summary of what you have been experiencing, rather than relying on memory.
For a broader view of what people on Mounjaro commonly report, our Mounjaro side effects hub covers the full picture in one place.
GLP Trackr is a tracking and journaling tool — it is not a substitute for your prescriber's advice, and any decision about your dose or how to manage a side effect should always be made with your healthcare professional, not on your own.
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Frequently asked questions.
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) share a broadly similar gastrointestinal side-effect profile. Individual experience varies significantly, and the right choice for any person depends on their full health history — a decision that belongs with their prescriber or doctor, not a general comparison.
Yes, gastrointestinal effects such as nausea, constipation, and indigestion are among the most commonly reported experiences on GLP-1 weight loss injections. They tend to be most noticeable after starting or after a dose adjustment, and many people find they ease as the body adapts over several weeks.
If a side effect is severe, persistent, or feels unusual compared to your normal experience, contact your healthcare professional promptly. Do not wait for a routine appointment. Symptoms such as severe abdominal pain, persistent vomiting, or signs of an allergic reaction warrant prompt medical attention.
Logging each symptom — its type, severity, and timing relative to your dose — helps you and your prescriber see whether side effects are improving or following a pattern. GLP Trackr includes a dedicated side-effect tracker that records this over time and can generate a summary for your prescriber appointments.
Many people report that gastrointestinal side effects, particularly nausea, are most prominent early in treatment or after a dose increase and become less noticeable over time. This is not universal — experience varies — and if effects are ongoing or distressing, discussing them with your prescriber or doctor is always the right step.