What are the known risks and side effects of slimming injections?
Both Mounjaro and Wegovy act on appetite-regulating pathways in the body, and that mechanism can produce a range of side effects — most commonly gastrointestinal. The most frequently reported include nausea, vomiting, diarrhoea, constipation, indigestion, and reflux. These tend to be most noticeable after a new dose or a dose adjustment, and many people find they ease over time.
Gastrointestinal effects
Slower stomach emptying — a common effect of this class of medicine — is often the underlying reason for nausea and feelings of fullness. This can make eating uncomfortable, particularly with larger meals or foods that are high in fat. These effects are commonly reported and are documented in the official patient information for both medicines.
Other commonly reported effects
Beyond the gut, people often report fatigue, headache, dizziness, and reactions at the injection site such as redness, itching, or mild swelling. Rotating your injection site between the abdomen, thigh, and upper arm with each dose is widely recommended to reduce localised irritation — your prescriber or doctor can advise on the right approach for you.
Less common but serious risks
Rarer but more serious concerns associated with this class of medicine include pancreatitis, gallbladder problems, and, in people with diabetes, hypoglycaemia when used alongside other medicines. There are also potential risks in people with a personal or family history of certain thyroid conditions. These are the reasons these medicines are prescription-only and require clinical supervision. According to NHS guidance, any severe or unusual symptoms should be reported to a healthcare professional promptly.
Mental health and mood
Some people report changes in mood while taking these medicines. If you notice low mood, anxiety, or other mental-health changes, speak to your prescriber or doctor without delay — do not wait until your next scheduled appointment.
If you experience any symptom that is severe, persistent, or worrying, contact your healthcare professional promptly. Any suspected serious side effects should also be reported to your national reporting scheme (such as the UK's Yellow Card scheme) and to your prescriber.
For a broader overview of how these medicines work and what to expect, see our guide on weight loss injections. You may also find it useful to read about slimming injections generally and how slimming injections are typically prescribed.
average weekly weight change in the first 16 weeks of Mounjaro & Wegovy use*
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How tracking side effects with GLP Trackr helps
One of the most practical things you can do when monitoring the risks of slimming injections is keep a consistent record of how you feel — and that is exactly where GLP Trackr is built to help.
The app's side-effect tracker lets you log symptoms as they happen: what you experienced, when it started, how severe it felt, and how long it lasted. Over days and weeks, patterns emerge — for example, you might notice that nausea reliably peaks on the day after your injection and settles by day three, or that certain foods make it worse. That kind of personal data is genuinely useful to bring to your prescriber or doctor, and far more reliable than trying to recall it from memory at a clinic appointment.
GLP Trackr also includes an injection-site tracker that logs each site you use and suggests your next rotation — helping reduce the risk of localised injection-site reactions over time. And the doctor report feature pulls your logged symptoms and trends into a clear summary, so you arrive at appointments with the full picture rather than fragments.
GLP Trackr is a tracking and journaling tool — it is not a substitute for the advice of your prescriber or doctor, and nothing logged in the app should replace a clinical conversation. If a symptom concerns you, always contact your healthcare professional directly.
Want to understand more about the injection experience? Our page on what to expect from slimming injections covers further practical detail.
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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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Frequently asked questions.
The most commonly reported side effects of prescribed slimming injections such as Mounjaro or Wegovy include nausea, vomiting, diarrhoea, constipation, and injection-site reactions. Most gastrointestinal effects are associated with slower stomach emptying and often ease over time. Always discuss concerns with your prescriber or doctor.
Most reported side effects are mild to moderate and tend to ease as the body adjusts. However, rarer but more serious risks — including pancreatitis and gallbladder problems — exist. That is why these medicines are prescription-only and require clinical supervision. Contact your healthcare professional promptly if any symptom is severe or persistent.
Some people report mood changes, low mood, or anxiety while taking prescribed slimming injections. If you notice any mental-health changes, speak to your prescriber or doctor without delay rather than waiting for a scheduled appointment.
Logging symptoms as they happen — including severity, timing, and duration — helps you and your prescriber identify patterns. GLP Trackr includes a side-effect tracker and a doctor report feature so you can bring a clear, organised record to your next clinical appointment.
Yes — as well as telling your prescriber or doctor, serious or unexpected side effects from prescription medicines should be reported to your national reporting scheme (such as the UK's Yellow Card scheme). Your prescriber can advise on the correct reporting route in your country.