Side Effects of Weight Loss Injections: What People Commonly Experience

The most frequently reported side effects sit in the gastrointestinal category. Nausea is the one most people mention first, often peaking in the day or two after an injection and easing before the next dose. Alongside nausea, people commonly report:

• Vomiting — usually short-lived, most often tied to dose increases
• Diarrhoea or constipation — both can occur; the pattern varies from person to person
• Indigestion, reflux, or bloating — linked to slower stomach emptying, an effect of how these medicines act on GLP-1 (and, in the case of tirzepatide, GIP) pathways
• Fatigue and headache — reported especially in early weeks
• Injection-site reactions — redness, mild swelling, or itching at the site of the injection, which typically resolve quickly
• Burping — a less-discussed but fairly common experience

These medicines work partly by slowing gastric emptying and reducing appetite signals in the brain. According to information held in the electronic Medicines Compendium (eMC), gastrointestinal effects are among the most commonly listed in the product information for both medicines. They are not universal, and not everyone will experience all — or any — of them.

Timing matters too. Side effects tend to cluster around the first few days after each injection and are often more noticeable after a dose step-up. Many people find they become less prominent as weeks go on. If you want to understand how side effects have been reported across the wider GLP-1 experience, the general guide to side effects of weight loss injections covers this in more detail, as does the overview of Mounjaro-specific side effects.

It is worth knowing that both medicines carry a Black Triangle (▼) designation, meaning healthcare professionals and patients are encouraged to report any side effects through their national pharmacovigilance scheme — such as the UK's Yellow Card scheme, or the equivalent reporting system in your country. Your prescriber or doctor can advise on this.

Keeping a clear record of when side effects happen, how severe they feel, and how long they last can be genuinely useful — both for your own understanding and when discussing your experience with your prescriber.

GLP Trackr's side-effect tracker is built exactly for this. You can log symptoms as they arise — nausea, fatigue, indigestion, injection-site reactions, or anything else you notice — and the app organises them over time so you can see whether there is a pattern. Does nausea always hit on day two after your injection? Does fatigue correlate with a poor night's sleep? These are the kinds of connections that are hard to spot in the moment but become visible when the data is in one place.

The daily morning check-in feature lets you capture mood, energy, and how you're feeling in under a minute, adding context to the side-effect log. And if you want to bring a clear picture to your next appointment, the doctor report feature pulls your tracked data into a format you can actually share.

For more on what Mounjaro users in particular report, see the detailed look at Mounjaro side effects over time, or visit the full Mounjaro side effects hub for a comprehensive overview.

GLP Trackr helps you record what you are experiencing so you can have better, more informed conversations with your prescriber — it is not a substitute for their guidance. If any side effect feels severe, does not ease, or worries you, contact your healthcare professional promptly. Do not adjust your dose based on side effects alone; that decision belongs with your prescriber.

Start logging your symptoms today — GLP Trackr is free to download on the App Store and Google Play, with a 7-day free trial to explore everything it offers.