Slim Injection Side Effects: What People on GLP-1 Medicines Commonly Experience

GLP-1 receptor agonists like semaglutide, and dual GIP/GLP-1 agonists like tirzepatide, work partly by slowing the rate at which the stomach empties. This mechanism is behind many of the gastrointestinal side effects that people notice, particularly early in their treatment or after a dose step-up.

Commonly reported gastrointestinal effects

• Nausea — often the most frequently mentioned; tends to peak shortly after an injection and settle over time
• Vomiting — less common than nausea; if persistent, discuss with your prescriber or doctor
• Diarrhoea or constipation — either can occur; staying well hydrated and eating smaller meals may help many people
• Indigestion, reflux, or bloating — related to slower gastric emptying
• Burping — commonly reported and often linked to changes in digestion

Other commonly reported effects

• Fatigue or low energy, particularly in the days immediately following an injection
• Headache or dizziness
• Injection-site reactions such as redness, mild bruising, or itching at the site used

These effects are described as common in the product information for both medicines. They are not universal — people's experiences vary considerably. For a broader overview, see our guide to managing side effects on Mounjaro, or if you are specifically interested in reactions at the injection site, that page covers site-specific experiences in more detail.

If you are on semaglutide (Wegovy), our page on side effects of Wegovy covers what people commonly report on that medicine.

Any side effect that is severe, persistent, or feels unusual should be raised with your healthcare professional promptly. In particular, seek medical attention if you experience severe abdominal pain that radiates to your back, persistent vomiting, or any symptoms that concern you — these may need clinical assessment. According to NHS guidance, serious side effects should always be reported to a healthcare professional, and you can also report them to your national pharmacovigilance scheme (such as the UK's Yellow Card scheme or the equivalent in your country).

One of the most useful things you can do when you notice a side effect is to log it — when it happened, how strong it felt, and how long it lasted. Patterns that seem invisible day-to-day become clear when you look back over a few weeks of data.

GLP Trackr's side-effect tracker is built exactly for this. Each time you notice nausea, fatigue, digestive discomfort, or any other symptom, you can log it in seconds. Over time the app surfaces those patterns — whether symptoms tend to cluster in the 24 hours after your injection, whether they are becoming less frequent, or whether something new has appeared. That record becomes genuinely useful when you sit down with your prescriber.

The app's daily check-in also lets you note your energy levels and general wellbeing each morning, so the broader picture of how you are feeling through your treatment is captured automatically alongside your symptom logs.

If you want to bring a clear summary to your next appointment, GLP Trackr can generate a doctor report from your logged data — a straightforward way to show your prescriber exactly what you have been experiencing and when.

For more on injection-related experiences specifically, the page on Mounjaro injection side effects covers what is documented in clinical guidance. And for a full overview of side effects across the treatment journey, visit our Mounjaro side effects hub.

GLP Trackr helps you record and organise your experience — it is not a substitute for advice from your prescriber or doctor, who knows your full medical history. If symptoms are severe, persistent, or worrying, contact your healthcare professional promptly rather than waiting for your next scheduled appointment.

Start logging your side effects and see your patterns clearly — download GLP Trackr free on the App Store or Google Play and explore it with a 7-day free trial.