Side Effects of Weight Loss Injections: What People Commonly Experience

GLP-1 receptor agonists and dual GIP/GLP-1 medicines work partly by slowing the rate at which the stomach empties and by acting on appetite-regulating pathways in the brain. These same mechanisms are behind many of the gastrointestinal effects people experience, particularly early in treatment or after a dose change.

Commonly reported effects

• Nausea — often the most frequently mentioned; tends to peak in the hours after an injection and typically lessens over several weeks
• Vomiting — less common than nausea but reported by some people, especially at higher doses
• Diarrhoea or constipation — both can occur; drinking enough water and eating adequate fibre can help, though any persistent change is worth discussing with your prescriber
• Indigestion and reflux — slower stomach emptying can make these more noticeable
• Burping and bloating — a common companion to slower gastric transit
• Fatigue and headache — reported by some people, particularly in the early weeks
• Injection-site reactions — redness, mild swelling or itching at the site are occasionally reported

These lists are not exhaustive. Every person responds differently, and the pattern of side effects can vary between medicines. For a closer look at the side-effect profile of specific medicines, see our pages on common Mounjaro side effects and side effects reported with Wegovy.

What about newer injectable medicines such as retatrutide?

Research into newer injectable agents — including retatrutide, a triple GIP, GLP-1, and glucagon receptor agonist — is ongoing. Because these medicines are at an earlier stage of evaluation, the long-term side-effect profile is less established than for medicines already in wider clinical use. If you have questions about any medicine you have been prescribed, your prescriber or doctor is the right person to ask. GLP Trackr is designed to support people tracking an already-prescribed injectable medicine; always rely on your prescriber for guidance on the specific medicine you are taking.

Some people also report effects relating to the urinary tract; if that is a concern for you, our page on urinary side effects associated with Mounjaro covers what is commonly reported in more detail.

One of the most useful things you can do when side effects arise is record them consistently — when they happen, how long they last, and how they relate to your injection schedule. A clear log turns vague recall into something concrete you can bring to your next appointment.

GLP Trackr's side-effect tracker is built exactly for this. Each time you notice a symptom — nausea after an injection, an afternoon of fatigue, an unsettled stomach — you can log it in seconds. Over time the app surfaces patterns: is nausea reliably appearing on injection day and clearing by day two? Does tiredness cluster around dose changes? Seeing those correlations laid out clearly is far more useful than trying to remember weeks later.

The app's daily check-in also captures mood and energy alongside your symptom log, so you get a fuller picture of how you are feeling across the week, not just on bad days. When you do see your prescriber, the doctor report feature lets you share a tidy summary of your tracked journey — symptoms, doses, and progress in one place — rather than piecing it together from memory.

If any symptom feels severe, is not improving, or is worrying you, contact your healthcare professional promptly. GLP Trackr helps you record what you experience so you can discuss it with your prescriber — it is not a substitute for their clinical advice, and any decisions about your treatment belong with them. For a broader overview of what to expect, our guide to managing side effects on Mounjaro goes into further detail.

You can log your side effects, spot your own patterns, and arrive at every appointment better prepared — GLP Trackr is free to try for seven days on the App Store and Google Play.