What's Actually in the Weight Loss Injections?

Understanding what's inside your pen helps make sense of the effects — and side effects — you may notice over time.

Semaglutide (Wegovy)

Semaglutide is a GLP-1 receptor agonist — a molecule that mimics glucagon-like peptide-1, a natural gut hormone involved in regulating appetite and blood sugar. By activating GLP-1 receptors in the brain and digestive tract, semaglutide can reduce appetite, slow stomach emptying, and increase the sense of fullness after eating. Many people describe a significant reduction in constant thoughts about food, sometimes called "food noise."

Tirzepatide (Mounjaro)

Tirzepatide acts on two receptors: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP is another gut hormone involved in appetite and metabolism. This dual action is what sets tirzepatide apart mechanically from semaglutide, though both medicines are in the same general class and share many of the same commonly reported effects.

What else is in the injection?

Beyond the active substance, each pen contains a sterile solution with inactive ingredients — such as buffers to maintain stability and a preservative or solubiliser depending on the formulation. The exact excipient list for each product is published on the electronic Medicines Compendium (eMC), and your patient information leaflet will list everything in your specific pen. If you have any known allergies or sensitivities, check the full ingredient list with your prescriber or pharmacist before starting.

What these injections are not

• They are not insulin — although some of their effects overlap with diabetes management, they are not the same class of medicine as insulin.
• They are not a permanent change to your body's chemistry — they work while you are taking them, under the supervision of your prescriber.
• They are not suitable for everyone — eligibility is determined by your prescriber or doctor based on your individual health history.

Both Mounjaro and Wegovy are prescription-only medicines subject to additional monitoring. If you notice any unexpected reactions, report them to your healthcare professional and to your national reporting scheme (such as the UK's Yellow Card scheme).

For broader context on how these medicines are used and what to expect, the complete guide to weight loss injections covers the full picture in one place.

If you are researching the financial side of treatment, you may also find it useful to explore how much the weight loss injection typically costs and factors that affect weight loss injection pricing.

Knowing the ingredients is one thing — understanding how the medicine is affecting you personally is something else entirely, and that's where tracking becomes genuinely useful.

The most relevant feature here is side-effect and symptom logging. Because both tirzepatide and semaglutide act on the digestive system, many people notice gastrointestinal effects — nausea, reflux, changes in appetite — particularly in the days following a dose. GLP Trackr lets you log symptoms as they happen and view them over time, so you can start to see patterns: does nausea peak on the day after your injection? Does it ease by day three or four? That kind of personal data is far more useful to bring to a prescriber appointment than a vague recollection.

The app also tracks your injection site and rotation history. Each week when you log your shot — recording the date, dose strength, and injection site — GLP Trackr keeps a running history and suggests your next rotation site, which helps reduce localised irritation over time.

If you ever want to share a clear summary of your journey with your prescriber — symptoms logged, doses recorded, progress noted — the doctor report feature brings it all together in one place.

Always follow the guidance of your prescriber or doctor regarding your medication — they know your full medical history and are the right person to advise on anything related to your dose, your suitability, or any symptoms you are experiencing.

Ready to start making sense of how your injection is affecting you, week by week? Download GLP Trackr on the App Store or Google Play — your first 7 days are completely free.