What Are the Active Ingredients in Weight Loss Injections?

Understanding what is in your prescribed injection can help you follow your treatment more confidently and have better-informed conversations with your prescriber.

Tirzepatide (Mounjaro)

Tirzepatide is the active ingredient in Mounjaro. It acts on two receptors — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — both of which are involved in regulating appetite and blood sugar. Many people prescribed Mounjaro report a noticeable reduction in appetite and in what is often described as "food noise". Tirzepatide is supplied in pre-filled pens at dose strengths that are typically started low and adjusted over time by the prescriber. According to the electronic Medicines Compendium, tirzepatide is the first dual GIP and GLP-1 receptor agonist approved for weight management.

Semaglutide (Wegovy)

Semaglutide is the active ingredient in Wegovy. It acts specifically on the GLP-1 receptor, mimicking a naturally occurring hormone that signals fullness to the brain and slows stomach emptying. This commonly contributes to a reduced appetite. Like tirzepatide, the dose is typically introduced at a low starting level and adjusted according to the prescriber's plan. For more on how these medicines compare in terms of what they are, visit our page on Mounjaro weight loss injections.

What else is in the pen?

Beyond the active ingredient, both pens contain excipients — inactive ingredients such as buffers, preservatives, and carriers that keep the medicine stable and safe to inject. These vary between products. If you have any allergies or sensitivities, always check the full ingredients list in the patient information leaflet that came with your pen, or ask your prescriber or pharmacist. The complete composition for each product is also listed on the electronic Medicines Compendium (eMC), which holds the official product information.

It is worth noting that both Mounjaro and Wegovy carry a black triangle (▼) designation, meaning they are subject to additional monitoring. Any side effects should be reported to your healthcare professional and, where applicable, to your national reporting scheme (such as the UK's Yellow Card scheme).

If you are curious about what to expect in terms of weight loss injections generally, our main guide covers the broader landscape in more detail.

Knowing what is in your injection is just the start — staying consistent with your weekly schedule and monitoring how your body responds are what make a real difference over time. GLP Trackr's injection-site tracking and rotation feature is built precisely for this: each time you inject, you log the site (abdomen, thigh, or upper arm), and the app keeps a full rotation history so it can suggest where to inject next. Rotating sites is generally recommended to support skin and tissue health, and having an automatic log removes the guesswork.

Alongside site tracking, the app's dose logging lets you record the strength and date of every injection, so you always have a clear timeline to refer back to — useful both for your own peace of mind and when attending a review with your prescriber. The side-effect tracker is equally relevant here: if you notice a reaction around an injection site, or a pattern of symptoms after each dose, logging it in the app means you have something concrete to discuss at your next appointment rather than relying on memory.

If you have questions about what you are paying for your prescription, our page on weight loss injection costs addresses the topic in a factual, neutral way.

Always follow the guidance of your prescriber or doctor regarding your injection schedule and technique — they know your full medical history. If you experience anything severe, persistent, or concerning after an injection, contact your healthcare professional promptly. GLP Trackr is here to support your tracking; it is not a substitute for clinical advice.

Ready to bring your injection schedule together in one place? GLP Trackr is free to try for 7 days on the App Store and Google Play — log your sites, track your doses, and carry a clear summary into every prescriber appointment.