Where to Jab Mounjaro: the Three Licensed Injection Sites

What the SmPC says about site selection, and why it matters for absorption

The Mounjaro SmPC, published on the electronic Medicines Compendium (eMC), specifies the abdomen, thigh and upper arm as equivalent licensed sites for subcutaneous injection. "Equivalent" is important: studies across GLP-1 medicines have found no clinically meaningful difference in systemic absorption between these three sites when injection technique is correct. The practical implication is that you do not need to favour one site over another for the medicine to work, the choice comes down to comfort, access and consistency of technique.

What does affect absorption is the tissue you inject into. Subcutaneous fat sits just under the skin; if the needle goes too deep and enters muscle, or if you inject into an area with very little subcutaneous tissue, the pharmacokinetic profile can change. This is why the leaflet specifies pinching a skin fold before inserting the needle at a 90-degree angle, a step that becomes especially relevant in leaner areas such as the thigh. If you are unsure which part of the body to use, our page on where to inject Mounjaro explains each licensed site in plain terms. For a closer look at the full injection process from start to finish, our guide to using the Mounjaro KwikPen walks through each step.

A question our prescribers hear most weeks is whether injecting into the abdomen is somehow more effective than the thigh. The answer, based on the licensed instructions and the clinical evidence behind them, is no, provided technique is sound at both sites.

Rotating sites: the practical rule for protecting your skin

Rotation is not optional. Repeatedly injecting the same spot can cause lipohypertrophy, a thickening of subcutaneous tissue that is both visible and capable of impairing drug absorption. The guidance is to change both the broad site (abdomen one week, thigh the next, for example) and the specific spot within that site. In the abdomen, this means working around the navel at least 5 cm away from it; in the thigh, using the outer front surface rather than the inner side.

Many people find it useful to keep a simple rotation log, a note on their phone, or a pattern they repeat in sequence. Some find the thigh the easiest site to reach and see clearly, which is one reason it is popular for self-injection. If you want detail specifically on the thigh as a site, our thigh injection guide covers positioning and technique. For a broader overview of all three sites with diagrams, the injection sites overview is worth reading alongside this page.

Avoid areas with active skin conditions, recent bruising or any scar tissue. Tattoos are also listed as skin to avoid, since the altered tissue can interfere with consistent needle depth.

Needle length, pen mechanics and getting the angle right

The Mounjaro KwikPen uses a fixed needle that does not need to be changed between doses, a single pen contains four weekly doses. However, the needle is attached to a disposable tip, and each injection requires a fresh tip for safety and sterility reasons. If you are using replacement needle tips, ensure they are compatible with the KwikPen before use. Our needle tips page explains compatibility.

At the injection site, clean the skin with an alcohol swab and allow it to dry fully before inserting the needle; wet skin increases discomfort and reduces the swab's antiseptic effect. Hold the pen firmly against the skin, press the injection button, and keep the pen in place for the full count recommended in the leaflet (typically around ten seconds) to ensure the full dose is delivered before withdrawing. The used pen should go into a sharps container immediately after use. NHS guidance and local council websites explain how to dispose of sharps containers safely once full.

Storage matters too: Mounjaro pens should be kept refrigerated between 2–8°C until needed. For the exact room-temperature window that applies once a pen is removed from the fridge, always check the Patient Information Leaflet directly rather than relying on any secondary source. Our Mounjaro storage guide covers the key points.

When to raise injection-site concerns with your prescriber

Mild redness or tenderness immediately after an injection is common and usually settles within a day or two. Persistent swelling, a lump that does not resolve, or skin that feels hard at the injection site after several weeks are worth mentioning at your next clinical review. These can be signs that rotation needs adjusting or that technique needs a small correction.

More significant reactions (spreading redness, warmth, pus, or a lump that grows rather than shrinks) should be assessed by a healthcare professional promptly rather than at the next routine review. Severe systemic reactions such as difficulty breathing, swelling of the face or throat, or a rapid rash following an injection require emergency medical attention. The MHRA Yellow Card scheme allows patients to report unexpected injection-site reactions as suspected side effects, which helps the regulator monitor safety signals in real-world use.

If you have questions about your injection sites as part of a broader conversation about starting treatment, you can start a free consultation with our prescribers, who review every patient personally.