A bubble under the skin after a Wegovy injection — what's happening and what to do

What the clinical trials recorded about skin reactions at the injection site

The STEP 1 trial (the landmark phase 3 study of semaglutide 2.4mg that informed Wegovy's UK licence) documented injection-site reactions as one of the recognised side-effect categories for subcutaneous GLP-1 medicines, published in the New England Journal of Medicine. Reactions including redness, bruising, and localised raised areas were noted but were generally mild and did not lead to significant numbers of participants stopping treatment. The NHS medicines page for semaglutide lists injection-site reactions (swelling, redness or a lump at the site) as a common side effect of Wegovy, one that typically resolves without intervention.

A bubble or bleb specifically describes what happens when fluid sits in the dermis or very superficial subcutaneous layer rather than dispersing into the fat beneath. The skin there has less capacity to absorb the volume quickly, so it sits proud for a short time. This is not unique to semaglutide; it is a well-recognised finding across all subcutaneous injections. The Patient Information Leaflet (the PIL inside your Wegovy pack) and the full Summary of Product Characteristics on the electronic Medicines Compendium both carry injection technique guidance that helps prevent it.

Why a bubble forms, and what it tells you about technique

Subcutaneous injection is meant to place medicine in the layer of fatty tissue sitting beneath the skin. When the needle enters at too shallow an angle, or when it has not been inserted deeply enough, the tip can end up in the dermis itself (the inner layer of the skin) rather than in the fat below it. The result is a small, dome-shaped bleb that is visible and sometimes tender.

A few things make a shallow deposit more likely. Injecting into an area with very little subcutaneous fat, not pinching the skin to lift the fat away from the muscle, releasing the pinch before the needle is fully out, or pulling back slightly during the injection can all contribute. If you are using a Wegovy pen and find yourself unsure whether an air bubble in your semaglutide injection is playing a role, that is a separate question, a small air bubble in the pen does not cause a skin bleb, though it is worth understanding what those look like too.

Rotating sites helps: the abdomen, outer thigh, and upper arm are the three licensed injection sites. Using the same spot repeatedly can cause localised tissue changes that make absorption less predictable and surface deposits more likely. A site that has been used recently, is bruised, or feels firm is worth avoiding that week.

When a bleb is normal and when to seek advice

Most blebs flatten within a few minutes as the medicine disperses, and if you are wondering whether a bubble visible in your Wegovy pen is related to what you see at the injection site, those are two distinct things worth understanding separately. A small amount of mild redness or a faint pink ring around the site is normal and usually fades within an hour or two. If you notice what feels less like a bubble and more like a firmer lump that persists for days, it is worth reading more about persistent lumps at the Wegovy injection site, which can indicate lipohypertrophy or a localised inflammatory response rather than a simple technique issue.

Contact your prescribing team (or a healthcare professional) if the site becomes hot, increasingly painful, or shows spreading redness over the course of a day or more. These could point to an infection at the injection site, which is uncommon but does require attention. The same applies if you feel generally unwell after an injection or notice a reaction that goes beyond the site itself. You can also report any suspected side effects through the MHRA Yellow Card scheme, which helps regulators monitor real-world safety data for medicines like Wegovy.

One practical note from patients who have been through this: checking technique first thing in the morning before the day gets busy tends to work better than a rushed injection. The 12pm same-day dispatch cut-off at our pharmacy exists for exactly that reason, giving people time to prepare rather than rushing.

Adjusting technique, and when to talk to a prescriber

If blebs are happening regularly, the fix is usually straightforward: slow down the injection, hold the pen completely still until a count of ten after the full dose is delivered, then withdraw cleanly. Pinching a generous fold of skin before inserting, keeping the pen at a 90-degree angle (or the angle specified in your PIL for your needle length), and checking the needle is fully inserted all make a difference. Fresh needles matter too, a needle used more than once blunts quickly, and a blunt tip is more likely to create surface trauma. Replacing the needle for every injection is standard practice, and if you need a reliable supply, compatible replacement needles are available through our pharmacy.

If you are new to Wegovy or have recently moved to a higher dose and found injection-site reactions are increasing, it is worth reviewing the technique section of your PIL. Patients who are wondering about evening injections sometimes find that a consistent routine (same time, same general area rotation) reduces site variability over weeks. For a broader sense of how Wegovy works and what the full side-effect picture looks like, our Wegovy treatment guide covers the evidence in detail. And if cost or access is a question alongside technique, there is honest context on what Wegovy costs privately in the UK.

If you have ongoing concerns about your injection sites, or want a clinical eye on your technique, our prescribers review patients seven days a week. Speak to our prescribers through a free consultation, there is no charge for aftercare support once you are a patient.