What are the signs of a potential allergic reaction to Wegovy?

Seek urgent help for widespread hives, facial, lip, or tongue swelling, wheeze or breathing difficulty, dizziness or faintness, or a rapidly spreading rash. Local redness the size of a coin is common and usually settles; large, hot, or painful swelling needs clinical review. Stop using the pen until you’ve been assessed if you suspect a true allergic reaction.

Understanding allergic reactions to medicines

Allergic reactions happen when the immune system mistakes part of a medicine for a harmful substance. With Wegovy (semaglutide), this response is rare but possible, just as with any injectable medication. NICE TA875 and the MHRA’s product safety sheet both note that mild local irritation at the injection site is common, but serious allergic reactions (such as anaphylaxis) occur in fewer than 1 in 1,000 cases. It’s important to distinguish between expected local effects — like brief redness or itching — and the more serious signs of a systemic allergic response. Recognising this difference helps people react quickly but not panic unnecessarily.

Typical mild skin reactions

After injecting Wegovy, it’s not unusual to see a small area of redness or slight itching at the site. These mild reactions usually appear within minutes and fade within 24–48 hours. They happen because the body is responding to the needle puncture and the medication’s presence under the skin, not because of a true allergy. STEP trial data and NHS guidance both confirm that such local effects are short-lived and do not require stopping treatment. A cool compress or mild antihistamine cream can soothe mild irritation if needed, though most people find it settles on its own.

Signs of a possible allergic response

A genuine allergic reaction usually looks and feels different. Key warning signs include:

• Sudden or widespread hives or raised, itchy welts.

  
  

• Swelling of the face, lips, tongue, or throat.

  
  

• Wheezing, shortness of breath, or chest tightness.

  
  

• Dizziness, fainting, or a feeling of pressure in the chest.

  
  

• A rapidly spreading rash or severe itching over large areas of the body.

  
  

If any of these occur, stop using the pen and seek emergency help immediately by calling 999 or attending A&E. MHRA safety updates classify these symptoms as potential signs of anaphylaxis, a serious but treatable allergic reaction. Prompt medical attention is essential, even if symptoms seem to ease.

When to contact your clinician instead of emergency services

Not all skin changes require urgent help. Redness or itching around the injection site, up to about the size of a two-pound coin, can be watched at home. However, if the area becomes larger, warm, firm, or painful over a few days — or if the redness continues to expand — contact your clinician or pharmacist. This may indicate local infection or inflammation rather than an allergy. The NHS and NICE both advise pausing treatment until a healthcare professional has reviewed the area. They may recommend an alternative injection site, a change in needle length, or temporary suspension of treatment while checking for other causes.

Understanding why reactions occur

Semaglutide itself is unlikely to cause an immune reaction, as it closely resembles a natural human hormone (GLP-1). Most allergic-type symptoms arise from sensitivity to other components in the pen, such as preservatives or the needle material. Some patients have found it helpful to rotate sites or change from one body area to another to reduce irritation. The MHRA’s 2024 device safety review also identified that incorrect injection depth — particularly injecting into muscle instead of the fatty layer — can lead to local inflammation that mimics an allergy. This is why injection training with an NHS nurse or pharmacist is so important before and during the early stages of treatment.

Steps to reduce risk

A few simple habits can help prevent confusion or avoid unnecessary discomfort. Always inspect the pen before use; if the solution looks cloudy, discoloured, or contains particles, do not inject it. Clean the skin gently with soap and water or an alcohol swab and allow it to dry completely before injection. Rotate sites each week and avoid areas that are already red or irritated. Use a fresh needle every time, and never share pens or needles, as sharing can introduce irritants or bacteria. NHS training materials also recommend noting the date, site, and any reaction in a treatment diary to help your clinician identify triggers.

Managing anxiety about reactions

Understandably, people sometimes feel anxious about the possibility of an allergic reaction — especially when starting an injectable medicine. NICE guidance and NHS support frameworks emphasise the importance of reassurance and clear communication. Most skin reactions are mild, temporary, and manageable. Talking to your clinician before starting treatment can help you understand what’s normal and what’s not. NHS weight-management programmes often include nurse-led helplines, where you can describe symptoms and get same-day advice on whether to continue or pause injections. This practical support helps maintain confidence and safety without unnecessary worry.

Looking at the whole story

True allergic reactions to Wegovy are rare, while mild local irritation is relatively common and usually harmless. Knowing what to watch for — and what doesn’t need alarm — helps you stay calm and act quickly if needed. NICE, MHRA, and NHS guidance all align: mild redness and itching can be managed at home, but widespread swelling, rash, wheezing, or dizziness require immediate medical help. Looking at the whole story, the key to safety lies in awareness, proper technique, and open communication with your healthcare team — ensuring that confidence, not concern, shapes your treatment experience.