What the labels and EU product information report
Each US label says serious hypersensitivity reactions such as anaphylaxis and angioedema have been reported. If one occurs, the medicine should be stopped, the reaction treated and the person watched until it settles [1][2][3][4][5][6]. Section 4 of each label lists a previous serious hypersensitivity reaction to the active ingredient, or to any inactive ingredient, as a contraindication [1][4][5][6]. The table compares the trial figures and the EU frequency classes.
- In the EU classes, uncommon means up to 1 person in 100 and rare up to 1 in 1,000 [7].
- The Ozempic® label gives no trial rate; its serious reactions come from post-marketing reports, which cannot give a reliable frequency [1].
| Document | Warning section | Allergic reactions in trials | Serious reactions |
|---|---|---|---|
| Ozempic®, US label [1] | 5.8 | No trial rate given | Anaphylaxis, angioedema, rash and hives in post-marketing reports |
| Wegovy®, US label [2] | 5.7 | Adolescent trial: rash 3% and hives 3%, against 0% on placebo | Anaphylaxis and angioedema reported |
| Mounjaro®, US label [4] | 5.4 | 3.2% vs 1.7% on placebo, sometimes severe | Anaphylaxis and angioedema in post-marketing reports |
| Zepbound®, US label [3] | 5.6 | 5% on each dose vs 3% on placebo | Severe reactions 0.1% vs none on placebo |
| Saxenda®, US label [5] | 5.8 | Hives 0.7% vs 0.5% on placebo | Anaphylaxis in trials and marketed use |
| Foundayo™, US label [6] | 5.6 | 0.5% vs 0.3% on placebo, including anaphylactic reaction | Included in the trial figure |
| Ozempic®, EU product information [7] | 4.8 | Rash, itching or hives: uncommon | Anaphylaxis and angioedema: rare |
| Mounjaro®, EU product information [8] | 4.8 | Common; 5.0% vs 3.8% on placebo in weight trials | Anaphylaxis and angioedema: rare |
| Saxenda®, EU product information [9] | 4.8 | Allergic reactions including rash: uncommon | Anaphylaxis: rare |
Timing, antibodies and earlier reactions
The Zepbound® label separates reactions within a day of an injection, 2.1% against 0.4% on placebo, from later ones, 3.5% against 2.7%; most were skin reactions such as rash or itching [3]. MedlinePlus explains that a drug allergy may cause no problems the first time, because the immune system first has to make antibodies; later doses can then trigger symptoms, either right away or hours afterwards [13].
The labels report the same link with antibodies to the medicine itself. With Wegovy®, allergic reactions occurred in 16% (8 of 50) of adults who developed anti-semaglutide antibodies, against 7% (114 of 1,659) of those who did not [2]. With Zepbound® the figures were 6.2% against 3%, and with Mounjaro® 4.1% against 3.0% [3][4]. The labels report antibodies in 64.5% of people in the main Zepbound® weight trials and in about 1% in Ozempic® trials, but warn that different test methods make such figures hard to compare [1][3]. The labels do not suggest testing for them, and most people with antibodies did not react.
If you have had anaphylaxis or angioedema with one GLP-1 medicine, the labels ask for caution with another, because it is unknown whether the risk carries over [1][2][3][5][6]. This is worth raising before any switch between brands.
Allergic reaction or injection site reaction?
Redness, itching, bruising or a small lump where you injected is counted separately, as an injection site reaction. They are frequent in trials, often more so than allergic reactions: 6 to 8% on Zepbound® against 2% on placebo, 13.9% on Saxenda® against 10.5%, 3.2% on Mounjaro® against 0.4%, and 1.4% on Wegovy® injection against 1% [2][3][4][5]. A meta-analysis of randomised trials found injection site reactions clearly more frequent with GLP-1 medicines than with comparators, while wider skin reactions were infrequent and not significantly raised [11].
A reaction that stays at the injection site and fades is usually a site reaction; the injection site reactions article covers those, and brand pages such as the Ozempic® and Zepbound® rash articles cover skin rashes. Hives or a rash that spreads, swelling of the face or mouth, or any breathing or circulation symptoms point to hypersensitivity [1][14].
What a large study found
A population-based cohort using UK and US health databases followed people with type 2 diabetes starting a GLP-1 medicine, compared with people starting a DPP-4 inhibitor or an SGLT-2 inhibitor [10]. Anaphylaxis was rare in every group, at roughly 30 to 40 cases per 100,000 person-years. Against DPP-4 inhibitors, the GLP-1 group showed a small increase that was not statistically clear; against SGLT-2 inhibitors, the risk was about 38% higher [10].
Observational studies can be skewed by differences between the people who receive each medicine, and they depend on how anaphylaxis is coded in records. The study covered GLP-1 medicines used between 2007 and 2019 in type 2 diabetes, not tirzepatide or weight-management doses [10].
Why it may happen
- Supported by the labels: serious reactions have been reported after approval, and reactions were more frequent in people who formed antibodies to the medicine [1][2][3][4].
- Not established: whether a reaction is caused by the active ingredient or by one of the inactive ingredients, which the contraindication also covers; and whether antibodies that cross-react with the body’s own GLP-1 have any clinical effect, which the Wegovy® label calls unknown [2].
- General: a drug allergy is an immune response that releases histamine and other chemicals; most drug allergies cause mild rashes or hives, and anaphylaxis is the severe, whole-body form [12][13].
What else could it be
MedlinePlus notes that many people mistake an unpleasant side effect, such as nausea, for a drug allergy [13]. Nausea and diarrhea on a GLP-1 medicine are side effects, not allergy. The picture changes when stomach symptoms come on suddenly together with hives, swelling or breathing trouble, because anaphylaxis can include abdominal pain, nausea and vomiting [12].
Angioedema has other causes too: foods, insect stings, pollen and medicines such as antibiotics, anti-inflammatory painkillers and ACE-inhibitor blood pressure medicines; in most cases no cause is found [14]. Hives can also follow an infection [14]. If a reaction appears soon after you start any new medicine or product alongside your GLP-1, tell your clinician about all of them.
What may help
The Medication Guides say not to use the medicine if you have had a serious allergic reaction to it or to any of its ingredients [1][2]. The full ingredient list is printed at the end of each Medication Guide, and it differs between products and between forms; Wegovy® injection and Wegovy® tablets, for example, contain different inactive ingredients [1][2].
MedlinePlus advises making sure every clinician, including dentists and hospital staff, knows about any drug allergy you have [13]. For mild symptoms such as an itchy rash, ask a pharmacist or your clinician before taking anything; MedlinePlus lists antihistamines among the usual options for mild drug allergy symptoms [13]. For any sign of anaphylaxis, the advice is to call emergency services straight away [12].
Ozempic®, Wegovy® and Saxenda® are registered trademarks of Novo Nordisk A/S. Mounjaro® and Zepbound® are registered trademarks and Foundayo™ is a trademark of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Novo Nordisk or Eli Lilly and Company.
When to get help
| Sign | Who to contact | How fast | Source |
|---|---|---|---|
| Difficulty breathing or swallowing, or a swollen tongue, lips, face or throat | Emergency services | Emergency | Ozempic® and Wegovy® Medication Guides [1][2]; MedlinePlus [12] |
| Fainting, dizziness or a very rapid heartbeat after a dose | Emergency services | Emergency | Ozempic® and Saxenda® Medication Guides [1][5] |
| Sudden widespread hives with vomiting, cramps or a hoarse voice | Emergency services | Emergency | MedlinePlus [12][13] |
| Severe rash or itching | Stop the medicine and get medical help right away | Emergency | Ozempic® Medication Guide [1] |
| A mild rash or hives that spreads beyond the injection site | Your prescriber | Today | Zepbound® and Mounjaro® labels [3][4]; MedlinePlus [13] |
| A past reaction to any GLP-1 medicine, before starting or switching | Your prescriber | At your next appointment | Labels, sections 4 and 5 [1][3] |
