Does Mounjaro cause hives? Mounjaro (tirzepatide) is a GLP-1 and GIP receptor agonist licensed in the UK for type 2 diabetes mellitus. Whilst generally well tolerated, hypersensitivity reactions—including hives (urticaria)—have been reported. According to the MHRA-approved Summary of Product Characteristics, urticaria, rash, and pruritus can occur, though serious allergic reactions remain rare. Most patients do not develop skin reactions, but recognising hives and knowing when to seek medical attention is essential. If hives appear alongside facial swelling, breathing difficulty, or dizziness, urgent assessment is required. This article explores the link between Mounjaro and hives, how to recognise allergic reactions, and what steps to take if symptoms develop.

Does Mounjaro Cause Hives?

Mounjaro (tirzepatide) is a glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus. Whilst Mounjaro is generally well tolerated, like all medications it can cause adverse effects in some individuals, including allergic and hypersensitivity reactions.

Hives (urticaria) have been reported as a potential adverse reaction to Mounjaro. According to the Summary of Product Characteristics (SmPC) approved by the Medicines and Healthcare products Regulatory Agency (MHRA), hypersensitivity reactions—including rash, pruritus (itching), and urticaria—can occur in patients receiving tirzepatide. The exact incidence varies, but clinical trial data suggest that serious allergic reactions are rare.

It is important to distinguish between common gastrointestinal side effects (such as nausea, vomiting, and diarrhoea, which affect approximately 10-20% of patients) and less frequently reported allergic manifestations like hives. Most patients tolerate Mounjaro without developing skin reactions. However, if hives do appear, particularly if accompanied by other symptoms such as facial swelling, difficulty breathing, or dizziness, this may indicate a more serious hypersensitivity reaction requiring urgent medical attention.

Patients should be aware of the signs of allergic reactions and know when to seek help from their GP or emergency services. If you experience side effects that you think may be related to Mounjaro, you can report them directly to the MHRA through the Yellow Card Scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

Understanding Allergic Reactions to Mounjaro

Allergic reactions to medications occur when the immune system mistakenly identifies a drug or one of its components as harmful, triggering an inflammatory response. With Mounjaro, hypersensitivity reactions can range from mild skin manifestations to severe reactions, although the latter are rare.

Mechanism of allergic response: Tirzepatide is a synthetic peptide that has the potential to provoke immune-mediated reactions in some individuals. The body may respond to the medication or its excipients (inactive ingredients), leading to symptoms such as urticaria, angioedema (swelling of deeper skin layers), or systemic reactions.

Types of hypersensitivity reactions associated with Mounjaro include:

• Immediate reactions (Type I hypersensitivity): These occur within minutes to hours of injection and may present as hives, itching, flushing, or in severe cases, anaphylaxis with respiratory compromise and cardiovascular collapse.

• Delayed reactions: These develop hours to days after administration and may manifest as skin rashes, eczema-like changes, or persistent urticaria.

Clinical trial data and post-marketing surveillance have identified hypersensitivity reactions in a small percentage of Mounjaro users. The MHRA and European Medicines Agency (EMA) continue to monitor safety signals. Healthcare professionals are encouraged to report suspected adverse drug reactions via the Yellow Card Scheme to enhance pharmacovigilance.

Importantly, Mounjaro is contraindicated in patients with known hypersensitivity to tirzepatide or any of the excipients. Patients with a history of severe allergic reactions to other GLP-1 receptor agonists should discuss the risks and benefits with their clinician before starting treatment.

Recognising Hives and Other Skin Reactions

Hives (urticaria) are raised, itchy welts on the skin that can vary in size and shape, often appearing suddenly and sometimes migrating across different body areas. They result from the release of histamine and other inflammatory mediators from mast cells in the skin. Recognising hives and differentiating them from other skin reactions is important for appropriate management.

Characteristic features of hives include:

• Appearance: Red or skin-coloured raised bumps (wheals) with well-defined edges, often with a pale centre

• Itching: Intense pruritus is typical, though some patients may experience burning or stinging sensations

• Transience: Individual lesions usually resolve within 24 hours, though new ones may continue to appear

• Distribution: Hives can occur anywhere on the body and may coalesce into larger plaques

Other skin reactions that may occur with Mounjaro include:

• Injection site reactions: Localised redness, swelling, itching, or bruising at the injection site have been reported and are usually mild and self-limiting. These are distinct from widespread hives and typically resolve without intervention.

• Angioedema: Swelling of deeper skin layers, particularly around the eyes, lips, tongue, or throat. This can be serious if it affects the airway and requires urgent medical assessment. If swelling affects the lips, tongue or throat, call 999 immediately.

• Generalised rash: Non-urticarial rashes, which may be maculopapular (flat red areas with small bumps), are less commonly reported but possible.

Patients should monitor for accompanying symptoms that might indicate a more serious allergic reaction, such as difficulty breathing, wheezing, chest tightness, rapid heartbeat, dizziness, or swelling of the face or throat. Photographic documentation of skin changes can be helpful when consulting healthcare professionals, particularly if the rash is intermittent. For urgent but non-emergency advice, contact NHS 111.

What to Do If You Develop Hives on Mounjaro

If you develop hives whilst taking Mounjaro, prompt assessment and appropriate action are essential to ensure your safety and determine whether you can continue the medication.

Immediate steps:

• Assess severity: If hives are accompanied by difficulty breathing, swelling of the face or throat, dizziness, or feeling faint, call 999 immediately or go to your nearest A&E department, as these may be signs of anaphylaxis—a medical emergency.

• Stop the medication temporarily: Do not administer your next dose of Mounjaro until you have spoken with a healthcare professional. Continuing the drug during an active allergic reaction may worsen symptoms.

• Symptomatic relief: Over-the-counter antihistamines (such as cetirizine or loratadine) may help relieve itching and reduce hives. Speak to a pharmacist about suitable non-sedating options and potential interactions with your other medications. However, antihistamines should not replace medical evaluation, particularly if symptoms are severe or persistent.

When to contact your GP or diabetes specialist:

• Hives that persist for more than 24–48 hours

• Recurrent episodes of urticaria after each Mounjaro injection

• Any associated symptoms such as joint pain, fever, or malaise

• Uncertainty about whether the reaction is related to Mounjaro or another cause

Clinical assessment and management: Your healthcare provider will take a detailed history, examine the rash, and assess the temporal relationship between Mounjaro administration and symptom onset. Depending on the severity, they may:

• Recommend discontinuation of Mounjaro and consider alternative diabetes therapies

• Refer you to an allergy specialist for further evaluation and assessment

• Report the reaction to the MHRA via the Yellow Card Scheme

According to NICE guidance NG28 for type 2 diabetes management, treatment should be individualised. If Mounjaro is discontinued due to hypersensitivity, your clinician will discuss alternative treatment options. The potential for cross-reactivity with other GLP-1 receptor agonists is uncertain, so any alternative should be initiated with caution and under specialist supervision. Patient safety is paramount, and no medication should be continued if it poses a significant allergic risk.

You can also report suspected side effects directly to the MHRA through the Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Frequently Asked Questions