Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes and weight management. Some patients wonder whether Mounjaro and cold sores are connected, particularly if outbreaks occur after starting treatment. Cold sores, caused by herpes simplex virus type 1 (HSV-1), affect over 70% of UK adults and can reactivate due to various triggers including stress, illness, and immune changes. This article examines the evidence regarding any potential link between Mounjaro and cold sore outbreaks, explores management strategies, and clarifies when medical advice is needed.

Understanding Mounjaro (Tirzepatide) and Its Mechanism of Action

Mounjaro (tirzepatide) is a prescription medicine licensed in the UK for the treatment of type 2 diabetes mellitus and, more recently, for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. It must be used as an adjunct to a reduced-calorie diet and increased physical activity. Mounjaro belongs to a novel class of medications known as dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists.

The mechanism of action involves mimicking two naturally occurring incretin hormones that regulate blood glucose levels and appetite. When administered via weekly subcutaneous injection, tirzepatide enhances insulin secretion in response to elevated blood glucose, suppresses inappropriate glucagon release, slows gastric emptying, and promotes satiety through central nervous system pathways. These combined effects lead to improved glycaemic control and significant weight reduction in clinical trials.

Tirzepatide is metabolised primarily through proteolytic cleavage and does not rely heavily on hepatic cytochrome P450 enzymes. However, the delayed gastric emptying effect may affect the absorption of some oral medicines, including oral contraceptives. Women using oral contraceptives should use additional barrier contraception for 4 weeks after starting Mounjaro and after each dose increase.

Common adverse effects include gastrointestinal symptoms such as nausea, vomiting, diarrhoea, and constipation, particularly during dose escalation. Other reported effects include injection site reactions and hypoglycaemia when used in combination with insulin or sulphonylureas. Important safety considerations include the risk of pancreatitis (seek medical attention for severe abdominal pain), gallbladder disease, acute kidney injury with dehydration, and potential effects on diabetic retinopathy.

Understanding how Mounjaro works is essential when considering any potential relationship with other health conditions, including viral infections such as cold sores.

Cold Sores: Causes, Triggers and the Role of Immune Function

Cold sores, medically termed herpes labialis, are small, painful blisters that typically appear on or around the lips. They are caused by the herpes simplex virus type 1 (HSV-1), though herpes simplex virus type 2 (HSV-2) can occasionally be responsible. HSV-1 is extremely common in the UK, with estimates suggesting that over 70% of adults have been exposed to the virus by middle age.

Following initial infection—often asymptomatic or presenting as mild oral symptoms in childhood—the virus remains dormant in the trigeminal ganglion, a nerve cluster near the ear. Reactivation occurs when the virus travels back down the nerve to the skin surface, resulting in the characteristic vesicular lesions. Cold sore outbreaks are typically self-limiting, resolving within 7–10 days without treatment.

Common triggers for reactivation include:

• Physical or emotional stress

• Fatigue and lack of sleep

• Exposure to ultraviolet (UV) light or extreme weather

• Hormonal changes (menstruation, pregnancy)

• Fever or systemic illness

• Immunosuppression or immune system changes

• Local trauma to the lips or face

The immune system plays a critical role in controlling HSV-1. Cell-mediated immunity, particularly CD8+ T lymphocytes, suppresses viral replication and maintains latency. Any factor that temporarily weakens immune surveillance—whether physiological stress, illness, or certain medications—may permit viral reactivation.

Cold sores are highly contagious while the blisters are present. To prevent spreading the virus, avoid kissing, oral sex, or sharing items like lip balm, cutlery or towels. Take particular care around newborn babies, pregnant women and people with weakened immune systems. Never allow someone with a cold sore to kiss a baby, as neonatal herpes infection can be very serious.

Can Mounjaro Trigger or Worsen Cold Sores?

There is currently no established clinical evidence directly linking Mounjaro (tirzepatide) to an increased risk of cold sore outbreaks or worsening of herpes simplex virus infections. The Summary of Product Characteristics (SmPC) for Mounjaro, as approved by the MHRA and European Medicines Agency (EMA), does not list herpes labialis or HSV reactivation among the known adverse effects observed in clinical trials.

Tirzepatide does not have known immunosuppressive properties. Unlike corticosteroids, immunomodulatory biologics, or chemotherapy agents, GLP-1 and GIP receptor agonists do not directly impair immune cell function or suppress the body's ability to control latent viral infections. The pharmacological action is primarily metabolic, targeting glucose homeostasis and appetite regulation rather than immune pathways. While non-specific upper respiratory tract infections may be reported with incretin-based therapies, this does not indicate immunosuppression.

However, indirect factors warrant consideration. Patients initiating Mounjaro may experience significant metabolic changes, weight loss, and gastrointestinal side effects, which could constitute physiological stress. Additionally, the lifestyle changes accompanying diabetes management or weight loss programmes—such as dietary modifications, increased physical activity, or psychological stress—might theoretically act as cold sore triggers in susceptible individuals.

Anecdotal reports from patients occasionally mention cold sores developing after starting Mounjaro, but such observations do not establish causation. Cold sores are common in the general population, and their occurrence may be coincidental rather than medication-related. Without controlled studies specifically examining HSV reactivation rates in tirzepatide users versus comparator groups, any purported association remains speculative.

If you're concerned about cold sore patterns, discuss your individual circumstances with your healthcare provider, including any other medications or conditions that might genuinely affect your immune function.

Managing Cold Sores While Taking Mounjaro

If you develop cold sores whilst taking Mounjaro, standard management approaches remain appropriate and effective. There is no evidence suggesting that tirzepatide interferes with antiviral medications or topical treatments used for herpes labialis.

First-line management strategies include:

• Topical antiviral creams: Aciclovir 5% cream or penciclovir 1% cream, applied at the first sign of tingling (prodrome), may reduce lesion duration and severity. These are available over-the-counter in UK pharmacies.

• Oral antiviral therapy: For frequent or severe outbreaks, GPs may prescribe oral aciclovir, valaciclovir, or famciclovir. These systemic treatments are more effective than topical preparations, particularly when initiated early. Dosing regimens vary by product and should be adjusted in patients with kidney impairment.

• Symptomatic relief: Paracetamol or ibuprofen can alleviate pain. Avoid aspirin in children due to Reye's syndrome risk.

• Protective measures: Apply lip balm with SPF protection, avoid touching or picking lesions, and maintain good hand hygiene to prevent viral spread or secondary bacterial infection.

Preventative strategies are particularly important for individuals experiencing recurrent outbreaks:

• Identify and minimise personal triggers (stress management, adequate sleep, sun protection)

• Consider suppressive antiviral therapy if outbreaks occur frequently (≥6 per year), as per NICE Clinical Knowledge Summary guidance

• Maintain optimal overall health, including good glycaemic control if diabetic

• Stay well-hydrated and manage any gastrointestinal side effects from Mounjaro that might cause additional stress

To prevent spreading the virus:

• Avoid kissing or oral sex while lesions are present

• Don't share items that touch the affected area (lip balm, cutlery, towels)

• Wash hands thoroughly after touching the cold sore

• Take special care around newborns, pregnant women and immunocompromised people

Continue taking Mounjaro as prescribed unless specifically advised otherwise by your doctor. Cold sores do not constitute a contraindication to tirzepatide therapy. If you notice an unusual pattern of frequent or severe outbreaks after starting Mounjaro, document the timing and discuss this with your healthcare provider for individualised assessment.

When to Seek Medical Advice: Cold Sores and Mounjaro Treatment

Whilst most cold sores are self-limiting and manageable with over-the-counter treatments, certain circumstances warrant prompt medical attention. Patients taking Mounjaro should be aware of warning signs that require GP consultation or, in severe cases, urgent care.

Contact your GP if:

• Cold sores do not improve within 10 days or appear to be worsening

• You experience frequent recurrent outbreaks (more than 6 per year)

• Lesions spread beyond the lip area or become unusually large

• You develop signs of secondary bacterial infection (increasing pain, purulent discharge, spreading redness, fever)

• You have a weakened immune system due to other medical conditions or medications

• You are pregnant or breastfeeding and develop cold sores

Seek urgent medical attention if:

• You develop severe, widespread blistering or systemic symptoms (high fever, confusion, severe headache)

• Eye involvement occurs (pain, redness, vision changes, light sensitivity) – this requires same-day ophthalmology assessment via A&E or NHS 111

• You have difficulty swallowing or breathing due to oral lesions

• Signs of eczema herpeticum develop (widespread vesicular rash in patients with atopic eczema) – attend A&E

• A baby under 6 weeks has a rash or has been exposed to a cold sore – seek immediate medical advice

Regarding Mounjaro specifically, inform your healthcare provider if you notice any unexpected patterns or concerns after starting treatment. Your GP can review your medication history, assess whether any adjustments are needed, and provide appropriate antiviral therapy or referral if required. According to NICE Clinical Knowledge Summary guidance, management of recurrent herpes simplex should be individualised, with consideration of suppressive therapy for those significantly affected by frequent outbreaks.

If you suspect Mounjaro may be causing any adverse effects, including unusual patterns of cold sores, you can report this through the MHRA Yellow Card scheme. Never discontinue Mounjaro without medical advice, as abrupt cessation may affect your diabetes control or weight management goals.

Frequently Asked Questions