Mounjaro (tirzepatide) is a once-weekly injectable medication licensed in the UK for treating type 2 diabetes in adults. As a dual GIP and GLP-1 receptor agonist, it helps control blood glucose levels and supports weight management. Whilst Mounjaro is not classified as a photosensitising drug, patients often wonder about sun exposure safety whilst taking this treatment. Understanding how to protect your skin, manage common side effects in warm weather, and recognise when to seek medical advice ensures safe and effective diabetes management during outdoor activities and sunny conditions.

What Is Mounjaro and How Does It Work?

Mounjaro (tirzepatide) is a prescription medicine licensed in the UK for the treatment of type 2 diabetes mellitus in adults. It belongs to a class of medications known as dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists. By activating both GIP and GLP-1 receptors, Mounjaro enhances insulin secretion when blood glucose levels are elevated, suppresses glucagon release, slows gastric emptying, and reduces appetite—all of which contribute to improved glycaemic control and weight management.

Mounjaro is administered as a once-weekly subcutaneous injection, typically into the abdomen, thigh, or upper arm. Treatment usually starts with 2.5 mg once weekly for 4 weeks, then increases to 5 mg weekly, with further dose adjustments in 2.5 mg increments up to a maximum of 15 mg weekly as tolerated. This gradual titration helps minimise gastrointestinal side effects and optimise therapeutic benefit. Clinical trials have demonstrated significant reductions in HbA1c levels and body weight in patients with type 2 diabetes who have not achieved adequate control with other antidiabetic agents.

As with all medications, Mounjaro carries potential side effects. The most commonly reported adverse reactions include nausea, diarrhoea, vomiting, constipation, and abdominal discomfort. These gastrointestinal symptoms are usually mild to moderate and tend to diminish over time. Less common but important side effects include hypoglycaemia (particularly when used with insulin or sulphonylureas), injection site reactions, and a precautionary warning about thyroid C-cell tumours observed in rodents (though the human relevance remains uncertain). Other important safety considerations include risks of gallbladder disease, potential worsening of diabetic retinopathy with rapid glycaemic improvement, and the need for renal monitoring if severe gastrointestinal symptoms occur.

While there is no official link between Mounjaro and increased sensitivity to sun exposure documented in the Summary of Product Characteristics (SmPC) or MHRA guidance, understanding how the medication works and its side effect profile is essential for comprehensive patient care and safety.

Sun Safety Precautions While Taking Mounjaro

Although Mounjaro is not classified as a photosensitising medication, patients taking this treatment should still observe general sun safety measures to protect their skin and overall health. This is particularly important given that individuals with type 2 diabetes may have an increased risk of skin complications and delayed wound healing.

Key sun protection strategies include:

• Apply broad-spectrum sunscreen with SPF 30 or higher and a 4 or 5-star UVA rating to all exposed skin, reapplying every two hours and after swimming or sweating

• Seek shade during peak UV hours (11am to 3pm in the UK) when the sun's rays are strongest

• Wear protective clothing, including wide-brimmed hats, long-sleeved shirts, and UV-protective sunglasses

• Stay hydrated by drinking plenty of water, especially important as Mounjaro can cause gastrointestinal fluid loss through diarrhoea or vomiting

• Monitor injection sites for any unusual reactions, redness, or irritation and avoid injecting into irritated or sunburned skin

Patients with diabetes should be particularly vigilant about skin care, as diabetes can impair wound healing and increase susceptibility to skin infections. Regular skin checks are advisable to identify any changes early. If you notice any new skin lesions, persistent redness, or unusual reactions following sun exposure, consult your GP or diabetes specialist nurse.

It is worth noting that some other diabetes medications (such as certain sulphonylureas) may increase photosensitivity according to the British National Formulary. If you are taking Mounjaro alongside other treatments, discuss your complete medication regimen with your healthcare provider to understand your individual risk profile and receive tailored sun safety advice.

Managing Side Effects During Sun Exposure

The most common side effects of Mounjaro—nausea, vomiting, and diarrhoea—can be particularly problematic during hot weather or prolonged sun exposure due to the increased risk of dehydration. Dehydration can worsen these gastrointestinal symptoms and may also affect blood glucose control, creating a cycle that impacts overall wellbeing.

Practical strategies for managing side effects in warm weather:

• Maintain adequate hydration by sipping water regularly throughout the day, even before feeling thirsty. Oral rehydration solutions may be beneficial if experiencing significant fluid loss

• Eat smaller, more frequent meals rather than large portions, choosing bland, easily digestible foods if nausea is present

• Avoid alcohol and caffeine, which can contribute to dehydration and may worsen gastrointestinal symptoms

• Monitor blood glucose levels more frequently during hot weather, as heat can affect glucose metabolism (and insulin absorption if you also use insulin)

• Plan outdoor activities for cooler parts of the day when gastrointestinal symptoms tend to be less troublesome

• Carry glucose tablets or snacks to manage potential hypoglycaemia, particularly if appetite is reduced due to nausea

If you experience persistent vomiting or severe diarrhoea lasting more than 24 hours, this constitutes a medical concern requiring prompt attention, as it can lead to significant dehydration, electrolyte imbalances and potential acute kidney injury. Your healthcare provider may need to check your kidney function and consider temporarily pausing Mounjaro until you recover. Follow local 'sick day rules' for managing diabetes during illness.

Heat exhaustion shares symptoms with some Mounjaro side effects (dizziness, nausea, weakness), so it is important to recognise warning signs and move to a cool environment if these develop.

Patients should also be aware that injection site reactions—such as redness, swelling, or itching—may occasionally occur with Mounjaro. While these are typically mild, rotating injection sites as recommended and avoiding injecting into areas that are already irritated, inflamed or sunburned can help minimise discomfort.

When to Seek Medical Advice

While Mounjaro is generally well-tolerated, certain situations warrant prompt medical attention. Patients should be aware of warning signs that require contact with their GP, diabetes specialist nurse, or emergency services.

Contact your GP or diabetes care team if you experience:

• Persistent nausea or vomiting lasting more than 24–48 hours, particularly if unable to keep fluids down

• Signs of dehydration, including dark urine, dizziness, dry mouth, reduced urination, or confusion

• Severe or persistent abdominal pain, especially if accompanied by vomiting, which could indicate pancreatitis (a rare but serious side effect)

• Right upper abdominal pain, fever or yellowing of the skin/eyes, which may indicate gallbladder problems (cholelithiasis or cholecystitis)

• Symptoms of hypoglycaemia that do not resolve with usual treatment, or frequent episodes requiring assistance

• Unusual skin reactions at injection sites, including spreading redness, warmth, swelling, or discharge suggesting infection

• New or worsening symptoms following sun exposure that seem disproportionate to the level of exposure

Seek immediate medical attention (call 999 or attend A&E) if you develop:

• Severe allergic reactions, including difficulty breathing, swelling of the face or throat, or widespread rash

• Signs of severe dehydration, such as extreme weakness, rapid heartbeat, very low blood pressure, or altered consciousness

• Symptoms of diabetic ketoacidosis, including excessive thirst, frequent urination, confusion, fruity-smelling breath, or rapid breathing (more common with insulin deficiency or in those also taking SGLT2 inhibitors)

• Severe, unrelenting abdominal pain with vomiting, which may indicate acute pancreatitis

Patients should maintain regular contact with their diabetes care team and attend scheduled review appointments. NICE guidance (NG28) recommends ongoing monitoring of HbA1c, renal function, and body weight in patients taking GLP-1 receptor agonists. If you experience any side effects, including those not listed in this article, you can report them via the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk or the Yellow Card app). Any concerns about medication tolerability, side effects, or the impact of environmental factors like sun exposure should be discussed openly with healthcare professionals to ensure safe and effective diabetes management.

Frequently Asked Questions