Mounjaro (tirzepatide) is a once-weekly injection licensed in the UK for treating type 2 diabetes. As a dual GIP and GLP-1 receptor agonist, it improves blood glucose control and promotes weight loss. Many patients wonder whether it is safe to take vitamin C supplements alongside Mounjaro. Vitamin C is a widely used supplement supporting immune function, collagen synthesis, and antioxidant defence. This article examines the safety of combining Mounjaro and vitamin C, potential interactions, and practical guidance for patients managing type 2 diabetes with tirzepatide therapy.

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. It is administered as a once-weekly subcutaneous injection and belongs to a novel class of medicines known as dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists.

The mechanism of action of Mounjaro involves mimicking two naturally occurring incretin hormones. By activating both GIP and GLP-1 receptors, tirzepatide enhances insulin secretion in a glucose-dependent manner, suppresses glucagon release when blood glucose is elevated, slows gastric emptying, and reduces appetite. These combined effects lead to improved glycaemic control in people with type 2 diabetes and significant weight loss.

Mounjaro is initiated at 2.5 mg once weekly for 4 weeks as a starting dose to improve gastrointestinal tolerability. This is followed by a 5 mg dose, with subsequent increases of 2.5 mg every 4 weeks based on clinical response and tolerability, up to a maximum of 15 mg weekly. Common adverse effects include gastrointestinal disturbances (nausea, vomiting, diarrhoea, constipation), reduced appetite, fatigue, and injection site reactions. Serious but rare risks include pancreatitis, gallbladder disease, and hypoglycaemia (particularly when used alongside insulin or sulfonylureas).

Importantly, Mounjaro can reduce the absorption of oral medications, including hormonal contraceptives. Women using oral contraceptives should consider using a non-oral method or adding a barrier method for 4 weeks after starting Mounjaro and after each dose increase. Mounjaro is not recommended during pregnancy or breastfeeding, and effective contraception is advised during treatment. Women planning pregnancy should discontinue Mounjaro at least 2 months before conception.

Patients prescribed Mounjaro should receive comprehensive education on injection technique, storage, and the importance of adhering to dietary and lifestyle modifications. Regular monitoring of HbA1c and renal function is recommended in line with NICE guidance. If severe abdominal pain occurs, especially with vomiting, Mounjaro should be stopped immediately and urgent medical assessment sought for suspected pancreatitis.

Can You Take Vitamin C While Using Mounjaro?

Vitamin C (ascorbic acid) is a water-soluble vitamin essential for numerous physiological processes, including collagen synthesis, immune function, antioxidant defence, and iron absorption. It is commonly obtained through diet—particularly from citrus fruits, berries, peppers, and leafy greens—or via over-the-counter supplements. Many people take vitamin C supplements to support general health, prevent colds, or address perceived deficiencies.

There is no official contraindication to taking vitamin C supplements while using Mounjaro. The Medicines and Healthcare products Regulatory Agency (MHRA) and the emc (electronic Medicines Compendium) do not list vitamin C as an interacting substance with tirzepatide in the Summary of Product Characteristics. Vitamin C is not known to interfere with the pharmacokinetics or pharmacodynamics of GLP-1 or GIP receptor agonists, and there is no evidence suggesting that concurrent use poses a direct safety risk.

For most patients using Mounjaro, continuing a moderate vitamin C supplement is considered safe and unlikely to affect the efficacy or tolerability of the medication. The UK Reference Nutrient Intake (RNI) for vitamin C is 40 mg per day for adults, which most people can achieve through a balanced diet. Supplemental intakes up to approximately 1,000 mg per day are unlikely to cause harm in most adults, according to UK guidance.

However, it is important to recognise that individual circumstances vary. Patients with specific medical conditions—such as renal impairment, a history of kidney stones, or iron overload disorders—should exercise caution with vitamin C supplementation, regardless of Mounjaro use.

As with any supplement, it is prudent to inform your GP or diabetes specialist nurse about all vitamins and over-the-counter products you are taking. This ensures a holistic approach to your care and allows healthcare professionals to identify any potential issues related to your overall treatment plan, including Mounjaro therapy.

Potential Interactions Between Mounjaro and Vitamin C

While there is no established pharmacological interaction between Mounjaro and vitamin C, it is important to consider indirect effects and theoretical considerations. Mounjaro slows gastric emptying as part of its mechanism of action, which can alter the absorption kinetics of orally administered substances, including vitamins and supplements. Although vitamin C is absorbed primarily in the small intestine via active transport mechanisms, the actual impact of delayed gastric emptying on vitamin C bioavailability has not been specifically studied with tirzepatide.

One theoretical concern relates to gastrointestinal tolerability. High doses of vitamin C (typically above 1,000 mg daily) can cause gastrointestinal side effects such as nausea, abdominal cramps, and diarrhoea. Since Mounjaro itself commonly causes nausea, vomiting, and diarrhoea—especially during dose escalation—taking large amounts of vitamin C concurrently may exacerbate these symptoms. Patients experiencing gastrointestinal disturbances on Mounjaro may benefit from moderating their vitamin C intake or splitting doses throughout the day to improve tolerance.

An important consideration involves vitamin C and blood glucose monitoring. Some continuous glucose monitoring (CGM) systems, particularly certain FreeStyle Libre models, can be affected by high-dose vitamin C supplementation (≥500 mg daily), potentially showing falsely elevated glucose readings. Patients should check their specific glucose monitoring device's user guide for information about vitamin C interference and discuss any concerns with their healthcare team.

It's worth noting that Mounjaro's effect on slowing gastric emptying can significantly reduce the absorption of oral medications, including oral contraceptives. Women using oral hormonal contraceptives should consider using a non-oral contraceptive method or adding a barrier method for 4 weeks after starting Mounjaro and after each dose increase.

There is no evidence that vitamin C affects the efficacy of Mounjaro in lowering blood glucose. Vitamin C does not influence incretin hormone activity, insulin secretion, or appetite regulation pathways targeted by tirzepatide. Therefore, patients can be reassured that standard vitamin C supplementation is unlikely to compromise their diabetes management outcomes while using Mounjaro.

Safe Vitamin C Intake for Mounjaro Patients

The UK Reference Nutrient Intake (RNI) for vitamin C is 40 mg per day for adults, an amount easily achieved through a balanced diet rich in fruits and vegetables. Many people choose to take supplements providing higher doses, commonly ranging from 200 mg to 1,000 mg daily. For individuals using Mounjaro, these doses are generally considered safe, provided there are no underlying contraindications.

Patients should be aware that vitamin C is water-soluble, meaning excess amounts are typically excreted in urine rather than stored in the body. However, high doses (above 1,000 mg daily) can increase the risk of adverse effects, including gastrointestinal upset, kidney stones (particularly in susceptible individuals), and interference with certain laboratory tests and glucose monitoring devices. Given that Mounjaro already affects the gastrointestinal system, it is advisable to avoid high-dose vitamin C that could worsen nausea or diarrhoea.

Practical recommendations for Mounjaro patients include:

• Aim for dietary sources of vitamin C first, such as oranges, strawberries, kiwi, broccoli, and peppers.

• If supplementing, consider a moderate dose (e.g., 200–500 mg daily) to support general health without increasing gastrointestinal side effects.

• Take vitamin C supplements with food to improve tolerance, particularly during the initial weeks of Mounjaro therapy when nausea is most common.

• Stay well hydrated, as both Mounjaro and high-dose vitamin C can affect fluid balance and gastrointestinal function.

Patients with chronic kidney disease should discuss vitamin C supplementation with their healthcare team, as impaired renal function can reduce the excretion of vitamin C metabolites, potentially increasing the risk of oxalate accumulation and kidney stones. The risk of kidney stones is particularly relevant for those with a history of nephrolithiasis or reduced kidney function. Similarly, individuals with haemochromatosis or other iron overload conditions should use caution, as vitamin C enhances iron absorption.

When to Consult Your GP About Supplements and Mounjaro

It is important to maintain open communication with your GP, diabetes specialist nurse, or prescribing clinician about all medications, supplements, and over-the-counter products you are taking alongside Mounjaro. While vitamin C is generally safe, a comprehensive medication review ensures that your overall treatment plan is optimised and that potential interactions or contraindications are identified early.

You should contact your GP or healthcare provider if you experience:

• Persistent or worsening nausea, vomiting, or diarrhoea that interferes with daily activities or hydration.

• Severe, persistent abdominal pain, particularly if accompanied by vomiting. This could indicate pancreatitis and requires immediate medical attention. Stop taking Mounjaro and seek same-day medical assessment.

• Signs of dehydration, such as dizziness, reduced urine output, or dark-coloured urine. If you cannot keep fluids down for more than 24 hours, contact NHS 111 or seek urgent care.

• Symptoms of hypoglycaemia (low blood sugar), including sweating, tremor, confusion, or palpitations, especially if you are also taking insulin or sulfonylureas. Your insulin or sulfonylurea dose may need to be reduced when starting or increasing Mounjaro.

• Any new or unusual symptoms after starting a vitamin C supplement or changing the dose.

Women using oral contraceptives should discuss alternative contraception methods with their healthcare provider, as Mounjaro can reduce the effectiveness of oral hormonal contraceptives. If you are planning pregnancy, inform your healthcare team as Mounjaro should be discontinued at least 2 months before attempting conception.

Before starting any new supplement regimen, including vitamin C, it is advisable to discuss your intentions with your healthcare team. This is particularly important if you have pre-existing conditions such as kidney disease, a history of kidney stones, gastrointestinal disorders, or if you are taking other medications that may interact with supplements.

Your GP can provide personalised advice based on your medical history, current medications, and treatment goals. They may recommend blood tests to assess vitamin levels, renal function, or other relevant parameters. By working collaboratively with your healthcare team, you can safely incorporate vitamin C and other supplements into your routine while maximising the benefits of Mounjaro for diabetes management.

If you suspect you are experiencing side effects from Mounjaro, you can report these through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or via the Yellow Card app).

Frequently Asked Questions