Interest in "keto vs Mounjaro" has increased in the UK, but they are two very different approaches to weight management. Mounjaro is a prescription only medicine containing tirzepatide. It acts on GIP and GLP 1 receptors, which are involved in appetite and blood glucose regulation. Keto is a low carbohydrate diet that aims to put the body into a state called nutritional ketosis.
It is important to keep these approaches separate. Mounjaro does not work by putting the body into ketosis, and there is currently limited clinical evidence on using tirzepatide together with a ketogenic diet.
For UK adults considering or already using prescription weight loss treatment, there are several practical questions to consider. These include whether Mounjaro causes ketosis, how it differs from a ketogenic diet, what the evidence shows about each approach, and what safety considerations may apply when combining them.
Mounjaro is a Prescription Only Medicine containing tirzepatide. This page provides general information and does not replace advice from your prescriber. Whether Mounjaro is suitable for you depends on an individual clinical assessment.
Key things to know
- Mounjaro does not directly cause ketosis. It works by acting on GIP and GLP 1 receptors involved in appetite and blood glucose regulation rather than directly changing the body's main fuel source.
- Some people taking Mounjaro may experience mild ketosis if they eat much less food or fewer carbohydrates. This would be related to their reduced food intake rather than a direct effect of tirzepatide.
- Nutritional ketosis and diabetic ketoacidosis are different conditions. Diabetic ketoacidosis is a serious medical condition that requires urgent medical attention.
- There is currently no large randomised clinical trial specifically testing tirzepatide together with a ketogenic diet. This means there is not enough evidence to say that combining the two provides an additional weight loss benefit.
- Research suggests that different calorie controlled diets can support weight loss. A ketogenic diet is not required for Mounjaro to be used as part of a weight management plan.
Can You Do Keto on Mounjaro?
For some people, a low carbohydrate diet may be possible while taking Mounjaro. However, whether it is a suitable choice depends on your health, diet, current treatment and individual circumstances.
Mounjaro may reduce appetite and slow the rate at which food leaves the stomach. As a result, some people may find it easier to eat less. Adding a highly restrictive diet on top of this could make it harder to get enough calories and nutrients.
If you are considering keto while taking Mounjaro, focus on getting enough nutrition rather than simply trying to reduce carbohydrates as much as possible. A varied diet that provides adequate protein, fibre, vitamins, minerals and fluids is important during weight management.
People with diabetes, kidney or liver problems, a history of pancreatitis, or an eating disorder should speak to a healthcare professional before making major changes to their diet. The same applies to people who are pregnant or breastfeeding or who take medicines such as insulin, sulfonylureas or SGLT2 inhibitors.
If you are already taking Mounjaro and want to start a ketogenic diet, discussing your plans with your prescriber or a dietitian can help you choose an approach that is appropriate for you.
Does Mounjaro Cause Ketosis?
No, Mounjaro does not directly cause nutritional ketosis. Tirzepatide works by activating GIP and GLP 1 receptors. These receptors are involved in appetite, blood glucose regulation and the body's response to food.
Mounjaro can reduce appetite and slow gastric emptying, which may lead some people to eat less. If someone also eats much less carbohydrate, their body may produce more ketones as it uses stored fat for energy. This is related to reduced food and carbohydrate intake rather than Mounjaro directly putting the body into ketosis.
Nutritional ketosis happens when the body produces ketones because carbohydrate availability is low. The main ketones include beta hydroxybutyrate, acetoacetate and acetone.
This is different from diabetic ketoacidosis, which is a potentially serious medical condition. If you have diabetes and develop symptoms such as persistent vomiting, severe abdominal pain, unusual thirst, deep or rapid breathing, confusion or marked weakness, seek urgent medical advice.
If you are wondering whether Mounjaro will put you into ketosis, the answer is no. You would generally need to follow a low carbohydrate eating pattern to intentionally reach nutritional ketosis. If you notice ketones unexpectedly, particularly if you feel unwell, speak to a healthcare professional rather than assuming that it is simply part of weight loss treatment.
Nutritional ketosis and ketoacidosis are not the same thing
| Feature | Nutritional ketosis | Ketoacidosis |
|---|---|---|
| What causes it | Deliberate carbohydrate restriction, or a large sustained fall in food intake | Severe insulin deficiency, prolonged starvation, or illness on top of low intake |
| Blood ketone level | Typically low and stable | Very high and rising |
| Blood glucose | Usually normal | Can be high, or near normal in euglycaemic ketoacidosis |
| How you feel | Usually well, sometimes tired in the first week | Unwell, nauseated, vomiting, abdominal pain, rapid breathing, confusion |
| What to do | Nothing, if you feel well | Seek urgent medical care the same day |
When Ketones Stop Being Harmless
Nutritional ketosis is different from ketoacidosis. Ketoacidosis is a serious condition where ketones and acid build up in the blood and cause metabolic acidosis. In some cases, blood glucose may not be very high, which can make the condition harder to recognise.
There have been rare reports of ketoacidosis in people taking tirzepatide, including some people without diabetes. In these reports, very low food intake, vomiting or significant carbohydrate restriction were among the factors involved. These reports are rare and do not mean that Mounjaro commonly causes ketoacidosis.
The risk may be more concerning when several factors occur together, such as persistent nausea, vomiting, very low food intake and severe carbohydrate restriction. If you take Mounjaro and feel unwell while following a very low carbohydrate diet, do not simply continue the diet without seeking advice.
Seek urgent medical advice if you develop persistent nausea or vomiting, severe abdominal pain, unusual or deep and rapid breathing, drowsiness, confusion or a fruity smell on your breath, particularly if you feel seriously unwell.
If you take an SGLT2 inhibitor, speak to your diabetes team before making major changes to your carbohydrate intake. They can advise you about sick day rules and whether ketone testing is appropriate for you.
Two Different Mechanisms, One Shared Outcome
Understanding why keto and Mounjaro work differently can make it easier to decide what approach may be suitable for you.
How Tirzepatide Works
Mounjaro contains tirzepatide, which activates both the GLP 1 and GIP receptors. These receptors are part of the body's incretin system and are involved in appetite and blood glucose regulation.
Tirzepatide can reduce appetite, increase feelings of fullness, slow gastric emptying and support blood glucose control. These effects may help some people reduce their food intake and lose weight when Mounjaro is used as prescribed alongside a reduced calorie diet and increased physical activity.
Tirzepatide does not directly force the body into ketosis or cause ketone production. Its effects are different from the metabolic changes caused by a ketogenic diet.
If you want more information about Mounjaro, our page on Mounjaro injections covers dosing, storage, side effects and eligibility.
How a Ketogenic Diet Works
Keto is a type of low carbohydrate diet that uses very low carbohydrate intake to encourage the body to produce ketones. A ketogenic diet often limits carbohydrates to around 20g to 50g per day, although the amount can vary between individuals and different versions of the diet.
When carbohydrate intake is very low, the body uses stored glycogen and then increases its use of fat for energy. The liver produces ketones, which can then be used as an energy source. This state is known as nutritional ketosis.
The early drop in weight seen with a ketogenic diet can partly be due to changes in water and glycogen stores, rather than body fat alone. Some people may also experience temporary symptoms such as tiredness, headache or irritability while adapting to a major change in their diet.
A ketogenic diet is not the only way to lose weight, and you do not need to be in ketosis for Mounjaro to support a weight management plan. The most suitable eating pattern depends on your health, preferences and ability to follow it safely over time.
Keto vs Mounjaro: What the Evidence Shows
| Factor | Mounjaro (tirzepatide) | Ketogenic diet |
|---|---|---|
| Type | Prescription Only Medicine, weekly injection | Self-directed eating pattern |
| Mechanism | GIP and GLP-1 receptor activation, with weight loss driven mainly through appetite control, reduced calorie intake, and better blood sugar control rather than ketosis | Carbohydrate restriction and ketone production, where low carbohydrate intake creates a metabolic shift toward fat as a fuel source |
| Access | Clinical assessment and prescription required | Can be started without approval, though advice is sensible |
| Trial evidence | Clinical trials report average reduction of up to 20.9% of body weight over 72 weeks in adults with obesity or overweight with a weight-related condition | Meaningful short-term loss, with outcomes broadly comparable to calorie-controlled diets by twelve months |
| Daily effort | One injection per week, plus dietary and activity changes | Daily tracking, label reading, meal planning |
| Oversight | Prescriber review, dose changes and monitoring | Usually self-managed, or with a registered dietitian |
| Main drawbacks | Nausea, diarrhoea, constipation, vomiting, ongoing cost | Restrictive, socially difficult, early fatigue, adherence falls over time |
| Cost | Ongoing prescription cost | Variable food cost, often similar to a previous diet |
Head to head, clinical trials suggest that tirzepatide can lead to substantial weight loss, but comparisons with dietary approaches are not always like for like. Tirzepatide has also produced greater average weight loss than semaglutide in a direct randomised trial, with an average reduction of around 20% compared with around 14% in the study. Results from ketogenic diets can vary, and any early advantage over other diets may become smaller over time, with long term adherence being an important factor.
It is therefore more useful to see these as different approaches rather than direct alternatives. Mounjaro is a prescription medicine that is supplied following a clinical assessment. Keto is an eating pattern that someone may choose to follow with or without weight loss medication.
Keto and Mounjaro Together: What Is Actually Known
This is an area where it is important to separate evidence from theory. You may see claims that combining ketosis with tirzepatide creates an additional effect on appetite or weight loss. However, there is currently not enough clinical evidence to confirm that combining tirzepatide with a ketogenic diet provides an extra benefit.
The published research on combining GLP 1 medicines with keto is also limited. One small retrospective case series followed seven adults who followed a supervised ketogenic diet while taking low dose semaglutide for six months. The study reported improvements in some measures, but it involved only seven people, used semaglutide rather than tirzepatide and did not include a control group. This is not enough evidence to confirm that the same results would occur with Mounjaro.
One area worth considering when losing weight is muscle preservation. Weight loss can include both body fat and lean tissue, whether it happens through diet, medication or a combination of approaches. A suitable protein intake and regular resistance exercise can help support muscle during weight loss.
A ketogenic diet can provide enough protein, but this depends on how the diet is planned. If you are taking Mounjaro and eating very little because of reduced appetite, it can be harder to meet your nutritional needs. If you are concerned about maintaining muscle or getting enough nutrition, speak to your prescriber or a registered dietitian.
Practical Considerations Before Combining Keto and Tirzepatide
The Under Eating Trap
This is one of the main things to watch when combining a strong calorie restriction with Mounjaro. Mounjaro can reduce appetite, while keto removes or limits many carbohydrate rich foods. Together, these changes may make it easy to eat less than intended.
Signs that you may not be eating enough can include ongoing tiredness, dizziness, feeling faint, feeling unusually cold or changes in your sleep. If your appetite is very low, focus on regular, nutritious meals rather than trying to restrict your food intake further.
If you are struggling to eat or drink enough while taking Mounjaro, speak to your prescriber or pharmacist.
Protecting Muscle
Getting enough protein is important during significant weight loss. The amount you need depends on factors such as your body weight, age, activity levels and health. Rather than following a fixed target, your prescriber or dietitian can help you decide what is appropriate for you.
A ketogenic diet can include protein rich foods, but some versions focus heavily on fat and may not provide enough protein. Including a suitable protein source with meals can help you meet your nutritional needs.
Resistance exercise can also help support muscle strength during weight loss. If you are new to exercise, start gradually and choose activities that are suitable for your fitness level.
High Fat Meals and Slowed Digestion
Mounjaro can slow gastric emptying, and high fat meals may also take longer to digest. This means a large, high fat keto meal may feel uncomfortable for some people while taking Mounjaro.
Smaller portions and eating slowly may make meals easier to tolerate. If certain foods make your nausea worse, you may find it helpful to avoid them while your symptoms settle.
If nausea continues, you are vomiting or you cannot keep fluids down, speak to your prescriber or pharmacist for advice.
Fluids, Fibre and Electrolytes
Constipation and dehydration can be concerns during weight loss treatment. A ketogenic diet can also reduce fibre intake if foods such as whole grains, pulses and some fruit are heavily restricted. Mounjaro can cause gastrointestinal side effects, including constipation, in some people.
Try to drink regularly throughout the day and include suitable sources of fibre, such as leafy vegetables, avocado, chia seeds and flaxseed, if they fit your diet.
Do not make large changes to your salt or electrolyte intake without appropriate advice. If you have kidney disease, heart failure or have been told to limit your fluids or salt, speak to your healthcare professional before changing your diet or fluid intake.
Diabetes Medicines and Ketoacidosis Risk
Mounjaro is less likely to cause low blood sugar when used on its own because its effect on insulin release is linked to blood glucose levels. However, the risk of hypoglycaemia can increase when it is used with insulin or sulfonylureas.
If you have diabetes and take insulin or another diabetes medicine, do not change your dose yourself when starting Mounjaro or changing your diet. A healthcare professional can review your medicines and advise you on any changes that may be needed.
If you take an SGLT2 inhibitor, speak to your diabetes team before starting a very low carbohydrate diet. SGLT2 inhibitors can increase the risk of diabetic ketoacidosis, including cases where blood glucose is not very high.
People with diabetes should follow the ketone testing and sick day advice provided by their healthcare team. If you feel unwell and are concerned about ketoacidosis, seek urgent medical advice rather than relying only on your blood glucose reading.
Who Should Take Specialist Advice First
Speak to a healthcare professional before starting a ketogenic diet if you have type 1 or type 2 diabetes, chronic kidney disease, liver disease, a history of pancreatitis or gallbladder problems, or a history of an eating disorder.
You should also seek medical advice if you are pregnant, planning to become pregnant or breastfeeding before making major changes to your diet or treatment.
A ketogenic diet can also be used as part of specialist treatment for some forms of drug resistant epilepsy. This is different from following keto as a weight loss diet and should be medically supervised.
Is Keto the Best Diet With Tirzepatide?
There is currently no good evidence that one specific diet is the best choice for everyone taking tirzepatide. What matters is getting enough nutrients and following an eating pattern that you can maintain over time.
Many people may find a balanced diet easier to follow alongside Mounjaro. A suitable eating pattern should provide enough protein, fibre, fluids, vitamins and minerals without creating unnecessary restrictions.
Some people may prefer a moderate low carbohydrate approach rather than strict keto. Others may prefer a Mediterranean style diet based on vegetables, lean protein, healthy fats and whole grains. A protein focused approach can also help people who have a reduced appetite make the most of their meals.
The NHS generally recommends creating a calorie deficit for weight loss rather than requiring one specific diet or macronutrient split. Your prescriber or a registered dietitian can help you choose an approach that fits your health, preferences and treatment plan.
If you are weighing up medical rather than dietary options, our guide to prescription weight loss treatments explains the options available in the UK and how eligibility is assessed.
Keto Drinks, Keto Supplements and Orlistat
A few related questions come up regularly, so here are the key points to know.
Exogenous ketone drinks and ketone salts can temporarily increase the level of ketones in your blood. However, this is different from reaching nutritional ketosis through a very low carbohydrate diet. There is also not enough evidence to show that ketone supplements provide a meaningful weight loss benefit.
You may also see products marketed as combining keto with GLP 1 medicines. GLP 1 receptor agonists such as tirzepatide are prescription only medicines. A supplement cannot legally contain a prescription GLP 1 medicine without meeting the relevant requirements. Be cautious of products that suggest otherwise.
Orlistat, sold as Xenical and at lower strengths as Alli or Orlos, works differently from Mounjaro. It reduces the amount of dietary fat your body absorbs. It is intended to be used with a reduced fat diet.
For this reason, combining orlistat with a high fat ketogenic diet may increase gastrointestinal side effects. These can include oily stools, loose stools, flatulence and an urgent need to use the toilet. If you are taking orlistat, follow the dietary advice provided with your treatment and speak to your pharmacist or prescriber if you are unsure what to eat.
If you are looking for a refund or cancellation for a commercial keto app or subscription, that is a separate consumer issue and should be dealt with directly with the company providing the service. It is not related to prescribed Mounjaro or other weight loss medicines.
Eligibility and Clinical Review in the UK
Mounjaro is authorised in the UK for weight management alongside a reduced calorie diet and increased physical activity. It is also authorised for the treatment of type 2 diabetes in certain circumstances.
For weight management, eligibility may generally be considered for adults with a BMI of 30 or above, or a BMI of 27 or above where at least one weight related health condition is present. These can include high blood pressure, type 2 diabetes, abnormal blood fat levels, obstructive sleep apnoea or cardiovascular disease. Different BMI thresholds may apply to some ethnic groups.
Meeting a BMI threshold does not automatically mean that Mounjaro is suitable for you. A prescriber will consider your medical history, current medicines, previous weight management efforts and other relevant factors before deciding whether treatment is appropriate.
The prescribing information also includes important warnings and precautions. For example, Mounjaro should not be used by people with a personal or family history of medullary thyroid carcinoma or by people with Multiple Endocrine Neoplasia syndrome type 2. Your prescriber will check your medical history as part of the assessment.
NICE guidance for tirzepatide for weight management recommends reviewing treatment after a period of use. Continued treatment may be considered where the person has lost at least 5% of their initial weight after 6 months on the highest tolerated dose. This assessment is based on weight loss and overall treatment response, not on whether you have reached nutritional ketosis.
What Mounjaro Costs Compared With Eating Keto
Mounjaro is supplied in a KwikPen containing four weekly doses, so one pen generally provides four weeks of treatment when used as prescribed.
The cost can change as your prescribed dose changes. Mounjaro treatment usually starts at a lower dose and may be increased gradually if your prescriber considers this appropriate. This means the price of your first pen may not be the same as the cost of treatment at a later dose.
When comparing Mounjaro with the cost of following a ketogenic diet, it is therefore helpful to look beyond the first month. Consider the ongoing cost of your prescribed dose, repeat orders, consultations and any other treatment related costs.
A ketogenic diet may have its own costs, depending on the foods and products you choose, but it does not replace the clinical assessment required for prescription treatment. The best option for you will depend on your health, preferences, treatment goals and the advice of your healthcare professional.
| Strength | Price per pen | Cost per weekly dose |
|---|---|---|
| 2.5mg | £174.99 | £43.75 |
| 5mg | £204.99 | £51.25 |
| 7.5mg | £254.99 | £63.75 |
| 10mg | £274.99 | £68.75 |
| 12.5mg | £304.99 | £76.25 |
| 15mg | £323.99 | £81.00 |
Prices shown are standard website prices and are correct at the time of writing. Prices can change, so the live pricing table at checkout should always be treated as the most up to date source.
One useful point to remember when planning the cost of Mounjaro is that one year contains 52 weekly doses. As each KwikPen provides four weekly doses, 52 doses would require 13 pens rather than 12. This is because four week treatment periods do not always match calendar months. If you are planning a longer term budget, it can be helpful to work from the number of weekly doses rather than simply multiplying a monthly price by 12.
A ketogenic diet does not have a prescription cost, but your food costs may change. Foods such as meat, oily fish, cheese, nuts and specialist low carbohydrate products can cost more than some of the foods they replace. Eggs, lower cost cuts of meat and simple vegetables can help keep the diet more affordable. Your overall food bill will depend on your usual shopping choices, so there is no single cost that applies to everyone.
Questions Worth Asking Before You Buy
- Is the price the standard price, or is it a first order or promotional price that may change for your next pen?
- Does the price include the clinical assessment, prescription and delivery, or are any of these charged separately?
- Is there an additional fee for an expedited clinical review?
- What is the price of the strength you may need later in treatment, rather than only the starting strength?
- Is the pharmacy registered with the General Pharmaceutical Council, and can you check its registration on the public register?
Ordering, Clinical Assessment and Delivery
Treatment usually starts with an online consultation. This may ask about your height, weight, BMI, medical history, current medicines, allergies, previous weight management treatment and pregnancy status where relevant.
A UK registered prescriber reviews the information provided and may ask for further details before deciding whether Mounjaro is suitable. Completing a consultation does not guarantee that you will receive a prescription. If the prescriber decides that treatment is not appropriate, they may recommend another option or advise you to speak to your GP.
If a prescription is issued, Mounjaro should be dispensed by a registered pharmacy. The medicine requires appropriate temperature controlled handling during delivery. Unopened Mounjaro pens should be stored in a refrigerator between 2°C and 8°C and must not be frozen. Once a pen is in use, follow the storage instructions supplied with your medicine.
Delivery times can vary depending on the service, location and other delivery conditions. Some remote areas and parts of Northern Ireland may have different delivery times for medicines that require temperature controlled transport. Check the current delivery information before placing your order.
Medical Disclaimer
This article is for general information only and does not constitute medical advice, diagnosis or treatment. Mounjaro is a Prescription Only Medicine and should only be used following an appropriate clinical assessment and under the supervision of a qualified prescriber.
Whether Mounjaro is suitable for you depends on your individual circumstances, and treatment results can vary from person to person. Do not start, stop or change a prescribed medicine based on this article. You should also speak to your GP, pharmacist or prescriber before making a major dietary change, such as starting a ketogenic diet.
It is particularly important to seek professional advice if you have diabetes, kidney or liver disease, a history of pancreatitis or an eating disorder, or if you are pregnant, planning a pregnancy or breastfeeding.
If you feel seriously unwell or develop symptoms that could indicate diabetic ketoacidosis, seek urgent medical advice. If you are unsure what to do, NHS 111 can provide advice. Suspected side effects from medicines can also be reported through the MHRA Yellow Card scheme.
