Does Mounjaro make you tired?

Tiredness can make everyday life harder, from getting through a busy day at work to finding the energy to cook or go for a walk. If you’ve been feeling it more since getting your medication, you might wonder: does Mounjaro make you tired? It can.

Feeling tired (fatigue) is listed as a common side effect in the Mounjaro patient leaflet, affecting up to 1 in 10 people taking it for weight management. So if you've been thinking “Mounjaro is making me tired”, you're not alone.

The tiredness may be linked to changes that can come with treatment, like eating less, losing fluids, or changes in your blood sugar. In this guide, we’ll cover why tiredness on Mounjaro can happen, how to support your energy levels, and when to get advice.

Why does Mounjaro make you tired?

There isn't always one clear reason. The patient leaflet lists fatigue as a side effect, but it doesn't explain why it happens. If you're wondering “why am I so tired on Mounjaro?", a few changes that come with treatment may play a part.

You’re eating less

Mounjaro affects how your body regulates appetite, which may help you eat less food. This is part of how it helps reduce your body weight. But if your meals become very small, or you skip them entirely, it may be harder to get all the nutrients your body needs. Two to keep an eye on are iron and vitamin B12. Low levels of iron or vitamin B12 can cause tiredness and a lack of energy, according to the HSE.

For more information on how to eat well while on your treatment, our Mounjaro diet guide explains which foods to prioritise, and which to eat less of, and why.

You’re losing fluids

When starting Mounjaro, people affected by certain common side effects like vomiting, nausea, or diarrhoea may become dehydrated. Feeling tired is one of the signs of dehydration, along with feeling thirsty, dark yellow pee, and feeling dizzy, according to the HSE.

You’re drinking alcohol while on Mounjaro

Drinking too much alcohol can also cause of dehydration. Alcohol is also one of the most common causes of night sweats, which can cause you to lose even more fluids. The HSE advises avoiding alcohol within four hours of bedtime to help you sleep well.

It might not be Mounjaro at all

Tiredness has lots of possible causes. Poor sleep and medical conditions such as iron deficiency anaemia, vitamin B12 deficiency, and type 2 diabetes can all leave you feeling tired.

Could blood sugar be making you feel tired on Mounjaro?

If you have type 2 diabetes, it could. Low blood sugar can make you feel tired or sleepy, and type 2 diabetes itself can cause tiredness.

Mounjaro lowers the level of sugar in your body, according to the patient leaflet. But low blood sugar (also known as hypoglycaemia, or a “hypo”) is very common when Mounjaro is used with insulin or a sulphonylurea (another type of diabetes medicine).

Signs of low blood sugar listed in the leaflet include:

• drowsiness
• weakness
• dizziness
• headache
• feeling hungry
• confusion
• irritability
• a fast heartbeat
• sweating

The HSE also lists feeling tired, a lack of energy, and sleeping more than usual among the symptoms of type 2 diabetes.

If you take insulin or a sulphonylurea, test your blood sugar as your doctor, nurse, or pharmacist has told you. Your doctor may need to lower the dose of these medicines while you use Mounjaro. The HSE lists feeling tired or sleepy as a sign of a hypo, and advises talking to your diabetes care team about any hypos you have. Do not drive or use machines if you notice signs of low blood sugar.

How to manage tiredness on Mounjaro

A few small, realistic changes to how you eat, drink, sleep, and move may help you feel more like yourself. Here’s what we recommend:

Prioritise nutrient-dense foods

When you’re eating less, it helps to choose nutrient-dense meals. The HSE recommends eating a variety of foods from each food group daily, and spreading meals and snacks across the day. It also suggests eating something every 3 to 5 hours.

Vitamin B12 also plays an important role in feeling energised. A deficiency can cause tiredness, so it is important to obtain enough through your diet. You can find it in meat, fish, eggs, dairy products, yeast extract, and fortified foods, according to the HSE. Many of these are also good sources of protein.

Iron and zinc are also important in maintaining good energy levels. A review of iron and zinc nutrition notes that, in single-meal studies, vitamin C helped iron absorption, while tea and coffee reduced it. If you're at risk of low iron, try having vitamin C-rich foods like oranges with your meals, and tea or coffee between meals instead. Good sources of iron include meats, pulses, cereals, and dark green leafy vegetables, while zinc can be found in beef, shellfish, dairy products, and nuts and seeds (especially pumpkin seeds).

Stay hydrated

The patient leaflet advises drinking plenty of fluids to avoid dehydration. The HSE advises drinking enough during the day so your pee is pale yellow. Additionally, drinking watter may also support you in your weight loss journey. To find out more, read our guide to the benefits of drinking water.

Practise good sleep hygiene

Adults need 7 to 9 hours of sleep a night, says the HSE. If sleeplessness is adding to your tiredness, the HSE suggests:

• going to bed and waking up at the same times each day
• avoiding screens for at least one hour before bed
• avoiding caffeine and alcohol within four hours of bedtime
• not eating a large meal less than two hours before bedtime
• keeping your bedroom dark, quiet, and not too hot or cold
• not napping within eight hours of bedtime or for more than 30 minutes

Stay active

When you're tired, physical activity might be the last thing you feel like. But the HSE recommends doing at least 30 minutes of moderate activity, like brisk walking, five days a week as part of a good sleep routine. Just avoid exercising within two hours of bedtime – this can make it more difficult to fall asleep.

Ask about supplements

If you think you might be low in certain nutrients, speak to our clinicians or your GP before taking supplements. A blood test can show whether you're low in iron or vitamin B12. Vitamins and minerals that may be worth asking about include:

• Iron: iron tablets prescribed by a GP are stronger than the ones you can buy in pharmacies and supermarkets, says the HSE
• Vitamin B12: experts recommend a daily B12 supplement for people following a plant-based diet, as unfortified plant foods are not a reliable source
• Vitamin D: a letter in a German medical journal notes that low vitamin D can also contribute to fatigue, especially in people who work long hours indoors

“If you’re feeling tired, start with the basics: eat regularly, stay hydrated and make space for rest. If that doesn’t seem to help, speak to your clinician or GP. Rather than guessing which supplements you need, they can investigate whether something like an iron deficiency could be contributing to how you feel.

Niya MansuriPrescribing Pharmacist

How long does tiredness from Mounjaro last?

There's no set time frame, as the patient leaflet doesn't say how long tiredness lasts. If your tiredness is related to dehydration caused by nausea, vomiting or diarrhoea, it may improve once fluid levels are restored and these symptoms settle. However, fatigue can have other causes, so speak to your clinician or GP if it persists or worsens

Do not stop using Mounjaro without talking to your doctor first.

When should you speak to a doctor about tiredness?

Speak to your GP or our clinicians if you:

• feel tired all the time
• have tried changing your sleep routine and it hasn't helped
• often fall asleep during the day
• have had sleep problems for months
• find it hard to cope with daily life because of tiredness
• often wake up with soaking wet sheets from night sweats
• have other symptoms, such as shortness of breath, heart palpitations, or pale skin

This is based on HSE advice on sleep problems, night sweats, and iron deficiency anaemia. Tiredness can have many causes, so always see a doctor for a proper diagnosis.

Call 112 or 999 if you have confusion, severe dizziness, or a fast heartbeat, or have not peed all day. The HSE warns these can be signs of serious dehydration.

Get support with your Mounjaro side effects

Feeling tired on Mounjaro can be frustrating, but there's plenty you can try to support your energy levels while losing weight. For more information on other side effects, check out our guide to Mounjaro side effects.

And if you’re a myBMI patient, you don't have to manage side effects alone. Every myBMI treatment plan includes ongoing clinical support from our IMC-registered doctors, so help is there if side effects get in the way.

Sources

• Dehydration in adults. (2024) HSE. [Accessed 6 October 2026]
• Hypoglycaemia (hypos) - type 2 diabetes. (2024) HSE. [Accessed 6 October 2026]
• Iron and zinc nutrition in the economically-developed world: a review. (2013) Nutrients. [Accessed 6 October 2026]
• Iron deficiency anaemia: symptoms, causes, prevention and treatments. (2021) HSE. [Accessed 6 October 2026]
• Lifestyle changes to treat obesity. (2025) HSE. [Accessed 6 October 2026]
• Mounjaro: package leaflet: information for the patient. (2026) medicines.ie. [Accessed 6 October 2026]
• Night sweats: causes, diagnosis and treatments. (2024) HSE. [Accessed 6 October 2026]
• Sleep problems - tips and self-help. (2025) HSE. [Accessed 6 October 2026]
• Symptoms of type 2 diabetes. (2024) HSE. [Accessed 6 October 2026]
• The importance of vitamin B12 for individuals choosing plant-based diets. (2022) European Journal of Nutrition. [Accessed 6 October 2026]
• Vitamin B12 deficiency as the cause. (2022) Deutsches Arzteblatt International. [Accessed 6 October 2026]
• Vitamin B12 or folate deficiency anaemia: symptoms, diagnosis and treatments. (2024) HSE. [Accessed 6 October 2026]