Does Mounjaro Make You Tired? Causes & What Helps

Written by the ClariDay clinical team. Medically reviewed by Ali Salih, GPhC-registered Pharmacist Independent Prescriber.

Reviewed 9 July 2026. Next review 9 January 2027.

Yes. Mounjaro can make you feel tired, particularly during the first few weeks or after a dose increase. In most people it’s mild and temporary. The most common reason is eating much less than before, although dehydration and in some people, low blood sugar can also contribute [1].

Mounjaro (tirzepatide) is a prescription-only medicine, licensed in the UK and recommended by NICE for type 2 diabetes and, in eligible adults, weight management [1][5]. This page explains why the tiredness happens and what to do about it. It is not advice to start, stop or change your dose.

Is the tiredness actually caused by Mounjaro, or something else?

Yes, it can be, and tiredness is listed as a side effect on the medicine’s label. So if your energy has dropped since you started it is recognised as a common side effect [1][4].

But it is usually caused indirectly, which is genuinely good news, because it means there is normally something you can do about it. Mounjaro’s main job is to turn down your appetite. When it does that well you eat less, sometimes a lot less, and most of the tiredness people feel comes from eating less rather than from the drug itself.

The numbers are reassuring, too. In the trials, about 7 in 100 people on Mounjaro reported tiredness, against about 3 in 100 on a dummy injection [2]. The medicine does add a little, but the large majority never feel it and it was rarely the reason anyone stopped. The trials don’t tell us exactly why the few who were tired felt that way, but in practice the reason is usually simple.

So for most people the tiredness is mild, comes early and points to eating less rather than a warning that something is wrong.

Why does Mounjaro make you feel tired?

The most likely reason is that you are eating a good deal less than you were. That is the first thing to look at, because it is easy to assume the injection is draining you when the real driver is relatively simpler. Reducing your appetite is what Mounjaro is designed to do, so eating less isn’t a side effect sitting next to the main effect; it is the main effect, and feeling low on energy is a natural knock-on effect.

The change can be bigger than people expect. In the trials, people lost around 16 to 22 percent of their body weight over about seventeen months [6], which for someone starting at 100kg, close to 15 stone 10lb, is roughly 16 to 22kg, or 2 stone 7lb to 3 stone 7lb. That is a measure of weight, not energy, but it tells you how far eating has changed. Some people find their appetite falls so far that they go from three meals and snacks to one small meal without really noticing and only feel it as low energy a week or two later.

The next most likely cause is not drinking enough. Mounjaro’s most common side effects mainly affect the guy, feeling sick and loose stools, and either can quietly leave you short of fluid [1]. Because your appetite is down, your drinking often drops along with your eating without you clocking it. Even mild dehydration feels like tiredness, a heavy head and trouble concentrating, which is exactly why it is so easy to blame the medicine.

The third only applies if you take Mounjaro with certain diabetes medicines: a sulfonylurea, such as gliclazide, or insulin. On its own, or with metformin, Mounjaro rarely lowers your blood sugar. But paired with those two, low blood sugar becomes common [1], and it can feel almost exactly like tiredness, with shakiness, sweating and a foggy head [4]. If you don’t take them, you can set this one aside.

Put simply, eating less and not drinking enough explain most cases. The trials tell us tiredness happens; they don’t tell us why any one person feels it. But the three causes are easy to tell apart, and each points to a different fix.

When does the tiredness start, and how long does it last?

It usually starts in the first week or two, and comes back for a few days after each dose increase. That timing is a clue in itself. Mounjaro is stepped up slowly on purpose, and your body takes a little while to settle at each new dose, so a short dip after a dose rise fits the normal pattern rather than signalling a problem.

For most people it lifts once eating and drinking find a steadier routine, usually within the first few weeks. Our guide on how long Mounjaro takes to work sets that out week by week.

The tiredness worth a second look is the kind that doesn’t follow this pattern: that turns up out of the blue on a steady dose, or simply doesn’t ease. That is the one to raise, precisely because it is behaving differently from the common causes.

What actually helps?

Start by working out which of the three is behind it, because the fix follows from the cause. None of this is about changing your dose, which is a conversation for your prescriber.

If you are barely eating, that is usually the main thing, and the fix runs against the grain: eat a bit more regularly, not less. Aim for meals with enough protein, because when you are eating this little the body tends to lose muscle as well as fat, and protein helps limit that. Skipping meals to make the appetite drop “work” is a common way people make the tiredness worse.

If the tiredness came after a spell of feeling sick or loose stools, treat it as fluid first. Sip steadily through the day, more while that lasts, and remember your drinking has probably dropped along with your appetite. And if you have been eating very little as well, plain water on its own may not be enough, because when food intake falls your salts fall with it, and the body needs those salts to hold on to the fluid you drink. An oral rehydration sachet from any pharmacy, dissolved in water, replaces both at once.

If you take a sulfonylurea or insulin, keep something sugary to hand and tell your prescriber about the tiredness. Those medicines, not Mounjaro, are the usual cause of a low, so it is their dose that may need a look, which is their call to make.

If it is worst in the days just after a dose increase, plan for it and give yourself an easier stretch.

If you are not sure which one fits, the eating and drinking basics are the safest place to start, because they cover the two commonest causes at once and do no harm if you have guessed wrong.

When should you speak to your prescriber?

Speak to your prescriber if the tiredness is severe, hasn’t eased after the first few weeks or a dose change, or is starting to shape your day. The common causes settle on a fairly predictable timescale, so tiredness that ignores that timescale is the kind most likely to have another explanation.

Get in touch sooner if you take a sulfonylurea or insulin and go shaky, sweaty or confused, since that points to low blood sugar rather than ordinary tiredness [1][4]. And treat it as urgent if the tiredness comes with a racing heart, fainting, chest pain, breathlessness or clear signs of dehydration; the NHS advises calling 111 if you think a side effect has turned serious [4].

Part of what a prescriber does here is check the simple causes, and part is making sure nothing unrelated is hiding behind a symptom that is easy to pin on the medicine. Our overview of Mounjaro side effects and our main Mounjaro guide have the wider picture.

If you are not sure whether yours has crossed that line, it is fine to ask. Working out whether tiredness is the medicine or something else is exactly the kind of thing a prescriber would rather check early.

In short, tiredness on Mounjaro nearly always has a cause behind it, most often eating too little, not drinking enough, or a dip in blood sugar in people on certain other medicines. Finding the cause is what finds the fix. The tiredness to act on quickly is the kind that is severe, that does not settle, or that arrives with other symptoms.

If you are considering treatment, you can book a consultation with a prescriber to discuss your own situation.

This article is general information, not medical advice, and is not a substitute for a consultation with a qualified prescriber.

Frequently asked questions

Does Mounjaro make everyone tired?

No, most people are not affected. In the trials, about 7 in 100 people on Mounjaro reported tiredness, against about 3 in 100 on a dummy injection [1][2]. When it does happen, it is usually mild and early.

Why does Mounjaro make me feel tired and drained of energy?

Most often because you are eating much less and running on less fuel. Less commonly because gut side effects have left you slightly dehydrated, or, if you also take a sulfonylurea or insulin, because your blood sugar has dropped [1]. More than one can be involved, so it makes sense to check all three.

Will the tiredness go away?

For most people, yes. It clusters in the first weeks and around each dose increase, then eases as your eating and fluids settle. Tiredness that is severe or simply won’t shift is one to raise with your prescriber.

Can Mounjaro cause low blood sugar that feels like tiredness?

On its own or with metformin, low blood sugar is uncommon. With a sulfonylurea or insulin it is common, and it can feel like tiredness alongside shakiness, sweating and dizziness [1][4]. If that fits your medicines, speak to your prescriber.

Should I stop Mounjaro if I feel tired?

Do not start or stop a prescription medicine on your own. Tiredness was rarely a reason to stop treatment in the trials [2]. If it is troubling you, speak to your prescriber, who can find the cause and advise.

References

1. Eli Lilly and Company Limited. (2025). Mounjaro (tirzepatide) Summary of Product Characteristics, sections 4.4 and 4.8. electronic medicines compendium (emc). Retrieved 9 July 2026, from https://www.medicines.org.uk/emc/product/15481/smpc

2. Wadden, T. A., Chao, A. M., Machineni, S., et al. (2023). Tirzepatide after intensive lifestyle intervention in adults with overweight or obesity (SURMOUNT-3). Nature Medicine, 29, 2909–2918. https://doi.org/10.1038/s41591-023-02597-w

3. Systematic review and meta-analysis of the efficacy and safety of tirzepatide for weight loss in adults without diabetes. (2024). Retrieved 9 July 2026, from https://pmc.ncbi.nlm.nih.gov/articles/PMC11576767/

4. National Health Service. (n.d.). Tirzepatide (including Mounjaro): side effects. NHS.uk. Retrieved 9 July 2026, from https://www.nhs.uk/medicines/tirzepatide/

5. National Institute for Health and Care Excellence. (2024, updated 2025). Tirzepatide for managing overweight and obesity (TA1026). Retrieved 9 July 2026, from https://www.nice.org.uk/guidance/ta1026

6. Jastreboff, A. M., Aronne, L. J., Ahmad, N. N., et al. (2022). Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 387(3), 205–216. https://doi.org/10.1056/NEJMoa2206038