Feeling wiped out the day after your injection? You're not imagining it. Mounjaro tiredness is a recognised side effect of Mounjaro, and so is the nausea that can make a workout feel like the last thing you want to do.1 It's a similar story with Wegovy, where feeling weak or tired is listed as very common.2
You don't have to choose between your treatment and staying active, though. This guide covers why exercise can feel harder on GLP-1 medicines, how to plan your week around your injection, what to eat before and after a session, and when tiredness is worth a chat with your GP.
Lower energy days are a common part of taking GLP-1s. Planning your week around them works far better than trying to push through.
Does Mounjaro make you tired?
It can. Tiredness is listed as a common side effect for people using Mounjaro for weight management, which means it affects up to 1 in 10 people.1 With Wegovy, feeling weak or tired is classed as very common, affecting more than 1 in 10.2
These medicines work by helping you feel fuller and less hungry, so you eat less food.1 That's the point, but less food also means less fuel for your day, and for your workouts.
Many people describe their Mounjaro tiredness as worse the day after their injection, or in the week after a dose increase. Dehydration can play a part too, especially if you've been sick or had diarrhoea. Spotting how your body reacts is the first step to planning around it.
Does it last forever? Usually not. The Wegovy patient information leaflet says its very common side effects, tiredness included, usually go away over time.2 And for Mounjaro, the gut side effects that often come with the tiredness are most common while your dose is going up, then tend to ease.3
Tiredness tends to be at its worst in the early weeks and after a dose increase. For most people, it's a short phase rather than the norm.
Why does exercise feel harder on GLP-1 medicines?
There's three things that tend to stack on one another.
Slower digestion. Tirzepatide, the active ingredient in Mounjaro, slows down how quickly your stomach empties.3 Food sits around for longer, so bending, bouncing or lying flat soon after eating can feel uncomfortable.
Nausea. In the weight management trials for Mounjaro, between a quarter and a third of people taking it reported nausea, compared with under 1 in 10 on a placebo (dummy treatment).3 It's hard to give a workout your all when you feel queasy.
Dizziness. Feeling dizzy is a common side effect of both medicines.1 With Wegovy, it mainly happens in the first 3 months of treatment.2 Mounjaro can also lower blood pressure in some people, which is why standing up quickly after floor exercises can leave you light-headed.1
This doesn't mean you should stop exercising. It means the way you train might need to change for a bit.
If a session feels tougher than it should, it's usually your body adjusting to the medicine, not a sign you've lost fitness.
When is the best time to exercise around your injection?
There's no strict clinical rule. Both patient information leaflets say you can inject at any time of day, with or without food, and neither tells you to avoid exercise afterwards.1,2
What we do know is that gut side effects are most likely while your dose is being stepped up.3 Beyond that, it comes down to how you feel. Many people notice they feel roughest in the first day or two after their injection, then pick up as the week goes on. Others feel no difference at all.
A simple way to plan your week:
- Injection day and the day after - if you don't feel great, keep it light. A gentle walk, some stretching or a mobility session all count.
- The rest of the week - this is often when energy picks up, so it's a good slot for your strength sessions.
- Dose increase weeks - drop the intensity for a week or so while your body adjusts, then build back up.
Following our 4-week beginner workout plan? Just swap your hardest session away from your injection day if you need to. The plan is built to flex.
Want to move your injection day to suit your routine? Both Mounjaro and Wegovy allow this, as long as it's been at least 3 days since your last dose.1 2 Check with your prescriber if you're not sure.
Do not skip a dose, take a double dose or change your dose to fit around your workouts. Speak to your prescriber first.
Can you do core exercises on Mounjaro or Wegovy?
Yes, but you might want to change which ones. Moves where you lie flat or fold at the waist, like crunches, sit-ups and leg raises, put pressure on your stomach. With digestion running slower than usual, some people find these bring on nausea.
Standing and upright core work tends to be easier to manage. Try these swaps:
- Instead of crunches - try a forearm plank, or an incline plank with your hands on a bench or worktop.
- Instead of sit-ups - try a bird dog, which works your core from all fours without folding your stomach.
- Instead of leg raises - try standing knee raises, holding a wall for balance if you need to.
- For something harder - try a suitcase carry. Hold a weight in one hand and walk slowly, keeping your body upright.
You'll find more moves, with beginner tweaks, in our guide to the best exercises to do on Wegovy or Mounjaro.
Your core works hard in plenty of standing moves. You don't have to be lying flat on the floor to train it.
What should you eat before and after exercise on Mounjaro?
Getting your food right around a workout makes a real difference, especially when your appetite is lower than it used to be.
Before you train
Go small and light. Rich food tends to take longer to digest, and your stomach is already emptying more slowly than usual.3 A banana, a slice of toast or a few crackers an hour or two beforehand gives you some fuel without weighing you down.
Training on a completely empty stomach can leave you feeling shaky, especially when you're eating less overall.
After you train
Aim to get some protein in. Eating enough protein alongside strength training can help you hold on to muscle while you lose weight. We explain why in our guide to GLP-1 medicines and muscle loss.
Eggs, Greek yoghurt, fish or a protein shake are all easy options when you're not very hungry.
Drink plenty
Feeling sick, being sick or having diarrhoea can all lead to dehydration, and the instructions for both treatments stress the importance of drinking enough fluids.1,2
Sweating during exercise adds to that, so sip water before, during and after a session.
On insulin or a sulphonylurea? Low blood sugar is more likely when you add a GLP-1 medicine, so check your exercise plans with your diabetes team first.
Taking diabetes medicines too? If you use insulin or a sulphonylurea alongside Mounjaro, your risk of low blood sugar goes up.1 Exercise can lower blood sugar too, so speak to your team before making big changes to your activity.
When is tiredness normal, and when should you see a GP?
Mounjaro tiredness or Wegovy fatigue that comes and goes with your injection week, eases as the weeks go on and still lets you get on with your day is usually your body adjusting. A quick note on your phone of when you inject, how you sleep and how you feel can help you spot the pattern, and it's useful to show your prescriber if things don't settle.
Speak to your GP or prescriber if:
- Your tiredness isn't improving, or it's getting worse
- You often feel dizzy or faint, during exercise or at rest
- You can't keep fluids down, or you have vomiting or diarrhoea that won't settle
- You have diabetes and notice signs of low blood sugar, such as shaking, sweating, confusion or a fast heartbeat
- Exercise feels much harder than it should, even on your best days
Severe stomach pain that will not go away is never something to wait out. Stop your medicine and get urgent medical help.
Some symptoms need urgent help. If you get severe stomach pain that won't go away, with or without being sick, stop using your medicine and get urgent medical help.1,2 This can be a sign of an inflamed pancreas, which is serious.
If you get chest pain, feel very short of breath or feel faint while exercising, stop straight away and get medical help.
Frequently asked questions
Does Wegovy make you tired?
It can. Feeling weak or tired is listed as a very common side effect of Wegovy, which means it affects more than 1 in 10 people. The leaflet says these very common side effects usually go away over time. Eating regular small meals and drinking enough fluids can help in the meantime. If tiredness isn't improving or is affecting your daily life, speak to your prescriber.
Does Mounjaro fatigue go away?
For many people, yes. Tiredness is a common side effect of Mounjaro, and the gut side effects that often come with it are most common while your dose is going up, then tend to ease. Small regular meals, enough fluids and gentler exercise around your injection day can all help. If fatigue doesn't settle or gets worse, speak to your GP or prescriber.
How long does nausea last with Wegovy?
It varies from person to person. Feeling sick is a very common side effect of Wegovy, and the leaflet says it usually goes away over time. Many people find it's most noticeable after starting treatment or moving up a dose. Smaller, lower-fat meals can help. If you can't keep fluids down or the nausea doesn't settle, contact your prescriber.
Can Mounjaro cause dizziness?
Yes. Dizziness is a common side effect in people using Mounjaro for weight management, affecting up to 1 in 10. Mounjaro can also lower blood pressure in some people. Stand up slowly after floor exercises, drink enough fluids and avoid long solo workouts somewhere remote if you're prone to it. Speak to your GP if dizziness happens often or doesn't ease.
Why does Mounjaro make you tired?
Mounjaro helps you feel fuller and less hungry, so you eat less. That's how it works, but it also means less energy coming in. Gut side effects such as feeling sick or diarrhoea can add to tiredness, particularly if they leave you dehydrated. Many people find it's worst in the days after their injection or after a dose increase.
Can you lose weight on Mounjaro without exercise?
Mounjaro is licensed for weight loss alongside a reduced-calorie diet and increased physical activity, so exercise is part of the plan rather than an optional extra. Being active matters beyond the scales too. Strength training can help protect your muscle while you lose weight, and regular activity supports your heart health. Even a gentle walk on a low-energy day counts.
References
- Eli Lilly and Company Limited. Mounjaro KwikPen: Package leaflet: Information for the patient. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Novo Nordisk Limited. Wegovy 2.4 mg, FlexTouch solution for injection in pre-filled pen: Patient Information Leaflet. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Eli Lilly and Company Limited. Mounjaro KwikPen 2.5mg solution for injection in pre-filled pen: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.