Why Am I Not Losing Weight On Mounjaro?

If you haven’t lost weight recently, it’s natural to worry that Mounjaro isn’t working. In most cases, there’s a reason behind it, and it’s usually something we can fix. The most common causes are:

• being on too low a dose 

• eating more calories than you’re burning 

• hitting a normal weight-loss plateau 

• taking other medications that encourage weight gain 

• an underlying condition such as an underactive thyroid or PCOS 

Most people who experience slow progress can get things moving again by reviewing their dose, diet, or both — and we cover each cause below with practical, specific steps.

If you’ve been on a steady dose for 8 weeks or more and aren’t seeing a change, you don’t have to work it out alone – speak to your prescriber at The Family Chemist, and we’ll help you find out what’s going on and what to do next. 

How Much Weight Should I Be Losing on Mounjaro?

How much weight you lose with Mounjaro depends on your dose, your diet, and your metabolism, so it’s highly individual.

To give you a benchmark, the SURMOUNT-1 trial found people lost an average of 15% to 22.5% of their body weight over 72 weeks, depending on their dose. 

Weight loss happens gradually, and the early weeks are about your body adjusting. Here’s roughly what to expect at each stage:

Losing inches but not weight on Mounjaro? 

If the number on the scales has stalled but your clothes feel looser, try not to be disheartened – you’re very likely still making real progress; it’s just not showing up where you’re looking for it.

This is one of the most common and most misunderstood parts of any weight-loss journey, and it rarely means Mounjaro has stopped working. 

There are a few reasons fat loss can hide from the scales: 

Water retention

Our bodies hold and release water all the time, so your weight can swing by 1–2 kg from one day to the next for reasons that have nothing to do with fat. Hormonal changes around your period, a salty meal, or simply drinking more can all mask genuine progress for days at a time. 

You’re holding on to muscle as you lose fat

If you’ve added any strength or resistance training, your body can be losing fat and preserving or building muscle at the same time. The scales barely move, but your shape changes — which is exactly why your measurements drop even when your weight doesn’t. 

New exercise

When you start a new type of activity, your muscles temporarily hold a little extra water as they recover. This can nudge the scales up for a week or two before they settle and begin to fall again. 

The answer isn’t to weigh yourself more often — it’s to stop relying on the scales alone. Take your waist and hip measurements, take a monthly progress photo, and notice how your clothes fit and how much energy you have day to day.

Weigh yourself no more than once a week, at the same time of day, so the normal daily ups and downs don’t throw you off. If the tape measure is moving, Mounjaro is working — so keep going. 

Reasons you’re not losing weight on Mounjaro

Most cases come down to one of the following. We’ve included what to do about each because, in most cases, this is very fixable.

1. You’re still eating above your calorie needs.

Mounjaro reduces your appetite, but it doesn’t switch hunger off completely — and if your intake stays above your maintenance level, weight loss will stall regardless of the medication. This is the most common reason, especially in the first 4–8 weeks. What to do: track your intake honestly for 1–2 weeks and aim for a 500–750 kcal daily deficit, focusing on protein-rich, high-fibre foods that work with Mounjaro’s fullness effect. Our calorie deficit calculator can help you find your numbers.

2. You’re not exercising

As you lose weight, your metabolism naturally dips, so movement matters more than people expect — both for burning energy and for protecting muscle. What to do: build activity in gradually, aiming over time for the NHS recommendation of 150 minutes of moderate activity a week, including two strength sessions. You don’t need to start with a run — a daily walk you can stick to is a genuinely good start.

3. You’ve missed or delayed doses

Mounjaro relies on a steady level of tirzepatide in your system, so even the odd missed dose can blunt its effect. What to do: pick an injection day and time you can easily keep to. The dose can be taken at any time of day, with or without food. If you miss a dose by more than 4 days, don’t double up — take your next one on schedule.

4. An underlying condition may be involved

An underactive thyroid, PCOS or Cushing’s syndrome can all affect weight loss in their own right. What to do: if you haven’t lost weight after 8 weeks on a stable dose, it’s worth asking your GP for thyroid and hormonal tests. Managing these conditions often improves the effectiveness of GLP-1 medicines, and our clinicians can flag this and point you in the right direction.

5. You’ve hit a weight-loss plateau

A plateau of three or more weeks, with no change to your diet or activity, is completely normal — your body adapts to weight loss by using a little less energy. The most common hidden cause is that calorie intake has quietly crept up. What to do: double-check your intake first; if it’s genuinely controlled and the plateau continues beyond four weeks, speak to your prescriber about a dose adjustment.

6. Your dose may be too low

Mounjaro starts at 2.5 mg, and some people — particularly those with a higher starting weight or insulin resistance — may need a higher dose before they experience meaningful appetite suppression. In SURMOUNT-1, higher doses produced more weight loss. What to do: if you’ve been on the same dose for 4+ weeks with settled side effects and stalled weight loss, talk to your prescriber at The Family Chemist about stepping up to the next level.

Why am I not losing weight in a calorie deficit? 

This is one of the most frustrating places to be — you feel like you’re doing everything right, eating carefully, and the scales still won’t budge. It’s far more common than you might think, and there’s almost always a logical explanation behind it. 

You may not be in quite the deficit you think

This is the most common reason, and it’s nothing to feel bad about — it’s genuinely hard to judge. Splashes of oil, drinks, the odd taste while cooking and slightly larger portions all add up quietly. Before you conclude the deficit isn’t working, try logging everything honestly for a week or two, weighing your food where you can. 

Water retention is masking your progress

You can be losing fat steadily while the scales stay flat for a fortnight, then suddenly drop. Constipation — a common side effect of Mounjaro — can add to this, so staying hydrated and getting enough fibre really does help. 

You might be eating too little

It sounds back to front, but cutting calories too hard tends to backfire — you lose muscle, feel drained, and find it much harder to keep going. Aim for a steady deficit of around 500–750 kcal a day with plenty of protein, rather than trying to starve the weight off. A safe, sustainable pace is roughly 0.5- 1 kg per week . 

Something else

An underactive thyroid, PCOS or certain medications can all slow things down in their own right. If you’ve honestly been in a deficit for four weeks or more with no change, it’s worth getting reviewed. 

What to do next:

Check your deficit with an honest food diary, give it two to four weeks before judging it, put protein and fibre first, and resist the urge to crash-diet. If you’ve done all that and you’re still stuck, speak to your prescriber at The Family Chemist — your dose may need adjusting, or there may be another factor worth looking into. 

One gentle word of caution: losing weight very quickly, or eating far too little, isn’t the goal, and it can cost you muscle and leave you feeling unwell. If you have almost no appetite and are struggling to eat enough, or you’re losing more than around 1 kg a week, please tell your prescriber. 

Medications that promote weight gain

There are several commonly prescribed medications that can promote weight gain or insulin resistance which counteract the effects of Mounjaro. If you are taking any of the following alongside your Mounjaro prescription, this may be why your weight loss is slower than expected. 

• Corticosteroids (prednisolone, dexamethasone) – cause fluid retention and increased appetite

• Antidepressants (particularly mirtazapine, amitriptyline, some SSRIs) – associated with weight gain in a significant proportion of patients 

• Antipsychotics (olanzapine, quetiapine, clozapine) – among the highest weight-gain risk medications

• Insulin and some other diabetes medications (sulphonylureassuch asgliclazide) 

• Beta-blockers (propranolol, atenolol) – reduce metabolic rate and can cause fatigue

• Some hormonal contraceptives – particularly injectable contraception (Depo-Provera)

Do not stop any prescribed medication without medical advice.  

Speak to your GP or prescriber at The Family Chemist about whether alternatives exist or whether your Mounjaro dose should be adjusted.  

Our clinicians can review your full medication list for interactions and weight-related effects. 

Poor sleep and high stress 

It’s easy to overlook, but sleep and stress both have a real effect on weight — largely through the hormone cortisol, which can counteract Mounjaro’s appetite-reducing effect. 

Sleep 

When you regularly get fewer than 6–7 hours a night, your body makes more of the hunger hormone ghrelin and less of the fullness hormone leptin — so you feel hungrier even on Mounjaro, and tend to eat more. Aiming for 7–9 hours can make a noticeable difference. 

Stress  

Persistently high cortisol levels encourage your body to store fat, especially around the middle, and drive cravings for high-sugar, high-calorie foods. For anyone going through a stressful patch, this can quietly cancel out the appetite suppression. If stress is a real factor for you, it’s worth speaking to your GP or a counsellor — and our nutritional coaching service includes lifestyle support alongside your prescription. 

What to eat on Mounjaro to get the best results 

Mounjaro makes you feel fuller on less, but what you eat during those smaller windows of appetite really matters. Working with the medication rather than against it can genuinely speed up your results. 

Foods to eat more of

Foods to limit or avoid 

Practical eating tips on Mounjaro: 

• Eat protein first at every meal -it supports satiety and reduces total calories eaten.
• Stop eating as soon as you feel full – Mounjaro makes it easier to recognise this signal and use it.
• Avoid large meals late at night – Mounjaro slows gastric emptying and large late meals can worsen nausea.
• Stay well hydrated and have fibre in your diet – constipation is a common side effect and hydration and fibre intake significantly reduce it. 
• If nausea is an issue, eat smaller, more frequent meals and avoid fatty or spicy food on injection day. 

When should I speak to my prescriber? 

Get in touch with your prescriber if you’re doing everything right and still not seeing results after a couple of months.