Why Am I Putting On Weight With Mounjaro? (October 2026)

01 · The core answer

Is the weight gain real, or is it noise?

Most of the time it is noise. Body weight moves from day to day with fluid, food still in your gut, salt, hormones and even the time you weigh yourself. None of that is fat. A single higher reading tells you very little, especially in the first few weeks of treatment or after a dose change.

The way to tell is the trend. Weigh at the same time of day, on the same scale, in similar clothing, and compare a run of readings over several weeks rather than one morning against the last. If the trend line is flat or heading down, you are fine. If it heads steadily upwards, work through the likely causes one by one.

Remember what the medicine does. Tirzepatide activates two gut-hormone receptors, GIP and GLP-1, which help regulate appetite and slow how quickly the stomach empties. In practice most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ That support only helps if the diet side is still happening, which is where most unexpected gains come from.

02 · What to check

The most likely reasons the scale is going up

Work through these in turn; none replaces advice from your prescriber.

If you want a deeper walk-through of the plateau side of this, our guide to why you might be stalling on Mounjaro covers it in more detail.

03 · Your next step

What to do next

First, give it time and gather evidence: a few weeks of consistent weigh-ins, a short food diary and a note of any missed doses. Second, if the trend is still upward, contact the prescribing team rather than adjusting anything yourself. Our monthly reviews exist for exactly this, and the team can look at your dose, side effects and routine together.

If you are not yet a patient and are weighing up treatment, you can start with the Mounjaro pen and free online consultation, and our weight loss clinic page explains how the programme fits together. Treatment is only supplied after a review by a GPhC-registered prescriber.

04 · Normal weight fluctuation

Why the scale moves even when treatment is working

The scale measures everything in your body, not just fat. Water, food moving through your gut, hormonal shifts and inflammation after exercise or illness all change the number. Those swings can easily hide real progress for a few days at a time.

This matters because trial results are averages over a long period. In the SURMOUNT-1 trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages, and individual results vary. Nobody in a trial lost weight in a perfectly straight line.

Practical habits help. Choose one weigh-in time, ideally the same morning slot each week, and write it down. Many people find weighing less often calmer and more accurate than checking daily. A phone note or the free GLPTrackr companion app makes the trend easy to see.

05 · Eating and drinking changes

Eating and drinking: where gains often hide

Mounjaro does not remove calories on its own. It helps you feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.² If the eating side drifts, the scale can drift with it.

Eating patterns are often the practical cause rather than a medical one. When a full plate feels unappealing, people reach for easy options such as sweet drinks, milky coffees, biscuits, crisps or ready meals that are small but energy-dense. Alcohol is another quiet contributor. Because these do not feel like proper meals, they are easy to overlook in your head.

Portion sizes tend to creep back up once the novelty wears off. A short, honest food diary for a couple of weeks usually reveals the pattern faster than guesswork.

Focus on foods that keep you satisfied: protein at each meal, vegetables, and wholegrain or high-fibre carbohydrates. You do not need a special diet. If particular foods make you feel unwell, mention that to your prescriber so they can support you.

06 · Dose, timing and missed doses

Dose, timing and missed doses

Mounjaro comes as a pre-filled multi-dose KwikPen in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg, taken once weekly.¹ Treatment normally starts low and is stepped up over time, so in the early weeks the effect on appetite can be modest. Gaining or standing still at a starting dose is not unusual and does not mean the medicine cannot work for you.

Regular timing matters. If doses are late, skipped or irregular, appetite can feel stronger again before the next injection. If you have missed a dose, follow the instructions in the patient information leaflet and ask the prescribing team; do not take extra doses to catch up.¹³

Any change to your dose should come from your prescriber after a review. If you have been on the same dose for a long time with no response, that is a conversation to have with them, and our guide on not losing any more weight on Mounjaro explains what that review usually covers.

07 · Early weeks and 5mg

Gaining in the early weeks or at a lower dose

Early treatment is when the scale is least reliable. Your body is adjusting, your dose is still low, and a change in what you eat can easily outweigh the early effect of the medicine. Appetite suppression builds as the dose is stepped up, and some people feel little at first.

If you are at the start of treatment and not feeling much difference in hunger, our pages on not getting suppression on Mounjaro and not losing weight at 5mg go through what is normal at that stage. The key message is not to panic and not to adjust anything alone. Tell the prescribing team how you feel so they can decide whether to hold or step up your dose.

For a broader view of the usual reasons results stall, see our UK guide to not losing weight on Mounjaro and the shorter explainer on why you might not be losing weight.

08 · Gaining after stopping

If you have stopped or are thinking of stopping

Stopping Mounjaro can lead to weight regain: in SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average.¹⁴ Tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system.¹ Nothing needs doing while it clears, and there is no way to speed it up. As it leaves, hunger may return gradually.

Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load. Our page on putting on weight after Mounjaro covers this in more depth.

09 · When to get advice

When to speak to your prescriber

Speak to your prescriber if your weight rises steadily over several weeks despite regular doses, or if anything about your symptoms is new or lasting. If you have severe, persistent stomach pain that may spread to your back, pain in the upper right abdomen with fever or yellowing of the skin or eyes, swelling of the face or throat, or difficulty breathing, get medical help straight away. Your prescriber can review your dose, your routine and your overall health, and decide whether treatment is still right for you.

If a new symptom appears, do not assume it is the medicine and do not assume it is not. Describe it to the prescribing team. You can also report suspected side effects through the MHRA Yellow Card scheme.¹⁹

Our support team is available Monday to Friday, 9am to 5pm, by email at support@everydaymeds.co.uk, through the account portal chat or by phone on 0333 207 0413. Mounjaro is a prescription-only medicine supplied after a consultation reviewed by a GPhC-registered prescriber, and individual results vary. This page is information, not a substitute for medical advice.⁴