What to Do When Mounjaro Stops Working

What the trial data say about plateaus during tirzepatide treatment

The SURMOUNT-1 trial, published in the New England Journal of Medicine, followed 2,539 adults over 72 weeks. The data show a consistent pattern: the steepest weight loss occurs in the first six months, with the curve gradually flattening thereafter. At the highest dose, participants reached around 20–21% average body-weight reduction, but much of that was established by week 36, and the final months contributed meaningfully less per week than the first.

This is not a quirk of the trial. It reflects physiology. As body weight falls, the caloric deficit required to lose the same absolute amount each week also falls, and metabolic adaptation means resting energy expenditure can decrease. A plateau during tirzepatide treatment is the expected endpoint of a biological process, not a sign the medicine has quietly switched off. If you want to understand when Mounjaro starts working and what the typical timeline looks like, that context can help you judge whether what you are experiencing is normal before concluding the medicine has stopped working for you.

That said, plateaus that arrive earlier than expected, or weight that begins to creep back without any change in behaviour, are worth investigating. Those are different situations, and the right response to each differs too.

Common reasons Mounjaro's effect can reduce, and which ones are fixable

When people ask what to do if Mounjaro stops working, they are often describing one of several distinct situations. Knowing which one applies changes the answer considerably.

Dose hasn't reached an effective level yet. Treatment begins at 2.5 mg, a dose sized for tolerability rather than therapeutic effect. If you are still on a lower dose and progress has stalled, the answer may simply be a scheduled titration step. This is a conversation for your prescriber, not a conclusion to jump to alone.

Injection technique or storage. Tirzepatide must be kept refrigerated at 2–8°C; the SmPC specifies a limited window for room-temperature storage. A pen that has been left out too long, or one that has been injected at the wrong site or angle, may deliver less medicine than it should. Our guidance on what to do when taking Mounjaro, including how to store and inject it correctly, covers these practical steps in full so you can rule out one of the more avoidable causes of reduced effect.

Lifestyle factors accumulating. Tirzepatide works alongside a reduced-calorie diet and increased physical activity, that framing appears in the licence for a reason. Sleep debt, alcohol, ultra-processed food creep and reduced step counts can all blunt a medicine that operates partly through appetite signals. Our guidance on supporting Mounjaro's effect through lifestyle covers this in detail.

The plateau is real, and it's time for a clinical review. If you have been on the maximum tolerated dose for three or more months with no measurable progress, that is a genuine clinical question. NICE's appraisal of tirzepatide notes that continuing treatment should be reviewed if weight loss is less than 5% after six months at the highest tolerated dose. Your prescriber can look at the whole picture and advise on next steps, which might include optimising lifestyle support, considering a dose change, or discussing whether a different medicine might suit you better.

What a prescriber review actually looks like, and why it matters

A clinical review when Mounjaro stops working is not a tick-box exercise. At nume, every repeat prescription involves a prescriber reading your notes, not a system auto-approving based on the previous month. That means a plateau or a reversal in progress will be seen and discussed, not waved through.

The review will typically consider how long you have been on your current dose, what your weight has done over recent weeks, whether any new medicines or health changes could be interacting, and whether there are aspects of the wraparound support (diet, activity, sleep) worth addressing. Our page covering what to do when on Mounjaro, including the lifestyle habits that make the biggest difference, covers some of the lifestyle context that often comes up in these conversations.

If the conclusion is that tirzepatide is no longer the right fit, semaglutide (Wegovy) is the other GLP-1 class medicine licensed for weight management in the UK, and a prescriber can weigh up whether switching makes sense for you. More background on how the medicines compare is available on our weight-loss treatment overview. Every prescribing decision (including any change of direction) needs the same clinical assessment that started treatment in the first place.

If you are mid-treatment and wondering whether what you are experiencing is a normal plateau or something that needs a closer look, the most useful thing is to raise it directly rather than waiting. A prescriber at our pharmacy can help you read your own data clearly. You can find more detail on the specific question of whether Mounjaro stops working over time, or explore whether this is a known pattern in the broader evidence. When you're ready to start or continue treatment with clinical guidance, you can begin your free consultation, a named prescriber reviews it the same day.