How do I know if Mounjaro is working?

What the SURMOUNT-1 trial tells us about timelines, and why yours may look different

The most direct evidence for how Mounjaro performs comes from SURMOUNT-1, a 72-week clinical trial published in the New England Journal of Medicine, which followed 2,539 adults with obesity. At the 15 mg maintenance dose, participants lost an average of around 20 to 21% of body weight over the full study period. That figure is a population average across 72 weeks, it does not describe what week eight looks like for an individual person starting on 2.5 mg.

This matters because many people question whether Mounjaro is working when they are still on the starter dose. The 2.5 mg starting dose exists specifically to let your body adjust to the medicine; its job is reducing side effects, not delivering maximum appetite suppression. The same principle applies at 5 mg. Meaningful weight loss typically accelerates once a therapeutic dose is reached and settled, which for most people begins several months in. If you are wondering when effects typically begin, the honest answer is that the trajectory is gradual by design.

NICE's appraisal of tirzepatide, TA1026, notes that continuing treatment should be reviewed if less than 5% weight loss is achieved after six months at the highest tolerated dose. That threshold gives you a concrete benchmark, not for month one or two, but for the six-month mark at your working dose.

Appetite and behaviour changes: the signs that often come first

Weight on a scale is a lagging indicator. The earlier, softer signals that Mounjaro is working are changes in how you relate to food. People describe feeling full after noticeably less, losing interest in finishing a meal, thinking about food less between meals, and finding that previously hard-to-resist foods simply hold less pull. These are the mechanisms tirzepatide is intended to produce (slowed gastric emptying and reduced appetite signalling) and they tend to emerge in the first two to four weeks at any given dose.

A question our prescribers hear regularly is whether these appetite changes count as progress if the scales have not moved yet. They do. The NHS's patient information on tirzepatide makes clear that reduced appetite is an expected and intended effect of the medicine. Weight loss follows sustained calorie reduction over time; appetite reduction is the mechanism that makes that sustainable.

A practical note on routine: many people keep their Mounjaro pen in the fridge door and inject on the same day each week, which also makes it easier to spot whether a dose was missed. Missed doses can temporarily blunt the appetite effect, so if your hunger returns unexpectedly, that is worth reviewing before concluding the medicine has stopped working.

When the scales genuinely stall: distinguishing a plateau from the medicine not working

Weight loss rarely follows a straight line. Plateaus are common even when treatment is working as expected, the body adapts its metabolic rate over time, and factors like sleep, stress, fluid retention and muscle gain all affect what the scales show on any given day. A week or two of no movement is not, on its own, evidence that Mounjaro has become ineffective.

A longer plateau is different. If weight has not changed meaningfully over six to eight weeks at a stable dose, a prescriber should be involved. The right next step might be a dose increase, a review of lifestyle factors, or (if you are already at your highest tolerated dose and well past the six-month mark) a structured reassessment of whether to continue. This is exactly the kind of question a clinical review covers. If you are concerned that Mounjaro has stopped working for you, there is a detailed look at the evidence around tolerance and dose optimisation on that page. There is also a detailed look at what happens when progress slows at our page on why Mounjaro may stop working for some people, which covers the evidence around tolerance and dose optimisation.

If you have transferred from another provider or are considering starting treatment, the cost of a private prescription is worth understanding too. Our Mounjaro price comparison page sets out what private treatment typically includes and what to look for in a legitimate provider.

Side effects as a signal, and when to take them seriously

Mild nausea, a reduced appetite and some digestive change in the early weeks are signs the medicine is active, not signs something is wrong. They are common, particularly after a dose increase, and they generally settle within days to a couple of weeks. Most people find they ease considerably once they have been on a given dose for a few weeks.

A small number of people experience more significant effects. Severe, persistent stomach pain that radiates to the back is not a normal side effect and needs prompt medical attention, the MHRA flagged acute pancreatitis as a known but infrequent risk in its January 2026 Drug Safety Update for GLP-1 medicines. Any suspected serious side effect can also be reported via the Yellow Card scheme. Separately, if you notice your pen has arrived at room temperature and are unsure about storage conditions, our guidance on Mounjaro storage explains what is and is not acceptable.

If you are curious whether Mounjaro suits your situation and want a prescriber's view, you can start a free consultation, a named clinician reviews every submission the same day.