Dr Nick Fuller
Leading Obesity Expert at the University of Sydney and founder of Interval Weight Loss.
So you’ve been taking Mounjaro (tirzepatide) and doing everything ‘right’ but the scale refuses to budge. What is going on?
While this medication can be a powerful tool for weight loss, it’s not a silver bullet. For example, in one meta-analysis, the active ingredient tirzepatide reduced body weight by ~5–20 % depending on dose and context. What this tells us is: medication can give you a head-start, but your lifestyle, dose, body chemistry and follow-through matter a lot.
Mounjaro contains the active ingredient tirzepatide, which mimics two key hormones in your body: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). By activating those hormone pathways, it:
Slows down gastric emptying (so you feel fuller for longer)
Reduces appetite and cravings by acting on the brain’s hunger/fullness centres.
Improves insulin sensitivity and lowers blood glucose, which indirectly supports fat-loss metabolism.
Although many people take it for weight loss, in many regions (including Australia), Mounjaro is officially approved for type 2 diabetes, not specifically for weight loss. Your doctor will emphasise diet and exercise alongside the medication.
Essentially, Mounjaro gives you an appetite-and-metabolism assist, but it doesn’t do all the work by itself. If your lifestyle, dose or other factors are off, you may see less-than-expected results.
Let’s talk numbers so you have a realistic yardstick. Clinical trials for tirzepatide (the active in Mounjaro) show quite impressive results, but with caveats.
In the SURMOUNT- 4 trial, participants achieved a mean weight reduction of ~25.3 % over ~88 weeks (versus ~9.9 % for placebo) when using tirzepatide. Another study found that participants achieved ~10-25% reduction in body weight over ~72 weeks.
So yes, substantial weight loss is possible with Mounjaro, but the ‘how much’ depends on your dose, how your body responds, your lifestyle, any other conditions, and how long you’ve been on it.
If your weight is stuck even while on Mounjaro, here are ten common reasons that might be the culprit.
When you start Mounjaro your doctor will often begin you on a lower dose and gradually increase it, to reduce side-effects like nausea. If you’re still on a low dose, the effect on appetite and weight loss might be modest compared to higher doses.
Also, slower dose escalation means slower full effect. Until you are at the optimal dose (as judged by your doctor), you may not see maximal weight-loss.
Mounjaro helps you eat less and feel fuller, but you still need to be at less energy in than energy out to lose weight. If you’re eating at maintenance or above (even if you feel less hungry), weight loss won’t happen. Think of the drug as a helper not a license to eat freely. Studies of tirzepatide always include diet + lifestyle changes.
You might be underestimating how much you’re actually eating. Drinks, sauces, snacks, ‘just one bite’ add up. Even with suppressed appetite, if you’re unknowingly consuming more calories than you think, weight loss will stall.
Tip: track everything for a week using apps or simply pen-and-paper, to spot sneaky calories.
Even though Mounjaro helps reduce appetite, combining it with physical activity makes a big difference. Activity burns calories, helps preserve lean muscle mass (crucial during weight loss) and improves metabolism.
If you’re mostly sedentary, weight loss will be slower. Think of moving more as investing in your ‘weight-loss bank’.
Sleep isn’t just rest; it’s a metabolic regulator. Poor sleep increases hunger hormones (ghrelin), lowers satiety hormones (leptin), reduces insulin sensitivity. All of that means your body fights weight loss. If you’re not sleeping well, improving sleep quality may unlock stuck weight loss.
Chronic stress raises cortisol. High cortisol can promote fat storage (especially around the midsection), increase appetite/cravings, and reduce your motivation for moving or eating well. Meditation, walk breaks, better stress management can improve how your body responds to the medication.
Thyroid issues, PCOS, menopause hormonal shifts: they all affect metabolism, fat storage and response to weight-loss efforts. Although Mounjaro is working, if your hormones are ‘out of whack’, you may see slower or less weight loss. Worth checking with your doctor if you suspect this.
Some medications can hinder weight loss (for example certain antidepressants, antipsychotics, steroids, etc). If you’re on other medication your doctor might need to review interactions or counter-effects. Inform your prescriber about all meds and ask if any might be blunting your weight-loss response.
After an initial drop, many people hit a plateau. Your body adjusts: lower calories, slower metabolism, less novelty effect.
While taking Mounjaro, you might hit such a phase. That doesn’t mean failure. It means it’s time to adjust your approach (dose, diet, activity) and stay patient.
Because the dose matters, and because your response matters, regular follow-ups are key. Your doctor can adjust dose, check for side-effects, assess lifestyle factors, and review other conditions. If you’re self-managing without medical input, you may miss optimisation opportunities.
Curious whether everyday habits could be slowing your progress on Mounjaro? Read this article on how coffee interacts with weight loss injections to see how small choices might affect your results.
You’re reading this because you want that extra push. Here are actionable, lifestyle-focussed tips to squeeze more from your Mounjaro treatment (with your doctor’s supervision, of course).
Stay consistent with your medication schedule. Take it at the same day/time each week (or as advised) to ensure steady hormone-levels and response. Don’t skip or delay doses unless your doctor advises.
Monitor your food portions. Even though appetite is reduced, ensure you’re not overeating relative to your needs. Use apps/food diaries or portion-control guides.
Choose nutrient-dense meals. Prioritise lean protein, fibre-rich vegetables, whole grains, healthy fats. These help satiety, maintain muscle and support metabolism.
Limit sugary and processed foods. These can spike insulin, trigger cravings, and reduce the benefit of appetite suppression. Keep processed food as occasional, not daily, if you can.
Drink enough water daily. Adequate hydration supports metabolism, digestion and may assist appetite control. Also replace sugar-laden drinks with water or low-calorie alternatives.
Prioritise good sleep. Aim for 7–9 hours, consistent bedtime, minimise screens before bed, create a restful environment. Better sleep helps your body respond to the drug and supports fat-loss.
Manage stress effectively. Include stress-reducing practices: mindfulness, walking, hobbies, social time. Lower stress may help your hormones, metabolism and overall lifestyle adherence.
Include regular physical activity. Aim for both:
Moderate-intensity aerobic activity (e.g., brisk walking, cycling)
Strength training 2–3 times/week to preserve muscle mass
Even small increases in NEAT (non-exercise activity thermogenesis) help (e.g., standing more, walking dog, taking stairs).
Track progress with measurements and logs. Use more than the scale: track waist circumference, body composition (if you have access), clothing fit, energy levels. Log food, sleep, activity, mood so you can spot patterns when things stall.
Stay in close communication with your healthcare provider. Review your dose, check for side-effects, evaluate other conditions or meds. Bring your logs/data to visits; ask ‘what can we tweak?’ rather than ‘why isn’t it working?’
Wondering how long it actually takes to see progress on weight loss injections? Read this article on how long you should stay on weight loss injections to understand the timeline and what real results look like.
If you’re not seeing the weight loss you expected on Mounjaro, don’t beat yourself up. The reality is that medication helps, but won’t replace the fundamentals: diet, movement, sleep, stress and consistent follow-through. Remember that Mounjaro works by reducing appetite and improving metabolism, but you still need a calorie deficit and healthy behaviour for best results.
Dr Nick Fuller is the founder of Interval Weight Loss and is a leading obesity expert at the University of Sydney with a Ph.D. in Obesity Treatment. Dr Fuller is also the author of three best-selling books and his work been published in top ranked journals in the medical field, including JAMA, Lancet and American Journal of Clinical Nutrition.