It is the first question almost every patient asks. The honest answer: it depends heavily on the dose you reach, the changes you make to what you eat, and taking it as prescribed.
What the clinical trials show
SURMOUNT-1 (Jastreboff AM et al, N Engl J Med 2022, n=2,539, 72 weeks) remains the landmark trial for tirzepatide in weight management. These figures count everyone who started treatment:
| Dose | Average weight loss | ≥5% lost | ≥15% lost |
|---|---|---|---|
| 5mg | 15.0% | 85% | 48.0% |
| 10mg | 19.5% | 89% | 66.6% |
| 15mg | 20.9% | 91% | 70.6% |
| Placebo | 3.1% | 35% | 8.8% |
In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes who took tirzepatide 15mg alongside diet and activity advice lost an average of 20.9% of their body weight over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Among people who stayed on treatment, the average was 22.5%. On 15mg, 91% lost at least 5% of their weight, compared with 35% on placebo. Individual results vary.
The trial average, worked out for a weight
Based on SURMOUNT-1 clinical trial data, counting everyone who started treatment. This is an average from a trial, not a prediction for you.
A week-by-week timeline
Weeks 1 to 4 (2.5mg)
The starting dose exists to allow adjustment. It is not a therapeutic weight loss dose. Most patients should not expect dramatic weight loss here. What to notice: reduced appetite, feeling full more quickly, possibly less food noise. Weight loss varies significantly between individuals at this stage, the mechanism is working from day one, but clinical effects accumulate over weeks and months.
Weeks 4 to 8 (5mg)
The first therapeutic dose. Appetite suppression becomes more pronounced. Most patients notice eating substantially less without feeling deprived. Weight loss tends to accelerate during this phase.
Weeks 8 to 20 (titration, 7.5mg, 10mg, 12.5mg)
The most consistent period. Each dose increase brings further appetite suppression. Patients often report the most dramatic reduction in food noise during this phase. Based on the overall 72-week SURMOUNT-1 trajectory, patients are typically making meaningful progress by this stage, though the exact pace varies considerably.
Weeks 20 to 36 (maintenance dose)
Most patients tolerating escalation have reached 12.5mg or 15mg. Weight loss continues but the rate often slows as the body adapts and a new weight setpoint is established. This is normal and represents sustainable progress.
Weeks 36 to 72 (consolidation)
Weight loss continues through week 72 in clinical trial data. Meaningful additional loss occurred between weeks 36 and 72 for patients who remained on treatment. Dietary and lifestyle habits built during earlier phases become most important here.
What makes the biggest difference to your results
- The dose you reach: higher doses produce significantly more weight loss. Titrating slowly to minimise side effects is far preferable to stopping at a lower dose due to early discomfort.
- Dietary changes: SURMOUNT-1 required participants to follow dietary guidance alongside medication. The weight loss in the SURMOUNT trials was seen in people using tirzepatide alongside a reduced-calorie diet and more physical activity, which everyone in the trials was counselled to follow. Most weight loss pharmacists recommend 1,200 to 1,500 kcal/day for women and 1,500 to 1,800 kcal/day for men.
- Physical activity: 150 minutes moderate activity per week per NHS guidance. Resistance training preserves lean muscle mass during significant calorie deficit.
- Adherence: missing doses or stopping early significantly impairs outcomes. Mounjaro is a long-term treatment, not a short course.
- Individual variation: genetics, gut microbiome, and baseline metabolic health all affect response. Weight loss plateau for more than 8 weeks at a stable dose warrants clinical review.
What about weight regain after stopping?
In the SURMOUNT-4 trial (JAMA, 2024), 670 adults with obesity or overweight and without diabetes who had lost an average of 20.9% of their weight over 36 weeks on tirzepatide were randomly assigned to carry on or switch to placebo. Over the next 52 weeks, the weight of people who switched rose by an average of 14.0%, while those who carried on lost a further 5.5%. At 88 weeks, people who had switched were on average 9.9% below their starting weight, compared with 25.3% for those who carried on. Individual results vary.
This reflects the chronic, relapsing nature of obesity as a disease. Many patients benefit from ongoing treatment, similar to long-term antihypertensive therapy. This is not a failure. It is biology.
"In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes who took tirzepatide 15mg alongside diet and activity advice lost an average of 20.9% of their body weight over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Among people who stayed on treatment, the average was 22.5%. On 15mg, 91% lost at least 5% of their weight, compared with 35% on placebo. Individual results vary. The weight loss came alongside significant improvements in blood pressure, blood sugar, and lipid profiles."