Tirzepatide GLP-1 before and after: what real trial data tells us

What did participants actually look like at the end of 72 weeks?

The SURMOUNT-1 trial enrolled 2,539 adults with obesity but without type 2 diabetes. At 72 weeks, those on the 15mg dose had lost an average of roughly 20–21% of their body weight. To put that in everyday terms: someone starting at 110kg would, on average, have reached around 87–88kg by the end of the study. Some participants lost considerably more; others lost less. That spread is expected, body weight is shaped by genetics, gut physiology, baseline metabolic rate and how consistently the lifestyle components of the programme are followed.

A useful habit: take a photo of yourself on your starting date and note your weight somewhere simple, like the notes app on your phone. A monthly check-in takes under a minute and gives you a concrete reference point rather than relying on memory alone. You can read more about what those early months tend to look like in practice in our three-month tirzepatide results guide.

It is also worth knowing that the titration schedule means the 15mg maintenance dose is typically not reached until around month five of treatment. The early weeks at 2.5mg are about letting the body adjust, not about producing the largest results. Progress in the first month tends to be modest by design, the starting dose is a tolerability phase, not a therapeutic peak.

How does the GLP-1 mechanism translate into visible change?

Tirzepatide works by activating both GIP and GLP-1 receptors, the two pathways that regulate appetite signals after eating, influence how quickly food moves through the stomach, and affect how the brain registers fullness. Most people on treatment describe a gradual but significant reduction in hunger between meals and a lower interest in eating past the point of comfort. The physiological result is a sustained reduction in calorie intake without the constant effort of deliberate restriction.

That mechanism is part of why the before and after difference for many people on tirzepatide extends beyond the number on the scale. Changes in how food tastes, portion sizes that feel natural, and a different relationship with hunger are commonly reported. The NHS medicines page for tirzepatide explains the mechanism and expected effects in plain language and is worth reading before starting treatment.

For those curious about how these changes appear in the face specifically, the tirzepatide face before and after page covers what the evidence and patient reports say about facial changes during treatment.

Does the head-to-head evidence tell us anything useful for before and after comparisons?

SURMOUNT-5, published in the New England Journal of Medicine in 2025, was the first randomised head-to-head trial comparing tirzepatide directly with semaglutide 2.4mg (Wegovy) in adults with obesity but without diabetes. Over 72 weeks, tirzepatide produced a greater average weight reduction. This matters for before and after comparisons because it is the closest thing available to a direct benchmark between the two most widely used injectable weight-loss medicines in the UK.

NICE reviewed these findings as part of its appraisal of tirzepatide (guidance TA1026), noting that indirect comparisons also favoured tirzepatide. That said, NICE is careful (and right to be) about translating trial averages into individual predictions. Which medicine suits any particular person is a clinical question, not a rankings exercise.

If you are weighing up the two options, our weight-loss treatment overview covers what is currently licensed in the UK and how a prescriber approaches the choice.

What affects how much you personally might see in a before and after?

Starting BMI plays a role: people with higher baseline weights tend to see larger absolute losses, though percentage results can be broadly similar. Consistency with the titration schedule matters too, missing doses or pausing treatment interrupts the pharmacological effect. Diet quality during treatment influences results significantly; tirzepatide reduces appetite but does not replace the benefit of adequate protein and fibre, which help preserve muscle alongside fat loss.

Men and women can experience the trajectory of change differently. The tirzepatide results in men guide looks at what the trial data shows for male participants specifically. Meanwhile, early movers (people who respond well in the first four weeks) often see the largest eventual reductions, though this is not a reliable predictor for everyone.

One honest caveat: trial populations are not always perfectly representative of every real-world individual. Participants in SURMOUNT-1 followed structured lifestyle programmes alongside the medicine. That wraparound support contributed to the outcomes, and it is a component that matters in practice too.

If questions about cost are a factor in your planning, our guide to Mounjaro pricing in the UK covers what a legitimate private prescription actually includes and why the headline number is rarely the whole story. And when you are ready to find out whether tirzepatide is clinically suitable for you, the next step is a free consultation with our prescribers, check your eligibility here.