When the UK list price of tirzepatide (sold as Mounjaro) rose by up to 170% in September 2025, people paying privately for weight-loss injections moved quickly toward the cheaper alternative. A study of more than one million self-paying adults at a single UK online provider, published on 14 October in Diabetes, Obesity and Metabolism, found that the share of new patients starting semaglutide jumped by about 31 percentage points above its previous trend, and that switching from tirzepatide to semaglutide rose as well. For anyone paying out of pocket, it is unusually large evidence that price, and not only effectiveness, decides which drug people end up on.
What the study measured
The authors looked back at every adult who started semaglutide or tirzepatide through the UK telehealth provider MedExpress between 1 May 2024 and 1 January 2026. That came to 1,033,522 people: 932,659 (90.2%) started on tirzepatide and 100,863 (9.8%) on semaglutide. The provider reached more women than men.
To measure the effect of the price change, they used an interrupted time-series design. In plain terms, they projected how many people would have started each drug if the trend before September 2025 had simply continued, then compared that projection with what actually happened. In September 2025, semaglutide starts were 31.15 percentage points higher than projected, with a 95% confidence interval of 26.07 to 36.24. Every value in that range is far from zero, so the shift is not a statistical accident. Switching from tirzepatide to semaglutide rose by 9.26 percentage points (95% CI 7.09 to 11.42) and then faded, which suggests a wave of switching around the change rather than a lasting drift. A sensitivity analysis found the shift began in August, consistent with people acting before the increase took effect.
The study also estimated weight change. At 12 months, people on tirzepatide had lost an estimated 19.93% of their starting weight (95% CI 19.88 to 19.98) and people on semaglutide 16.40% (95% CI 16.16 to 16.64), a gap of about 3.5 percentage points. The authors note this was achieved with limited lifestyle support alongside the medication.
What it means for people paying out of pocket
Anyone who pays for these drugs privately is exposed to price changes that an insurer would otherwise absorb, and those prices are set by manufacturers and providers and can move at short notice. Our guide to GLP-1 costs without insurance sets out how the US self-pay market works, where the manufacturer cash-pay programmes play a similar role to the UK private market described here.
The weight figures point in the same direction as the head-to-head trial evidence. In the randomised SURMOUNT-5 trial, tirzepatide produced more weight loss than semaglutide, which we cover in Wegovy vs Zepbound. This study adds real-world evidence that the gap persists outside a trial. Put together, the two findings describe the trade-off that a price rise creates: in this cohort, people who moved to the cheaper drug moved to the one associated with somewhat less weight loss on average. The study does not tell us how individual switchers fared, and it does not settle which drug is the better choice for any particular person.
What is not yet known
Several limits matter. This is an observational study at one provider in one country, and its authors include staff of HeliosX and analysts at Lane Clark & Peacock LLP; the publisher’s full text was not reachable, so we could not check the funding statement or how weights were recorded. The weight figures rest on a small slice of the cohort: 30,763 tirzepatide users, about 3.3% of those who started it, and 979 semaglutide users, about 1.0%, contributed 12-month data. People who stay on treatment and keep reporting tend to be those doing well, so these numbers are likely nearer the top of the range than typical. The abstract also does not say how many people stopped treatment altogether after the price rise.
Sources: Dix S, Mulligan A, Ficarelli M, et al. Real-World Treatment Patterns and Weight Outcomes Before and After a Tirzepatide Price Increase in Over One Million Self-Paying UK Adults With Overweight or Obesity. Diabetes, Obesity and Metabolism, published online 14 October 2026 (PubMed 42875827).
