The researchers analyzed adults who started semaglutide or tirzepatide through MedExpress, a UK online prescriber, between May 1, 2024 and January 1, 2026, and used interrupted time-series models to see what changed around the tirzepatide price increase.[1]
What the data show
- 1,033,522 adults were included. Tirzepatide dominated before the change: 932,659 (90.2%) started tirzepatide and 100,863 (9.8%) started semaglutide.[1]
- Semaglutide starts jumped by 31.15 percentage points in September 2025 compared with the projected pre-change trend (95% CI 26.07 to 36.24).[1]
- Switching from tirzepatide to semaglutide rose by 9.26 percentage points (95% CI 7.09 to 11.42), then faded.[1]
- Some of the shift started early. Sensitivity analyses pointed to anticipatory changes in August, before the new price took effect.[1]
Weight loss, with a caveat
Among patients who had 12 months of follow-up, estimated weight loss was 19.93% on tirzepatide (n = 30,763) and 16.40% on semaglutide (n = 979).[1] These are not randomized comparisons. Only a small share of the million starters had a 12-month weight on record, and people who stay on a cash-pay drug for a year are not typical of everyone who starts one. The authors also note the results came with “limited adjunctive lifestyle support.”[1] For the randomized head-to-head evidence, see our review of switching from semaglutide to tirzepatide.
The study’s authors are affiliated with HeliosX and the consultancy Lane Clark & Peacock, and the data come from a single commercial provider, so the findings describe that provider’s customers rather than all UK patients.[1]
Why it matters in the US
Most people paying cash for a GLP-1 behave like the patients in this study: price decides which drug they start and whether they stay on it. US list prices have moved the other way recently. Lilly’s second-quarter 2026 earnings release attributes part of its revenue pattern to “lower realized prices, including previously announced reductions in cash-pay prices.”[2] Our 12-month GLP-1 price tracker follows what that has meant for compounded and brand-name prices, and the telehealth cancellation index covers how easy each service makes it to stop or switch.