This retrospective cohort study covered 1,033,522 UK adults initiating semaglutide or tirzepatide through the telehealth provider MedExpress. The period ran from May 2024 to January 2026. Initiation and switching were modelled with segmented interrupted time-series analysis around the September 2025 tirzepatide price increase. Weight trajectories used linear mixed-effects models. Over the whole period 90.2% initiated tirzepatide and 9.8% semaglutide, with higher reach among women than men. In September 2025 semaglutide initiation rose by 31.15 percentage points (95% CI 26.07-36.24) against the projected trend. Switching from tirzepatide to semaglutide rose by 9.26 percentage points (95% CI 7.09-11.42), then attenuated. Estimated 12-month weight loss was 19.93% for tirzepatide (n = 30,763) and 16.40% for semaglutide (n = 979). Follow-up weights covered only a small fraction of the cohort, so weight estimates rest on a selected subgroup.
Why it is interesting: Price alone moved treatment choice sharply in a self-paying market. One provider reaching over a million adults is itself a measure of GLP-1 uptake outside the NHS.
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