Tirzepatide vs semaglutide: head-to-head data and Cayman supply

For years, physicians compared semaglutide and tirzepatide across separate placebo-controlled trials. That changed with SURMOUNT-5, published in the New England Journal of Medicine in July 2025, which randomized adults with obesity to one drug or the other⁴. For physicians in the Cayman Islands, the trial answers the efficacy question. Access depends on something else: the public hospital system buys most of its medicines in the United Kingdom¹.

SURMOUNT-5: the first head-to-head obesity trial

SURMOUNT-5 randomized 751 adults with obesity to once-weekly tirzepatide or semaglutide for 72 weeks⁴. The least-squares mean change in weight was -20.2% with tirzepatide and -13.7% with semaglutide⁴. In both groups the most common adverse events were gastrointestinal, mostly mild to moderate⁴.

The result is consistent with the earlier placebo-controlled trials. In STEP 1, semaglutide 2.4 mg produced a mean change of -14.9% at 68 weeks versus -2.4% with placebo, with 4.5% stopping for gastrointestinal events⁶. In SURMOUNT-1, tirzepatide produced -15.0%, -19.5% and -20.9% at 5, 10 and 15 mg over 72 weeks versus -3.1% with placebo⁷. In type 2 diabetes, the open-label SURPASS-2 trial compared tirzepatide with semaglutide in 1,879 patients over 40 weeks: HbA1c fell by 2.01, 2.24 and 2.30 percentage points with the three tirzepatide doses against 1.86 with semaglutide⁵.

The World Health Organization's first guideline on GLP-1 medicines for obesity, issued on 1 December 2025, makes conditional recommendations covering liraglutide, semaglutide and tirzepatide, citing "limited data on their long-term efficacy and safety"⁹. It projects that even with expanding production, GLP-1 therapies "are projected to reach fewer than 10% of those who could benefit by 2030", and suggests "pooled procurement, tiered pricing, and voluntary licensing"⁹.

Where Cayman's medicines come from

The Health Services Authority (HSA), which runs the islands' public hospital pharmacies, explained its supply chain during a 2023 shortage. Its chief executive, Lizzette Yearwood, said about 75% of its drugs come from the UK, 10% from the US and 15% from Puerto Rico, the Caribbean and Latin America¹. Chief pharmacist Colin Medford said the HSA buys mostly generic drugs but must use brand-name products for some narrow-therapeutic-index medicines, that annual tendered contracts limit how quickly it can switch suppliers, and that partial deliveries can leave large orders outstanding for months¹. He added that demand for medication is much higher at the hospital than at private pharmacies, which is why private pharmacies may still have a drug in stock when the hospital does not¹. A 2019 scoping review found that medication shortages "resulted in negative patient clinical, economic and humanistic outcomes", with reports of drug errors, adverse events and higher out-of-pocket costs⁸.

UK prices and US shortages: the two shocks of 2024-2025

Both of the islands' main supply markets moved sharply. In the United States, the FDA declared the tirzepatide injection shortage resolved on 2 October 2024, reconfirmed on 19 December 2024, and the semaglutide shortage resolved on 21 February 2025³. Even so, the agency says "Patients and prescribers may still see intermittent localized supply disruptions"³. In the UK, Eli Lilly announced in August 2025 that it would raise the private list price of Mounjaro by up to 170% from September, taking the monthly supply of the highest dose from GBP 122 to GBP 330; Lilly said it was "aligning the list price more consistently", and the increase did not apply to NHS patients².

Shortages and price gaps feed illegal supply. On 14 October 2026 the UK's Medicines and Healthcare products Regulatory Agency and West Midlands Police seized over 12,000 fake retatrutide and tirzepatide pens worth approximately GBP 2 million in Birmingham¹².

What Cayman law adds

Under the Pharmacy Law, 1979, restricted pharmaceuticals may be imported or dealt in only by the Chief Medical Officer, medical practitioners, the Chief Agricultural Officer and pharmacists (section 11)¹³. The Pharmacy Bill, 2024, which became the Pharmacy Act, 2024 and would replace it, defined an "approved medicine" as one approved in a country prescribed by regulations, approved by the Pharmacy Council, or authorized for exceptional or emergency use, so the Council would not have to approve each medicine individually¹⁰¹¹. Which countries the regulations prescribe will decide which tirzepatide and semaglutide products count as approved. The Act is still listed as not in force¹¹.