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Pfizer’s Once‑Monthly GLP‑1 Shows Strong Weight Loss but Raises Dosing‑Safety Questions

Mid‑stage ADA data led Pfizer to plan slower dose escalation and an expanded Phase 3 program to test monthly dosing after trials showed meaningful weight loss with clustered gastrointestinal side effects.

The logo for AstraZeneca is seen outside its North America headquarters in Wilmington, Delaware, U.S., March 22, 2021.  REUTERS/Rachel Wisniewski/File Photo
The logo of German pharmaceutical company Boehringer Ingelheim is seen at its building in Shanghai, China February 1, 2019. Picture taken February 1, 2019. REUTERS/Stringer
Eli Lilly logo is shown on one of the company's offices in San Diego, California, U.S., September 17, 2020. REUTERS/Mike Blake/File Photo
A logo of Pfizer is seen next to its products on display at the company’s booth at the 8th China International Import Expo (CIIE) in Shanghai, China, November 6, 2025.REUTERS/Maxim Shemetov

Overview

  • Pfizer presented VESPER mid‑stage results at the American Diabetes Association meeting showing a 4.8 mg once‑monthly dose produced up to 12.3% placebo‑adjusted weight loss after 28 weeks and earlier VESPER‑1 weekly dosing showed about 15.9% weight loss over eight months.
  • Trial safety data reported mean nausea around 38% and mean vomiting about 23.3%, with an uptick in adverse events when patients switched from weekly to monthly dosing, prompting the company to adopt slower titration in late‑stage studies.
  • Pfizer has laid out an extensive late‑stage plan that includes more than 20 trials this year and roughly 10 active or planned Phase 3 studies to confirm efficacy, refine dosing transitions, and monitor longer‑term safety.
  • A separate observational study published in the Annals of Oncology found GLP‑1 use in obese, non‑diabetic adults was associated with a 41% lower incidence of obesity‑related cancers over about two years, but authors cautioned the result is an association and not proof of causation and noted subgroup differences including no observed reduction among Black patients.
  • The commercial and clinical landscape is shifting fast as rivals (Novo Nordisk and Eli Lilly) push oral pills and higher‑efficacy agonists and Medicare expands GLP‑1 coverage in July 2026, which could broaden access but will raise scrutiny of safety, long‑term outcomes, and real‑world adherence if monthly dosing is adopted.