No. People currently taking compounded semaglutide or tirzepatide are not wanted for Pfizer's VESPER-Switch weight-maintenance trial.
Compounded drugs are pharmacy-mixed copies of brand weight-loss shots, and the trial rules list current compounded use as ineligible. VESPER-Switch plans to enroll about 660 adults who already lost weight on weekly semaglutide or tirzepatide, according to ICHGCP reporting on the VESPER-Switch trial plan. It will test staying on weekly therapy versus switching to monthly investigational berobenatide.
Official resources:
- Read the official guidance from FDA — Use this primary source to verify the official guidance.
- Read the official guidance from Pfizerclinicaltrials — Use this primary source to verify the official guidance.
Table of Contents
- What does VESPER-Switch want?
- Why does compounded use block eligibility?
- Did any recent study include compounded users?
- How should you check eligibility before applying?
What does VESPER-Switch want?
Pfizer describes Vesper-Switch as recruiting adults who use weekly semaglutide or tirzepatide for weight-maintenance research. The focus is maintenance after weight loss, not starting treatment. Final eligibility is decided by the study team.
Participants would continue weekly therapy or switch to monthly berobenatide. Berobenatide is investigational, meaning it is still under study. The comparison tests whether a monthly shot can maintain weight loss.
Why does compounded use block eligibility?
The U.S. Food and Drug Administration warns that unapproved compounded GLP-1 drugs do not undergo FDA review for safety, effectiveness and quality, as detailed in the FDA safety warning. The agency cites hospitalizations from dosing errors and adverse events from doses beyond approved labels. Trials need consistent, approved dosing to compare results.
The broad shortage allowance for mass compounded copies also ended after transition periods in spring 2025. FDA resolved the tirzepatide shortage on Dec. 19, 2024 and the semaglutide injection shortage on Feb. 21, 2025, according to Healio reporting of FDA actions. That shift returned the market to brand supply as the expected baseline.
Did any recent study include compounded users?
Yes, but not VESPER-Switch. A separate online-survey validation study recruited U.S. adults through an online panel from October 2025 to January 2026.
Eligible people reported at least three months of once-weekly semaglutide or tirzepatide use. That study analyzed 322 people with mean age 45.06. Blinded photo review classified 83 of 322 participants, or 25.8%, as using compounded drugs, according to Medscape reporting on the survey validation findings. Among those 83, 54 people, or 65%, had self-reported a brand such as Ozempic, Wegovy, Mounjaro or Zepbound.
How should you check eligibility before applying?
Start with your current product, not the brand you think it is. Many survey participants mistook compounded vials for brands.
Check the pharmacy label, prescription record and box. Contact the study site directly to confirm current status and whether your medication history qualifies.
- List the exact drug name, dose, pharmacy source and start date
- Note whether the label names Ozempic, Wegovy, Mounjaro, Zepbound or a compounding pharmacy
- Ask the site whether past compounded use matters and when brand use must start
- Confirm recruiting status, because Pfizer lists the study as recruiting while the ClinicalTrials.gov record updated Sept. 29 still showed not yet recruiting
