Pfizer's Monthly GLP-1 Shot Hits 12% Weight Loss
Pfizer's VESPER-3 Phase 2b trial of its once-monthly injectable GLP-1 PF'3944 hit its primary endpoint with 12.3% placebo-adjusted weight loss at 28 weeks.
February 3, 2026 · 3 min read
Pfizer's once-monthly injectable GLP-1 receptor agonist, PF-08653944 (PF'3944), met its primary endpoint in the Phase 2b VESPER-3 trial on February 3, 2026, demonstrating up to 12.3% mean placebo-adjusted weight loss at 28 weeks with monthly dosing. The result positions the third large pharmaceutical company — alongside Novo Nordisk and Eli Lilly — as a serious contender in the obesity drug market, with a key differentiator: one injection per month rather than one per week.
What happened
VESPER-3 enrolled adults with obesity or overweight without type 2 diabetes and tested four monthly injection regimens of PF'3944 against placebo over 28 weeks. All four active dose arms achieved statistically significant weight reduction versus placebo. The highest-performing regimen produced 12.3% mean placebo-adjusted weight loss at week 28.
The safety and tolerability profile was consistent with the GLP-1 receptor agonist class — predominantly gastrointestinal adverse events (nausea, diarrhea, constipation), mostly mild to moderate in severity. Discontinuation due to adverse events was low, at approximately 4.6% across the active monthly arms.
Pfizer acquired PF'3944 as part of its 2024 deal with Metsera, a private biotech focused on long-acting GLP-1 formulations. Full data from VESPER-3 are scheduled for presentation on June 6, 2026 at the 86th Scientific Sessions of the American Diabetes Association.
Why it matters
The weekly injection schedule of Wegovy and Zepbound is already dramatically more convenient than the daily injections that preceded them. A monthly schedule represents another meaningful reduction in injection burden — four times fewer injections per year, or roughly 12 instead of 52. For patients who cite injection frequency as a barrier, or for those who struggle with consistent weekly adherence, a monthly option would change the practical equation.
The 12.3% placebo-adjusted weight loss at 28 weeks is harder to contextualize directly against the weekly agents, because the trial duration is shorter and the adjustment methodology differs. A rough reference point: weekly semaglutide at 2.4 mg produces roughly 5–7% weight loss in the first 28 weeks (with the full ~15% accumulating over 68 weeks at maintenance dose). PF'3944's 28-week number at 12.3% is competitive, though comparison across trials is inherently imprecise.
The competitive landscape Pfizer is entering is formidable. Lilly's Zepbound and Novo Nordisk's Wegovy are entrenched, and oral options (including Lilly's newly approved Foundayo and the Wegovy pill) are expanding the injectable alternatives further. Monthly injection frequency is PF'3944's primary differentiation; everything else will depend on Phase 3 efficacy and safety data at longer durations.
What to watch
Pfizer has indicated that 10 Phase 3 trials with PF'3944 are planned to advance in 2026. These will include obesity, type 2 diabetes, and likely cardiovascular outcomes programs. Regulatory filing and potential approval timelines are years away, but the pipeline is moving faster than Pfizer's previous GLP-1 attempt (oral danuglipron, discontinued in 2024 after tolerability concerns).
The ADA 2026 presentation on June 6 will include the full dataset, including subgroup analyses and longer-term follow-up from the extension phase. This is the number to watch before drawing firm conclusions about where monthly PF'3944 fits in the efficacy landscape.
For context on how the current leading agents compare, see the tirzepatide vs semaglutide guide, and the retatrutide pipeline overview for how the next wave of triple-agonist therapies stacks up.
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