Pfizer's monthly GLP-1 injection PF'3944 shows up to 12.3% placebo-adjusted weight loss in Phase 2b

Pfizer reported on Feb. 3, 2026, that an experimental once-monthly GLP-1 injection called PF'3944 produced placebo-adjusted weight loss of up to 12.3% at 28 weeks in a Phase 2b trial, and that participants were still losing weight at the end of that period ^[1].

PF'3944, also called PF-08653944 and previously MET-097i, is an ultra-long-acting injectable GLP-1 receptor agonist that Pfizer is developing both as a weekly and as a monthly treatment for adults with obesity or overweight ^[1]. VESPER-3 was designed to test whether weight loss could continue when people switched from weekly shots to monthly shots, cutting the number of injections four-fold ^[1]. Pfizer said the switch worked: no plateau in weight loss was seen at week 28, which the company says suggests further loss as the study runs to week 64 ^[1].

The two monthly maintenance regimens planned for Phase 3 — the low dose in Arm 1 and the medium dose in Arm 3 — showed 10% and 12.3% placebo-adjusted weight loss at week 28 ^[1]. Those are the company's "efficacy estimand" figures, which compare the drug arms with placebo; total weight loss among participants who received the drug is not stated in the release. The primary endpoint, weight reduction from randomization to week 28, was met in all four dose regimens tested, with P < 0.001 ^[1].

VESPER-3 is a 64-week, randomized, double-blind, placebo-controlled study with five arms of about 54 participants each. The arms were: 0.4 mg weekly / 0.8 mg weekly / 3.2 mg monthly; 0.8 mg weekly / 3.2 mg monthly; 0.4 mg / 0.8 mg / 1.2 mg weekly / 4.8 mg monthly; 0.6 mg / 1.2 mg weekly / 4.8 mg monthly; and placebo ^[1]. Participants were adults with obesity or overweight who did not have type 2 diabetes ^[1].

Gastrointestinal side effects, which are common with this drug class, were mostly mild or moderate, with no more than one case of severe nausea or vomiting in any dose group and no severe diarrhea, Pfizer said ^[1]. In Arms 1 and 3, five participants stopped treatment because of adverse events during the weekly phase and five stopped during the monthly phase; no one in the placebo group discontinued for that reason ^[1]. The company described the profile as consistent with GLP-1 medications generally ^[1].

"These topline results from the Phase 2b VESPER-3 study reinforce the potential of PF'3944 as a monthly treatment with competitive efficacy," said Jim List, Pfizer's chief internal medicine officer ^[1].

Why it matters for patients

For people taking or considering GLP-1 medications for weight, the practical question raised here is how often a shot is needed. Pfizer is testing PF'3944 as a monthly option and says it plans to include a higher 9.6 mg monthly maintenance dose in Phase 3 ^[1]. A monthly schedule would mean far fewer injections if it holds up, though VESPER-3 did not include a group that stayed on weekly dosing for the full 28 weeks, so this release does not show a head-to-head comparison of monthly against weekly dosing ^[1].

The numbers also come with limits. This is a mid-stage trial with roughly 54 people per arm, and the results cover 28 weeks, not the full 64 weeks of the study ^[1]. Pfizer calls PF'3944 investigational; whether it will be approved, what it would cost in the U.S. or when it might reach the market has not been disclosed ^[1]. The study enrolled people without type 2 diabetes, so the results do not speak to people who have both obesity and diabetes ^[1].

What happens next

Pfizer says the detailed VESPER-3 results will be presented on June 6, 2026, at the 86th Scientific Sessions of the American Diabetes Association ^[1]. The study itself continues to week 64 ^[1].

Pfizer says 10 Phase 3 trials of PF'3944 are expected to advance in 2026. Those include the already-started VESPER-4 study of once-weekly PF'3944 in people with obesity or overweight without type 2 diabetes, a planned VESPER-5 study in people with type 2 diabetes, a planned VESPER-6 study of once-monthly dosing, and at least seven more Phase 3 studies aimed at other conditions ^[1]. Across its obesity pipeline, the company says it plans 20 or more trials in 2026, including combinations of PF'3944 with an amylin analog and a GIPR agonist ^[1].