Pfizer Shares Dip After Weight Loss Drug Data

Pfizer, the pharmaceutical giant long known for its breakthroughs in medicine, found itself in the spotlight again on February 3, 2026. The company’s announcement of new data from a midstage clinical trial for its weight-loss drug, PF-08653944 (PF’3944), sent ripples through the market, but not all of them were positive. Investors responded with a 4% dip in Pfizer’s stock price, reflecting a mix of optimism and disappointment over what the results might mean in the high-stakes race to compete with Eli Lilly’s blockbuster Zepbound.

According to MarketWatch, the focal point of investor concern was the topline data from the Phase 2b VESPER-3 trial. This study evaluated PF’3944, an ultra-long-acting injectable GLP-1 receptor agonist, as a once-a-month treatment for adults with obesity or overweight who do not have Type 2 diabetes. The numbers were clear: after 28 weeks, participants saw weight reductions of 10% and 12.3%, depending on the dose. For the first 12 weeks, patients received a weekly shot, then switched to a monthly maintenance dose for the remaining 16 weeks. The transition was designed to test whether the drug could maintain its efficacy while reducing the frequency of injections—a key differentiator in an increasingly crowded field of obesity treatments.

“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, MD, PhD, Chief Internal Medicine Officer at Pfizer, in a statement released by the company. “Based on the monthly dosing efficacy and tolerability demonstrated in this trial, we remain confident in our plan to include a higher 9.6 mg monthly maintenance dose of PF’3944 in Phase 3. With PF’3944 as an anchor of Pfizer’s obesity pipeline, we are positioned to address critical gaps in obesity care and meet the diverse needs of patients.”

The VESPER-3 trial, as detailed in Pfizer’s announcement, is an ongoing 64-week, randomized, double-blind, placebo-controlled study. The trial design included about 54 participants per arm, randomized across four different titration protocols, plus a placebo group. The dosing regimens ranged from weekly titration phases to monthly maintenance doses, aiming to find the optimal balance between efficacy and convenience. Notably, the study met its primary endpoint: statistically significant weight reduction at 28 weeks, with no plateau in weight loss observed after switching to monthly dosing. This suggests the drug’s effects may continue to build over time—a tantalizing prospect for patients and clinicians alike.

Safety was also a key focus. The results showed that PF’3944 maintained a well-tolerated and favorable safety profile through week 28, consistent with other GLP-1 receptor agonists. Gastrointestinal side effects, such as nausea and vomiting, were mostly mild or moderate. There was no more than one instance of severe nausea or vomiting in any dose group, and no cases of severe diarrhea. Five participants discontinued due to adverse events during the weekly phase and five more during the monthly phase, but none in the placebo group. This safety profile, while not perfect, is considered competitive in a class where tolerability issues often hinder patient adherence.

Pfizer’s ambitions for PF’3944 extend far beyond this single study. The company announced plans to advance 10 Phase 3 trials of the drug in 2026, covering both weekly and monthly dosing regimens. These trials will include people with obesity or overweight, with and without Type 2 diabetes, and will target a range of comorbidities to increase patient options and access. The pipeline is robust: more than 20 planned and ongoing studies for PF’3944 and related obesity treatments are scheduled for next year, following Pfizer’s recent $10 billion acquisition of Metsera and a global collaboration and license agreement with YaoPharma.

Obesity remains a daunting global challenge. As Pfizer’s release notes, approximately 1.9 billion people worldwide were living with obesity or considered overweight in 2015, a number projected to climb to over 2.9 billion by 2030. Obesity is linked to more than 200 health conditions and is a major driver of chronic disease burden, shortened lifespans, and spiraling healthcare costs. For many patients, current therapies don’t go far enough—whether due to limited efficacy, tolerability issues, muscle mass loss, or barriers to access and affordability. Innovations like PF’3944, which promise effective weight loss with less frequent dosing, could make a significant difference if they clear regulatory hurdles and prove their worth in larger, longer-term studies.

Still, the market’s reaction to Pfizer’s announcement was mixed. As MarketWatch reported, some investors had hoped for even more dramatic results, especially given the $10 billion price tag attached to the Metsera acquisition. The company’s fourth-quarter results, released alongside the trial data, offered little comfort: Pfizer reiterated a 2026 profit outlook that anticipates a decline, underscoring the pressure to deliver on its obesity pipeline. The specter of competition looms large, with Eli Lilly’s Zepbound already making waves as a leading GLP-1-based weight-loss therapy.

Pfizer, for its part, remains undeterred. The company’s expansive development program is designed to address not just obesity itself but also its many related health conditions. PF’3944 is being studied as a single agent and in combination with other peptides, including an amylin analog and a GIPR agonist, which could further enhance its effectiveness. The company’s global resources and legacy of scientific innovation, as highlighted in its statement, position it to tackle the obesity epidemic on multiple fronts.

Of course, challenges remain. Regulatory approval is never guaranteed, and the road from promising midstage data to a successful commercial product is littered with pitfalls. The announcement included the usual caveats about risks and uncertainties, from the potential for unfavorable new clinical data to the ever-changing landscape of healthcare regulation. Pfizer’s own disclosure emphasized that the information was current as of February 3, 2026, and that future developments could alter the outlook.

For now, the story of PF’3944 is one of cautious optimism. The drug has shown it can deliver meaningful weight loss with a once-monthly injection, a convenience that could set it apart from competitors if larger studies confirm these early findings. With a vast clinical program ramping up and the company’s reputation for scientific rigor, Pfizer is betting big on its ability to reshape the landscape of obesity care. Whether that bet pays off remains to be seen, but one thing is clear: the race to develop the next generation of obesity treatments is far from over, and the stakes—both for patients and for Pfizer—couldn’t be higher.