The news: Pfizer’s experimental once-monthly weight loss drug (obtained through its recent acquisition of Metsera) delivered what the company said were encouraging results in a mid-stage trial. Pfizer announced the results in conjunction with its Q4 and full-year 2025 earnings report.

Digging into the details: The Phase 2b study demonstrated that patients could sustain weight loss and tolerate higher doses when switching from weekly to monthly injections.

The transition to monthly injections was tested over 16 weeks following a 12-week lead-in of weekly injections.

• Patients on the medium dose lost 12.3% placebo-adjusted weight at week 28, while those on the lower dose lost 10%
• Pfizer said that the higher dose the company plans to use in a phase 3 trial could result in 16% weight loss at week 28.
• The company reported that the drug’s side effects were “well-tolerated” and in line with other GLP-1 drugs.

For context, there isn’t an FDA-approved, once-monthly weight loss drug currently on the market. Amgen’s MariTide candidate, administered monthly or less frequently, demonstrated up to 20% average weight loss in people living with obesity without type 2 diabetes in a recent phase 2 study.

Why it matters: Pfizer needs its $10 billion Metsera acquisition to pay off. The company knows it must become a major player in obesity drugs—a market that could be worth up to $150 billion within a decade—and is planning 10 phase 3 trials of the monthly drug slated for 2026.

Capitalizing on the weight loss drug opportunity is critical for Pfizer as other parts of its business decline.

• For example, US revenues from its oral COVID-19 treatment Paxlovid fell 78% YoY in Q4, while US COVID-19 vaccine sales declined 42% over the same period.
• Pfizer executives cited lower COVID-19 infection rates in the US and abroad, along with narrower US health agency COVID-19 vaccine recommendations, as drivers of slowing sales growth.

Implications for obesity drugmakers: Pfizer’s stock was down about 4% on Tuesday, even with its “positive” study results—a signal that the bar for truly successful weight loss in trials is rising as more drugmakers enter the obesity treatment race.

The weight loss drug market is shifting from a two-company stronghold to a hyper-competitive arena where product differentiation matters. Pfizer’s monthly dosing is an advantage, but all players—including incumbents Novo Nordisk and Eli Lilly—must meet rising consumer expectations around convenience, durable weight loss, minimal side effects, muscle preservation, and broader health benefits that extend beyond the scale.

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