Pfizer’s new weight-loss drug may offer monthly alternative to Wegovy and Mounjaro

Pfizer’s experimental obesity drug PF’3944 helped people keep losing weight through 28 weeks of treatment in a mid-stage trial, even after switching from a weekly injection to just once a month.

This result, announced by the company in a statement, marks a potential breakthrough in how weight-loss drugs are delivered—and could challenge market leaders like Wegovy and Mounjaro, which are currently taken once a week.

In the trial, called VESPER-3, people started on weekly doses of PF’3944 for 12 weeks, then switched to monthly injections for the next 16 weeks. By the end of the 28-week study, people taking Pfizer’s drug lost up to 12.3% more body weight on average than those receiving a placebo.

Two versions of the monthly dose—planned for larger Phase 3 trials—showed 10% and 12.3% weight loss compared with placebo. Pfizer said people continued to lose weight at week 28, suggesting the drug’s effects had not yet peaked.

Side effects were similar to other drugs in the same class, with mild to moderate digestive issues like nausea and vomiting. Ten people in the treatment groups dropped out due to side effects, but there were no dropouts in the placebo group.

Why it matters

Today’s leading obesity drugs, like Wegovy (made by Novo Nordisk) and Mounjaro (made by Eli Lilly), have shown strong weight-loss results but must be taken every week. That can be inconvenient for some people, especially those who aren’t used to giving themselves regular injections.

Pfizer’s PF’3944 belongs to the same class of drugs—called GLP-1 receptor agonists—which help people lose weight by regulating appetite and slowing digestion. But unlike Wegovy and Mounjaro, PF’3944 is being developed as a monthly injection.

If that monthly dosing proves effective and safe in larger trials, it could make it easier for patients to stick with the treatment, improve convenience and reduce the burden on healthcare providers managing prescriptions and supplies.

"This is the first GLP-1 class drug with data showing people keep losing weight after switching to a monthly dose," said Jim List, Pfizer’s chief internal medicine officer. “We think this could become a key part of our obesity pipeline.”

What's next

Pfizer said the current trial will continue for a full year to track longer-term results. The company plans to present detailed data in June at the American Diabetes Association meeting and launch ten Phase 3 studies in 2026.

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The company said it aims to run more than 20 studies across its obesity drug pipeline next year.

If successful, PF’3944 could give patients and doctors a new option that delivers similar results to today’s top drugs but with far fewer injections—potentially giving Pfizer a foothold in a fast-growing and competitive market.

The Recap

• Phase 2b VESPER-3 showed continued weight loss to week 28.
• Arm 3 achieved 12.3% placebo-adjusted mean weight loss.
• Pfizer plans ten Phase 3 trials and 20+ studies.