Amgen's MariTide is the second monthly-dosed obesity injection now in Phase 3, and the readouts that will determine whether it competes with retatrutide and the GLP-1 weekly class are tracking to early 2027. Phase 2 data published in November 2025 and presented at ADA 2026 showed mean weight loss up to 20% at 52 weeks with once-monthly dosing — and no plateau. With Pfizer's PF-3944 monthly injection heading into Phase 3 this year and Eli Lilly's retatrutide TRIUMPH-1 readout due in Q2 2026, the obesity drug landscape is bifurcating fast: weekly maximum-efficacy on one side, monthly convenience on the other.
Research-context information only. Peptides and investigational drugs discussed below are research compounds. Doses and outcomes reported come from published clinical trial data. This article reports what has been documented, not what should be done. Consult a licensed physician for personal medical decisions.
What MariTide Actually Is
MariTide is the development name for maridebart cafraglutide, a peptide-antibody conjugate Amgen designed for monthly subcutaneous administration. Two structural elements drive the molecule's profile.
The peptide payload combines GLP-1 agonism with GIP antagonism. This is the structural feature that distinguishes MariTide from every other late-stage obesity drug. Tirzepatide is a GIP agonist; MariTide is a GIP antagonist. The mechanism rationale is that blocking GIP signaling — rather than activating it — may improve weight loss durability and reduce the rebound that sometimes follows GLP-1 monotherapy. The Phase 2 readout's lack of plateau at 52 weeks is the first clinical signal this hypothesis has merit.
The antibody backbone supports monthly dosing. Amgen attached the GLP-1 + GIP-antagonist peptide to an antibody scaffold engineered for an extended pharmacokinetic half-life. Weekly GLP-1s like semaglutide and tirzepatide have half-lives of roughly 5-7 days. MariTide's effective half-life is engineered for once-monthly subcutaneous administration without losing therapeutic plasma concentration between doses. This is the same engineering pitch as Pfizer's PF-3944, with a different molecular approach (antibody conjugate vs. fully-biased agonism).
The result is a once-monthly injection with a mechanism that is structurally different from anything currently on the market.
The Phase 2 Numbers
Amgen reported topline Phase 2 results in November 2025 and presented detailed data at the American Diabetes Association meeting in June 2025. Two cohorts ran in parallel:
| Population | Dose Range | Mean Weight Loss (52 weeks) | Placebo |
|---|---|---|---|
| Obesity, no T2D | Active arms | 12.3% to 16.2% | 2.5% |
| Obesity + T2D | Active arms | 8.4% to 12.3% | 1.7% |
| Obesity, top dose | High-dose arm | up to 20% | 2.5% |
Two structural details matter beyond the headline number.
No weight-loss plateau at 52 weeks. Most weekly GLP-1 trials show a plateau between weeks 40 and 60, after which additional weight loss requires either dose escalation or fundamental adherence changes. MariTide's curve was still descending at 52 weeks. Whether the trajectory continues through Phase 3's 72-week treatment period is one of the open questions.
Cardiometabolic improvements consistent with the weight loss. HbA1c, systolic blood pressure, and lipid markers all moved in the expected directions, sized appropriately for the weight reduction observed. There were no surprises — positive or negative — in the cardiometabolic readout.
For context, weekly semaglutide produced ~15% placebo-adjusted weight loss at 68 weeks in STEP 1, weekly tirzepatide produced ~21% at 72 weeks in SURMOUNT-1, and retatrutide hit 28.7% at 68 weeks in TRIUMPH-4. MariTide's Phase 2 numbers sit between semaglutide and tirzepatide on weekly-comparable timepoints — but on a once-monthly schedule.
Why Monthly Dosing Changes the Calculation
Raw efficacy numbers undersell MariTide's design pitch. Three concrete patient-experience changes matter when an injection schedule moves from weekly to monthly.
Adherence math. Real-world GLP-1 adherence is bad. Roughly 40-60% of patients who start weekly injectable semaglutide or tirzepatide stop within a year, with injection burden cited as a contributing factor in self-reported survey data. Once-monthly dosing collapses 52 injection events per year into 12. If discontinuation rates drop even modestly, the population-level weight-loss outcome can rival a more potent drug taken less consistently. A 16% drug at 80% adherence often beats a 20% drug at 50% adherence.
Side-effect time course. GI side effects — nausea, vomiting, diarrhea — typically peak in the first 24-72 hours after a dose escalation. A monthly schedule gives the body more time between exposures and may smooth the side-effect peaks across a full dosing cycle. Amgen's Phase 3 program adopted a three-step dose escalation specifically to manage this, after Phase 2 showed higher GI adverse event rates than weekly comparators at faster titration.
Logistics and cold-chain. Monthly refills mean fewer pharmacy interactions, fewer cold-chain shipments for direct-to-patient programs, and fewer scheduled prescribing appointments. The infrastructure savings flow directly to manufacturer margin and could support more aggressive cash-pay pricing at launch.
The trade-off: a severe side effect on day three of a monthly cycle leaves the patient with 25 more days of drug exposure. Acute tolerability problems are harder to manage when skipping a single weekly dose is not an option. Phase 3 has to characterize how often this matters.
How MariTide Reshapes the Obesity Drug Map
Three things change if MariTide hits its Phase 3 endpoints in early 2027.
A second monthly option intensifies pricing pressure. With Pfizer's PF-3944 and Amgen's MariTide both heading toward 2028+ launches, the monthly category becomes competitive rather than monopolistic. Both companies will face the same all-in monthly cost competition from compounded weekly semaglutide and tirzepatide ($80-$350 a month), and both will have to price near-parity for the convenience trade to land.
The Lilly weekly franchise loses its convenience moat. Tirzepatide and the new oral Foundayo (orforglipron) currently dominate the weekly and oral tiers. Once a monthly option clears Phase 3, the convenience pitch shifts decisively toward whichever monthly drug has the best efficacy-to-tolerability ratio. Lilly's response is likely to be its own monthly program — but the company has not disclosed one publicly.
Compounded research-grade peptides retain the cost-and-efficacy advantage. No monthly GLP-1 will be available through 503A compounding pharmacies in the foreseeable future. The patent and antibody-conjugate manufacturing complexity put MariTide and PF-3944 firmly in the branded-only category. Patients who prioritize cost per kilogram of weight loss will continue to choose weekly compounded semaglutide, tirzepatide, or retatrutide regardless of which monthly drug launches first.
