Maritide vs retatrutide compared: mechanisms, monthly vs weekly dosing, phase 2 weight loss results, side effects, and why neither drug is FDA-approved yet.
Maritide (Amgen) and retatrutide (Eli Lilly) are both investigational obesity drugs that are not FDA-approved and cannot be legally prescribed in the United States. MariTide is a once-monthly antibody-peptide conjugate that blocks GIP receptors while activating GLP-1 receptors; retatrutide is a once-weekly triple agonist that activates GLP-1, GIP, and glucagon receptors. In separate phase 2 trials, retatrutide produced slightly more average weight loss, but no study has ever compared the two drugs head-to-head.
What Is MariTide (AMG 133)?
MariTide is Amgen's weight-loss candidate, also known as AMG 133 and maridebart cafraglutide. It is not a standard peptide — it is a monoclonal antibody joined to two GLP-1-like peptides.
The antibody portion blocks the GIP receptor, and the peptide portion switches on the GLP-1 receptor. That makes MariTide a GIP antagonist, which is the opposite of tirzepatide and retatrutide, both of which activate GIP.
Amgen designed the drug for monthly subcutaneous injection. Phase 2 data reported in 2024 showed roughly 20% average weight loss at 52 weeks at the highest dose, and Amgen stated that weight loss had not plateaued at that point. The phase 3 MARITIME program is now testing MariTide in obesity, type 2 diabetes, and related conditions.
What Is Retatrutide (LY3437943)?
Retatrutide is Eli Lilly's triple agonist. It activates GLP-1, GIP, and glucagon receptors at the same time.
The glucagon component is the distinguishing feature. Glucagon receptor activation raises energy expenditure, which is one reason researchers expect triple agonists to outperform earlier dual agonists on total weight loss.
Retatrutide is injected once weekly. In a phase 2 study published in the New England Journal of Medicine in 2023, participants taking 12 mg lost about 24% of their body weight after 48 weeks. Lilly is studying the drug in the phase 3 TRIUMPH program, and it is not FDA-approved.
Maritide vs Retatrutide: Side-by-Side Comparison
| Feature | MariTide (AMG 133) | Retatrutide (LY3437943) |
|---|---|---|
| Developer | Amgen | Eli Lilly |
| Drug type | Antibody-peptide conjugate | Triple peptide agonist |
| Receptor activity | GIP antagonist + GLP-1 agonist | GLP-1 + GIP + glucagon agonists |
| Injection frequency | Once every 4 weeks | Once weekly |
| Phase 2 weight loss | About 20% at 52 weeks (highest dose) | About 24% at 48 weeks (12 mg) |
| Phase 3 program | MARITIME | TRIUMPH |
| FDA status | Not approved | Not approved |
MariTide's main advantage is dosing convenience, because a monthly injection is easier to sustain than a weekly one. Retatrutide's main advantage is broader receptor coverage plus the larger phase 2 weight reduction.
Neither advantage has been confirmed in a direct comparison. Cross-trial comparisons are always imperfect because patient populations, dose escalation schedules, and study durations differ.
Researchers mapping the obesity pipeline often move from this matchup to others, such as cagrilintide vs retatrutide vs tirzepatide, which pits an amylin-based combination against dual- and triple-agonist drugs. The comparison survodutide vs retatrutide also comes up often because both add glucagon activity to incretin signaling.
How the Weight Loss Numbers Compare
Retatrutide's phase 2 result of about 24% at 48 weeks is among the largest weight reductions reported for any injectable obesity drug in a phase 2 trial. MariTide's roughly 20% at 52 weeks is close but slightly lower.
Two caveats matter a lot when reading those percentages:
- MariTide's trial ran 52 weeks versus 48 weeks for retatrutide, and weight loss had not plateaued in the MariTide study.
- Amgen selected a lower dose range for phase 3 than the highest phase 2 dose, which could shift the final numbers in either direction.
Because tirzepatide is already FDA-approved and retatrutide is not, searches for retatrutide vs tirzepatide: which is better often come down to availability as much as efficacy.
Side Effects and Safety Considerations
Both drugs work through incretin and metabolic hormone pathways, so gastrointestinal side effects dominate: nausea, vomiting, diarrhea, and constipation. In both phase 2 trials, most of these events were mild to moderate and clustered during dose escalation.
Retatrutide's glucagon activity raises a specific question. Phase 2 data showed small dose-related increases in heart rate, a known effect of glucagon receptor activation, so cardiac monitoring is part of the ongoing phase 3 program.
Long-term safety data for both drugs remain limited because neither has completed phase 3 testing. Anyone considering these compounds should discuss the risks with a licensed healthcare professional instead of self-prescribing.
Can You Buy MariTide or Retatrutide Today?
No. Both drugs are investigational, and the only legal route to either one in the United States is enrollment in a clinical trial.
Gray-market vendors do list retatrutide as a "research peptide," which is a different product category with no FDA oversight of purity, sterility, or dosing. MariTide is far less likely to appear on those lists because it is an antibody-peptide conjugate that cannot be produced with standard peptide synthesis.
Handling questions come up constantly in that space, and the explainer on reconstitution solution vs bacteriostatic water for peptides covers why some diluents contain preservatives and how that changes storage. People weighing peptide-based fat loss approaches against incretin drugs also frequently search aod 9604 vs retatrutide.
The bottom line: MariTide offers monthly dosing with GIP blockade, retatrutide offers weekly dosing with triple receptor activation, and neither is available by prescription in the United States today.
Frequently Asked Questions
Is MariTide the same as retatrutide?
No. MariTide is Amgen's antibody-peptide conjugate that blocks the GIP receptor and activates the GLP-1 receptor, given once every four weeks. Retatrutide is Eli Lilly's triple agonist that activates GLP-1, GIP, and glucagon receptors, given once weekly. Neither drug is FDA-approved for human use.
Which causes more weight loss, MariTide or retatrutide?
In separate phase 2 trials, retatrutide produced about 24% average weight loss at 48 weeks with the 12 mg weekly dose, while MariTide produced about 20% at 52 weeks with its highest monthly dose. No trial has directly compared the two drugs, so those numbers come from different study populations and dosing schedules and are not a true head-to-head result.
Can I get a prescription for MariTide or retatrutide in the US?
No. Both drugs are still investigational, so the only legal way to receive them in the United States is through an enrolled clinical trial. Retatrutide sold online as a research peptide is not an FDA-approved medication, and such products are not verified for purity, sterility, or accurate dosing.
This page provides educational research information and does not replace medical advice, diagnosis, or treatment.