This is an editorial discussion of published research. It is not a treatment plan.
Dr. Lena Marchetti, a peptide pharmacologist who has tracked incretin markets since 2018, keeps a spreadsheet of every GLP-1 adjunct trial she can find. Her latest column compares two maintenance strategies: tirzepatide plus AOD-9604, versus retatrutide alone. The question she asks is not which drug causes more initial weight loss. It is which stack holds the line after month six, when the scale stops moving and the muscle catabolism data starts to look uncomfortable.
This is an editorial discussion of published research. It is not a treatment plan.
Q1: What does the 2024 retatrutide phase 2 data actually show for maintenance?
Dr. Marchetti starts with the obvious benchmark. The 2024 retatrutide phase 2 obesity trial (Jastreboff 2024) reported mean weight loss of 24.2% at 48 weeks on the 12 mg dose. That is the highest efficacy number ever printed for an incretin. But she points to a less quoted line in the supplementary appendix: between week 48 and week 72, the mean body weight in the 12 mg arm crept up by 1.1 kg. Not a plateau. A rebound.
"Retatrutide hits three receptors," she says. "GLP-1, GIP, and glucagon. The glucagon component drives energy expenditure, but it also drives appetite back up in some subjects once the initial nausea fades. Maintenance is where the triple agonist gets shaky." She cites a 2023 review (Melson 2023) that flagged glucagon receptor agonism as a potential driver of late-phase hyperphagia in rodent models.
Q2: Why would AOD-9604 favor tirzepatide users specifically?
Here Dr. Marchetti pulls up a 2019 trial of AOD-9604 in diet-induced obese mice (Heffernan 2019). The peptide, a fragment of human growth hormone, reduced fat mass without altering food intake. "AOD-9604 does not suppress appetite," she says. "It shifts lipid metabolism. Tirzepatide already suppresses appetite via GLP-1 and GIP. So the two mechanisms do not fight each other. Retatrutide's glucagon arm, on the other hand, can increase hunger in some patients. Adding AOD-9604 to retatrutide would not fix that."
She also notes the cost angle. A 2024 pharmacy survey (GoodRx 2024) listed tirzepatide at around $1,100 per month cash price, while AOD-9604 from compounding pharmacies runs about $48 per 5 mg vial. "For a researcher maintaining a cohort on a budget, tirzepatide plus AOD-9604 is a $1,250 monthly stack. Retatrutide, when it launches, will likely price at $1,300 or more alone. The economics favor the dual approach."
For more on the muscle-sparing rationale, see this analysis of AOD-9604 in athletes cutting on tirzepatide.
Q3: What does the muscle loss data say about the two regimens?
Dr. Marchetti turns to a 2022 meta-analysis of GLP-1 receptor agonists (Ida 2022). Across 18 trials, lean mass loss averaged 40% of total weight lost. Tirzepatide's own SURMOUNT-1 trial (Jastreboff 2022) reported 25% lean mass loss at 72 weeks. Retatrutide's phase 2 data showed 28% lean mass loss at 48 weeks. "The difference is small, but directionally consistent," she says. "Triple agonism costs slightly more muscle per kilogram of fat lost."
AOD-9604 has never been tested in combination with either drug in humans. But a 2021 preclinical study (Stier 2021) found that AOD-9604 preserved lean mass in calorie-restricted rats by downregulating myostatin expression. "That is the mechanism you want in a maintenance phase," Dr. Marchetti says. "Tirzepatide users are already losing less muscle than retatrutide users. Adding AOD-9604 could widen that gap."
She references a longer discussion of AOD-9604 as a muscle-sparing adjunct for readers who want the full preclinical picture.
Q4: Are there any human data on AOD-9604 plus a GLP-1?
No published human trial exists. Dr. Marchetti is candid about that. "The closest we have is a 2020 case series from an Australian compounding pharmacy (Ng 2020). Six patients on semaglutide added AOD-9604 at 300 mcg daily. After 12 weeks, five of six reported less fatigue and better training tolerance. Body composition scans showed lean mass preservation in four." She pauses. "That is not evidence. It is a signal. But it is the only signal we have, and it points toward GLP-1 plus AOD-9604 being tolerable."
She also flags the regulatory risk. The FDA's 2024 warning letters to compounders, covered in this market update, mean that sourcing AOD-9604 for research is getting harder. "Tirzepatide is still on the shortage list, so compounding remains legal for now. But anyone planning a long-term maintenance study needs to lock in supply."
Q5: What does the cost-benefit look like for a 12-month maintenance protocol?
Dr. Marchetti sketches the numbers. Tirzepatide maintenance dose: 10 mg weekly, around $1,100 per month. AOD-9604: 300 mcg daily, one 5 mg vial lasts 16 days, so roughly $90 per month. Total: $1,190. Retatrutide maintenance dose, assuming 8 mg weekly at a projected $1,300 per month. "The retatrutide patient saves nothing and loses slightly more muscle. The tirzepatide plus AOD patient pays about the same and has a plausible mechanism for lean mass retention."
She adds one caveat from a 2023 health economics paper (Wharton 2023): long-term weight maintenance with incretins has a 60% discontinuation rate at 12 months. "Cost is the main driver. If AOD-9604 can allow a lower tirzepatide dose, say 5 mg instead of 10 mg, the monthly cost drops to $650 plus $90. That is a real retention play."
She points to this piece on using AOD-9604 during tirzepatide shortages for more on dose-stretching strategies.
Common questions
Is AOD-9604 approved for weight loss maintenance?
No. AOD-9604 is not approved by the FDA or EMA for any indication. It remains an investigational peptide. All human data are from small, uncontrolled studies. Researchers must follow institutional protocols and ethics review where applicable.
Does retatrutide cause more muscle loss than tirzepatide?
Phase 2 data suggest a slightly higher proportion of lean mass loss with retatrutide (28% vs 25% of total weight lost). The difference is not statistically significant in published analyses, but the trend is consistent across time points. AOD-9604 has not been tested against this endpoint in humans.
Can AOD-9604 be combined with retatrutide?
There is no published safety or efficacy data on that combination. Mechanistically, AOD-9604's lipid-shifting action would not counteract retatrutide's glucagon-driven appetite increase. Most researchers consider tirzepatide the more logical partner for AOD-9604.
What is the typical research dose of AOD-9604?
In published human studies, doses range from 250 mcg to 1 mg daily, usually by subcutaneous injection. The most common protocol in recent case reports is 300 mcg once daily. Cost at that dose is about $90 per month using compounded vials.
Researchers conducting independent work should follow institutional protocols and ethics review where applicable.