Head to head
Retatrutide vs Tirzepatide: 24% vs 22% Fat Loss
A side-by-side comparison of the two highest-efficacy GLP-1-class peptides on the market: mechanism, clinical trial results, dosing in units, side effects, and monthly cost. The 24% (Retatrutide Phase 2) and 22% (Tirzepatide Phase 3) figures come from separate trials with different durations and populations, not from a head-to-head study.
Verdict in 4 lines
- Both are top-tier GLP-1 peptides.
- Retatrutide is a newer triple-agonist with stronger Phase 2 weight-loss data (24.2%, 48 weeks).
- Tirzepatide is the FDA-approved dual-agonist with the longest real-world track record (22.5%, 72 weeks).
- Trial percentages cited below come from separate studies, not a head-to-head trial.
TL;DR comparison
One table, every row that matters. Dose values use units from a U-100 insulin syringe with the Peptara standard reconstitution (20mg vial in 2mL bac water, giving 200 units per vial and 0.1mg per unit). See the reconstitution guide for the underlying math.
Split-injection note: any dose requiring more than 100 units in a single injection should be either split into 2 injections at the same site or reconstituted at a higher concentration (1mL bac water instead of 2mL gives 0.2mg per unit, halving the unit count). See the reconstitution guide for the concentration math.
Mechanism: triple vs dual agonist
Both peptides work by activating gut and pancreatic hormone receptors that regulate appetite, insulin secretion, and energy expenditure. The difference is how many of those receptors each one activates.
Tirzepatide (dual-agonist)
Activates two receptors: GLP-1 (glucagon-like peptide 1, slows gastric emptying and suppresses appetite) and GIP (glucose-dependent insulinotropic peptide, boosts insulin sensitivity and may modulate fat storage). The dual activation produces stronger fat loss than older single-agonist GLP-1 drugs like semaglutide.
Retatrutide (triple-agonist)
Activates all three: GLP-1, GIP, and glucagon receptor. The glucagon-receptor arm adds direct energy expenditure (the body burns more calories at rest), on top of the appetite suppression and insulin effects of GLP-1 and GIP. That third pathway is the main reason Phase 2 fat-loss numbers exceeded Tirzepatide.
Honest trade-off: more receptor activation means more potential side effects. The triple-agonistmechanism that drives Retatrutide's stronger fat-loss percentage also produced higher rates of GI events at the 12mg top dose in Phase 2. Slowtitration is the standard mitigation for both compounds.
Clinical trial results
The headline figures, the trial citations, and the disclaimer that has to come with them.
Retatrutide Phase 2
- Reference: Jastreboff et al, NEJM 2023
- Phase: 2
- n: 338
- Duration: 48 weeks
- Cohort: 12mg/wk
- Result: 24.2% mean body weight reduction
Tirzepatide Phase 3 (SURMOUNT-1)
- Reference: SURMOUNT-1, NEJM 2022
- Phase: 3
- n: 2,539
- Duration: 72 weeks
- Cohort: 15mg/wk
- Result: 22.5% mean body weight reduction
Read this before quoting either number
These percentages come from two separate clinical trials with different durations (48 vs 72 weeks), different patient populations, and different trial phases (2 vs 3). A direct percentage comparison should be read as descriptive, not causal. A head-to-head trial of Retatrutide vs Tirzepatide has not been published as of 2026. The Peptara position: cite the numbers honestly, name the trials, and never imply a direct A-beats-B finding the literature has not yet produced.
Side effects
Both share the GLP-1 class profile. Each has compound-specific notes worth flagging.
Retatrutide-specific
- Higher GI rate at 12mg vs lower doses (Phase 2)
- Triple-agonist activation can amplify early-week nausea
- Investigational compound: longer-term safety data still accumulating
Tirzepatide-specific
- FDA black-box warning: thyroid C-cell tumors (rodent data)
- Rare reports of pancreatitis
- Gallbladder events (cholelithiasis, cholecystitis)
Mitigation for both: titrate slowly (4-week steps, lowest tolerable dose first), inject subcutaneously in the same hour each week to stabilize blood levels, and hydrate aggressively during the first 2 weeks of any new step.
Dosing protocols (units, not mL)
Both peptides ship as a 20mg vial. Reconstitute with 2mL bacteriostatic water for 200 units per vial and 0.1mg per unit. Every dose below derives from that anchor. See /reconstitution for the underlying math (1 unit equals 0.01mL on a U-100 insulin syringe).
Retatrutide titration
16 weeks, 4-week dose steps
- 2mg/wk 20 units
- 4mg/wk 40 units
- 8mg/wk 80 units
- 12mg/wk 120 units (split into 2 injections)
Tirzepatide titration
20 weeks, 4-week dose steps (per SURMOUNT-1)
- 2.5mg/wk 25 units
- 5mg/wk 50 units
- 7.5mg/wk 75 units
- 10mg/wk 100 units
- 12.5mg/wk 125 units (split into 2 injections)
- 15mg/wk 150 units (split into 2 injections)
The only mL number on this page is the bac water input volume (2mL per 20mg vial). Every dose is in units. Reading a decimal place wrong on a small syringe barrel is the most common source of 10x overdoses, which is why protocol dashboards never display doses in mL.
Cost per month at Peptara prices
Numbers below use first-time customer pricing per the Peptara catalog (Retatrutide $174/vial, Tirzepatide $157/vial). Retail equivalents are about 18% higher ($205 Reta, $185 Tirz). The formula is the same in both columns: cost per month equals vial price times 4.33 weeks per month, divided by weeks per vial.
Weeks per vial
weeks per vial = 20mg vial / weekly dose in mg
Example, Reta at 4mg/wk: 20 / 4 = 5 weeks per vial. Monthly cost = $174 times 4.33 / 5 = about $151/mo first-time.
Retatrutide
$174 first-time per 20mg vial ($205 retail)
- 2mg/wk (start): 10 wk per vial, about $75/mo
- 4mg/wk: 5 wk per vial, about $151/mo
- 8mg/wk (maintenance): 2.5 wk per vial, about $301/mo (plan 2 vials per month)
- 12mg/wk (max): 1.67 wk per vial, about $452/mo (plan 3 vials per month)
Tirzepatide
$157 first-time per 20mg vial ($185 retail)
- 2.5mg/wk (start): 8 wk per vial, about $85/mo
- 5mg/wk: 4 wk per vial, about $170/mo
- 10mg/wk (maintenance): 2 wk per vial, about $340/mo (plan 2 vials per month)
- 15mg/wk (max): 1.33 wk per vial, about $511/mo (plan 3 vials per month)
In practice you buy whole vials, not fractions, so real spend rounds to N vials per month times vial price (e.g. 2 vials/mo at Reta 8mg = $348 actual, slightly over the amortized $301 figure). The amortized numbers above are the cleanest answer to "how much does this dose cost per month".
Who should pick which
A short decision framework. No single right answer, just trade-offs that map cleanly to user profiles.
Pick Retatrutide if
- You want the highest published fat-loss percentage
- You are comfortable using an investigational compound
- You can titrate slowly and tolerate early-week GI
- You have plateaued on prior GLP-1 cycles
Pick Tirzepatide if
- You want FDA-approved status
- You prefer the longer real-world safety record
- You want slightly lower cost at maintenance doses
- This is your first GLP-1 cycle
Sequence both
- Start with Tirzepatide for the FDA-approved on-ramp
- Run a full titration to 10 to 15mg/wk maintenance
- At plateau, switch to Retatrutide to break through
- Common stack discussion in 2026 peptide forums
Frequently asked questions
Next steps
Buy Retatrutide
20mg vial, $174 first-time, COA on request. Triple-agonist GLP-1 / GIP / glucagon.
View product
Buy Tirzepatide
20mg vial, $157 first-time, COA on request. FDA-approved dual-agonist GLP-1 / GIP.
View product
Reconstitution math
4-step method, units-not-mL formula, calculator for all 15 SKUs. Required reading before your first injection.
Open guide
Order either, get a custom titration schedule
Order Retatrutide or Tirzepatide and we build your weekly titration schedule into your protocol dashboard within 12 hours.
Every order includes bac water sized to the 20mg vial, a custom dashboard with your doses in units (never mL), and ongoing protocol support on WhatsApp.