Head to head
Semaglutide vs Tirzepatide: 15% vs 22% Fat Loss
A side-by-side comparison of the two most-used GLP-1-class weight-loss compounds: mechanism, clinical trial results, dosing, side effects, and monthly cost. The 15% (Semaglutide Phase 3 STEP-1) and 22% (Tirzepatide Phase 3 SURMOUNT-1) figures come from separate obesity trials with different durations and populations, not from a head-to-head study. Both are FDA-approved as brand pens. Peptara sells Tirzepatide as a research vial and does not sell semaglutide.
Verdict in 4 lines
- Both are top-tier GLP-1 weight-loss compounds, FDA-approved as brand pens.
- Tirzepatide is the dual-agonist with stronger Phase 3 weight-loss data (22.5%, 72 weeks) and a lower-cost research-vial format.
- Semaglutide is the single GLP-1 agonist with the longest real-world track record (14.9%, 68 weeks).
- Trial percentages cited below come from separate obesity studies, not a head-to-head trial.
TL;DR comparison
One table, every row that matters. Tirzepatide 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). Semaglutide doses are the brand-pen ladder in mg (no reconstitution; Peptara does not sell semaglutide). See the reconstitution guide for the Tirzepatide math.
Format note: semaglutide is sold only as a finished pre-filled pen (Ozempic, Wegovy) that requires no reconstitution. Tirzepatide as a Peptara research vial ships freeze-dried and is reconstituted with bacteriostatic water. Any Tirzepatide dose requiring more than 100 units in a single injection should be split into 2 injections or reconstituted at a higher concentration (1mL bac water gives 0.2mg per unit). See the reconstitution guide for the concentration math.
Mechanism: single vs dual agonist
Both peptides work by activating gut and pancreatic hormone receptors that regulate appetite, insulin secretion, and energy balance. The difference is how many of those receptors each one activates.
Semaglutide (single GLP-1 agonist)
Activates one receptor: GLP-1 (glucagon-like peptide 1, slows gastric emptying and suppresses appetite). It was the first GLP-1 agonist to drive large-scale weight loss in obesity trials and remains the most-prescribed compound in the class. Peptara does not sell semaglutide; it is referenced here as the brand-pen benchmark.
Tirzepatide (dual-agonist)
Activates two receptors: GLP-1 plus GIP (glucose-dependent insulinotropic peptide, boosts insulin sensitivity and may modulate fat storage). The second pathway is the leading explanation for why Tirzepatide trial fat-loss numbers exceeded single-agonist semaglutide.
Honest trade-off: more receptor activation can mean more potential side effects. The dual-agonistmechanism that drives Tirzepatide's stronger fat-loss percentage can amplify early-week GI events at higher doses. Slow titration 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.
Semaglutide Phase 3 (STEP-1)
- Reference: STEP-1, Wilding et al, NEJM 2021
- Phase: 3
- n: 1,961
- Duration: 68 weeks
- Cohort: 2.4mg/wk
- Result: 14.9% mean body weight reduction
Tirzepatide Phase 3 (SURMOUNT-1)
- Reference: SURMOUNT-1, Jastreboff et al, 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 obesity trials with different durations (68 vs 72 weeks) and different patient populations. A direct percentage comparison should be read as descriptive, not causal. A head-to-head obesity trial of Semaglutide vs Tirzepatide has not been published as of 2026. The only head-to-head data (SURPASS-2, NEJM 2021) is in a type 2 diabetes population, where Tirzepatide outperformed semaglutide on weight and A1c, but that is not an obesity-population result. The Peptara position: cite the numbers accurately, name the trials, and never imply a direct A-beats-B obesity finding the literature has not yet produced.
Head-to-head note: SURPASS-2
The one published head-to-head trial, SURPASS-2 (Frias et al, NEJM 2021), compared Tirzepatide against semaglutide 1.0mg in type 2 diabetes. Tirzepatide produced greater A1c reduction and greater weight loss at all three doses. Two caveats: it used the 1.0mg diabetes dose of semaglutide (not the 2.4mg obesity dose), and the population had type 2 diabetes, not obesity. So it supports the directional finding that Tirzepatide is stronger, but it is not a clean obesity-dose comparison.
Side effects
Both share the GLP-1 class profile. Each has compound-specific notes worth flagging.
Semaglutide-specific
- FDA boxed warning: thyroid C-cell tumors (rodent data)
- Rare reports of pancreatitis
- Gallbladder events (cholelithiasis)
- Largest cardiovascular outcomes dataset in class (SELECT)
Tirzepatide-specific
- FDA boxed warning: thyroid C-cell tumors (rodent data)
- Higher GI rate at top doses vs lower doses
- 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
Semaglutide is a brand-pen ladder in mg (no reconstitution). Tirzepatide as a Peptara research vial ships as a 20mg vial: reconstitute with 2mL bacteriostatic water for 200 units per vial and 0.1mg per unit. The Tirzepatide doses below derive from that anchor. See /reconstitution for the math (1 unit equals 0.01mL on a U-100 insulin syringe).
Semaglutide pen ladder (mg)
Wegovy obesity schedule, 4-week steps (brand pen, not sold by Peptara)
- 0.25mg/wk (weeks 1 to 4)
- 0.5mg/wk (weeks 5 to 8)
- 1.0mg/wk (weeks 9 to 12)
- 1.7mg/wk (weeks 13 to 16)
- 2.4mg/wk (week 17 onward (Wegovy target))
Tirzepatide titration (units)
20 weeks, 4-week dose steps (per SURMOUNT-1), research vial
- 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)
For the Tirzepatide research vial, the only mL number 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. Semaglutide brand pens are pre-dialed in mg and require no reconstitution.
Cost per month
Semaglutide is brand-pen only: US retail runs about $968/mo for Ozempic and about $1,349/mo for Wegovy without insurance. Tirzepatide numbers below use Peptara first-time customer pricing ($157/vial; retail about $185). The Tirzepatide formula: cost per month equals vial price times 4.33 weeks per month, divided by weeks per vial. Peptara does not sell semaglutide, so there is no Peptara semaglutide price.
Weeks per vial (Tirzepatide)
weeks per vial = 20mg vial / weekly dose in mg
Example, Tirz at 5mg/wk: 20 / 5 = 4 weeks per vial. Monthly cost = $157 times 4.33 / 4 = about $170/mo first-time.
Semaglutide (brand pens)
US retail, no insurance. Not sold by Peptara.
- Ozempic (T2D pen): about $968/mo
- Wegovy (obesity pen): about $1,349/mo
- Insurance, manufacturer coupons, and compounding pharmacies can change these materially. Listed figures are list-price retail references.
Tirzepatide (Peptara research vial)
$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 $510/mo (plan 3 vials per month)
Net: the Tirzepatide research-vial route runs roughly 3 to 10x cheaper than brand semaglutide pens depending on dose. The molecule is not the same (tirzepatide is a different compound from semaglutide); the format and supply chain are different too. The cost gap is a format-and-market difference, not a like-for-like discount.
Who should pick which
A short decision framework. No single right answer, just trade-offs that map cleanly to user profiles.
Pick Semaglutide if
- You want the longest real-world safety record in the class
- You have prescription access and insurance coverage
- You value the cardiovascular outcomes data (SELECT)
- You prefer a pre-filled pen with no reconstitution
Pick Tirzepatide if
- You want the higher published fat-loss percentage
- You want a much lower cost than brand semaglutide pens
- You are comfortable with a research-vial, units-based protocol
- You have plateaued on single-agonist GLP-1
Sequence them
- Start on semaglutide if that is your prescription on-ramp
- Run a full titration to 2.4mg/wk maintenance
- At plateau, switch to Tirzepatide to break through
- Common stack discussion in 2026 peptide forums
Frequently asked questions
Next steps
Buy Tirzepatide
20mg research vial, $157 first-time, COA on request. FDA-approved dual-agonist GLP-1 / GIP. The same molecule as Mounjaro / Zepbound.
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Tirzepatide vs Ozempic
The brand-pen comparison: research vial vs Ozempic / Wegovy, FDA status, and the full cost-delta breakdown.
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Reconstitution math
4-step method, units-not-mL formula, calculator for all 15 SKUs. Required reading before your first Tirzepatide injection.
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Order Tirzepatide, get a custom titration schedule
Order 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. Peptara stocks Tirzepatide as a research vial and does not sell semaglutide.