At a glance
- 2 mL of BAC water in a 5mg semaglutide vial yields 2.5 mg/mL. The STEP-1 ramp doses (0.25, 0.5, 1.0, 1.7, 2.4 mg weekly) read 10, 20, 40, 68, 96 IU on a U-100 syringe
- 1 mL gives 5 mg/mL where 0.25 mg = 5 IU; cleanest math at the early titration weeks
- 3 mL gives 1.67 mg/mL; better resolution for sub-0.25 mg microdose research
- Reconstituted shelf life is approximately 28 days at 2-8°C with bacteriostatic water (0.9% benzyl alcohol)
- STEP-1 (Wilding et al. NEJM 2021, PMID 33567185) titrated 0.25 mg over 4 weeks then doubled monthly to 2.4 mg as the maintenance dose
Short answer: for a 5mg semaglutide vial, 2 mL of bacteriostatic water gives the cleanest dosing math (2.5 mg/mL, where the STEP-1 ramp doses fall on round insulin units across the full 0.25 mg to 2.4 mg range). 1 mL is fine for protocols that stay in the 0.25-1.0 mg titration band; 3 mL or 5 mL trade slightly more injection volume for finer microdose precision.
This guide answers the specific reconstitution question for a 5mg semaglutide vial. For the broader reconstitution mechanics, see the peptide reconstitution complete guide. For the dosing schedule from the STEP-1 trial (Wilding et al. NEJM 2021, PMID 33567185), the stopping GLP-1s research and the tirzepatide vs semaglutide comparison cover what the trials actually used.
TL;DR chart
For the STEP-1 dose ladder (Wilding 2021):
| BAC water | Concentration | 0.25 mg | 0.5 mg | 1.0 mg | 1.7 mg | 2.4 mg |
|---|---|---|---|---|---|---|
| 1 mL | 5 mg/mL | 5 IU | 10 IU | 20 IU | 34 IU | 48 IU |
| 2 mL | 2.5 mg/mL | 10 IU | 20 IU | 40 IU | 68 IU | 96 IU |
| 3 mL | 1.67 mg/mL | 15 IU | 30 IU | 60 IU | 102 IU | 144 IU |
| 5 mL | 1 mg/mL | 25 IU | 50 IU | 100 IU | 170 IU | 240 IU |
The 2 mL row is the cleanest because every dose lands on a round IU mark across the entire titration ramp. The 5 mL row needs two-syringe injections at the maintenance dose (240 IU > 100 IU max per syringe).
The math
Reconstitution formula:
- Vial mass = 5 mg (printed on the lyophilized semaglutide vial)
- BAC water volume = your choice (1, 2, 3, or 5 mL are practical)
- Target dose = your protocol target
Concentration = vial mass / BAC water volume. Volume per dose = target dose / concentration. Insulin units (U-100) = volume per dose × 100.
Worked example for 2 mL BAC water and 1 mg dose:
- Concentration = 5 mg / 2 mL = 2.5 mg/mL
- Volume per dose = 1 mg / 2.5 mg/mL = 0.4 mL
- Insulin units = 0.4 mL × 100 = 40 IU
The same calculator with the same inputs runs at /calculators/reconstitution; the live tool handles non-standard doses.
Concentration cheat sheet for the 5mg vial
| BAC water added | Concentration | Notes |
|---|---|---|
| 1 mL | 5 mg/mL | Compact storage; 0.25 mg = 5 IU; clean for early titration |
| 2 mL | 2.5 mg/mL | Recommended default. Round IU across full 0.25-2.4 mg ramp |
| 3 mL | 1.67 mg/mL | Larger injection volumes; better for sub-0.25 mg microdose research |
| 5 mL | 1 mg/mL | Largest volume; 1 mL = 1 mg directly. Maintenance dose exceeds 100 IU |
The 2 mL choice is the cleanest because it produces a 2.5 mg/mL concentration where every STEP-1 dose lands on a round number on a U-100 insulin syringe.
STEP-1 dose ramp on a 2 mL recon
The STEP-1 trial (Wilding et al. NEJM 2021, PMID 33567185) titrated semaglutide weekly over 16 weeks before the maintenance phase began at 2.4 mg weekly. The schedule, with insulin units on a 2 mL reconstitution:
| Week | Weekly dose | IU per injection (U-100, 2 mL recon) |
|---|---|---|
| 1-4 | 0.25 mg | 10 IU |
| 5-8 | 0.5 mg | 20 IU |
| 9-12 | 1.0 mg | 40 IU |
| 13-16 | 1.7 mg | 68 IU |
| 17+ | 2.4 mg (maintenance) | 96 IU |
Slow titration is the entire point. The STEP-1 protocol takes 4 months to reach the maintenance dose because the GI tolerability profile improves substantially when the ramp is gradual (Wilding et al. 2021, PMID 33567185). Researchers running shorter ramps frequently report nausea, vomiting, and discontinuation that the trial protocol was specifically designed to avoid.
For the comparative ramp data against tirzepatide (SURMOUNT-1) and retatrutide (TRIUMPH-1/2), see the retatrutide vs tirzepatide vs semaglutide 2026 comparison.
Microdosing math
Some research protocols target sub-0.25 mg doses (e.g. 0.1 mg or 0.15 mg) for tolerability or pharmacokinetic studies. The microdose math at common reconstitution volumes:
| Dose | 1 mL recon | 2 mL recon | 3 mL recon | 5 mL recon |
|---|---|---|---|---|
| 0.05 mg (50 mcg) | 1 IU | 2 IU | 3 IU | 5 IU |
| 0.1 mg | 2 IU | 4 IU | 6 IU | 10 IU |
| 0.15 mg | 3 IU | 6 IU | 9 IU | 15 IU |
| 0.2 mg | 4 IU | 8 IU | 12 IU | 20 IU |
For sub-0.1 mg dosing, the 3 mL or 5 mL reconstitution is necessary because U-100 syringes don't resolve below 1 IU cleanly. A 30-IU or 50-IU barrel U-100 syringe gives the finest gradation for these protocols.
Step-by-step reconstitution
- Materials. 5mg semaglutide vial, BAC water (0.9% benzyl alcohol in sterile water), one U-100 insulin syringe to draw the BAC water, alcohol prep pads, sharps container.
- Sanitize stoppers. Wipe both rubber stoppers with an alcohol pad.
- Withdraw BAC water. Draw 2 mL (or chosen volume) into the syringe. Tap out air bubbles.
- Inject slowly. Tilt the semaglutide vial. Insert the needle through the stopper at an angle, let the BAC water run down the inside wall.
- Dissolve gently. Swirl or roll the vial. Most semaglutide lots dissolve within 60 seconds. Do not shake.
- Confirm clear solution. Reconstituted semaglutide should be clear and colorless. Yellow tint or visible particulates after several minutes is a quality flag.
- Refrigerate. 2-8°C. Do not freeze.
Storage and stability
- Lyophilized vial: stable at -20°C long-term; 2-8°C for shorter periods
- Reconstituted in BAC water: ~28 days at 2-8°C
- Reconstituted in plain sterile water: ~24 hours; not recommended
- Avoid freeze-thaw cycles on reconstituted solution
For one full STEP-1 maintenance month (4 weekly injections at 2.4 mg = 9.6 mg total), a single 5mg vial reconstituted at 2 mL falls a hair short of one month. Most active researchers reconstitute one new 5mg vial every 21-28 days during maintenance dosing.
Sourcing the 5mg vial
The semaglutide lab-tests page lists every vendor we track with public COAs. Ascension Peptides publishes per-batch certificates on every semaglutide 5mg lot (recent verified HPLC: 99.98% on lot 04-01260229, Feb 24 2026). Code ENHANCED takes 50% off at checkout. The stopping GLP-1s research covers what happens after the maintenance phase ends.
FAQ
How much bacteriostatic water do I add to a 5mg semaglutide vial?
2 mL is the cleanest default because it produces a 2.5 mg/mL concentration where every STEP-1 ramp dose (0.25, 0.5, 1.0, 1.7, 2.4 mg) lands on a round insulin unit number on a U-100 syringe. 1 mL works for early titration weeks; 3 mL or 5 mL give better resolution for sub-0.25 mg microdose research.
How many IU is 0.25 mg of semaglutide?
Depends on the reconstitution. With 2 mL BAC water (2.5 mg/mL), 0.25 mg = 10 IU on a U-100 syringe. With 1 mL (5 mg/mL), 0.25 mg = 5 IU. With 3 mL (1.67 mg/mL), 0.25 mg = 15 IU.
How many IU is 2.4 mg (the STEP-1 maintenance dose)?
With 2 mL (2.5 mg/mL), 2.4 mg = 96 IU on a U-100 syringe. With 1 mL, 48 IU. With 3 mL, 144 IU (requires two-syringe injection or upgrading to a larger barrel). With 5 mL, 240 IU (requires multi-injection split).
How long does reconstituted semaglutide last?
Approximately 28 days at 2-8°C with bacteriostatic water (0.9% benzyl alcohol). Sterile water without preservative shortens the shelf life to ~24 hours. Avoid freezing reconstituted solution.
Do I need to titrate or can I start at the maintenance dose?
The STEP-1 protocol titrated specifically because GI tolerability is dose-dependent and improves with slow ramping (Wilding et al. NEJM 2021, PMID 33567185). Starting at 2.4 mg without ramping produces nausea, vomiting, and discontinuation rates substantially higher than what the trial reported.
What syringe size do I use for semaglutide?
A U-100 insulin syringe is standard. For early titration (0.25-1.0 mg), a 30-IU or 50-IU barrel gives finer markings. For maintenance (1.7-2.4 mg) on a 2 mL reconstitution, a 100-IU barrel is necessary. The reconstitution calculator recommends a specific syringe based on the dose and reconstitution volume.
Can I use a 5mg semaglutide vial across multiple weeks?
Yes, that's the standard pattern. At maintenance (2.4 mg/week), a 5mg vial covers approximately two weekly injections (5 / 2.4 ≈ 2 weeks). Reconstitute the full vial at once, store at 2-8°C, and use within 28 days.
Continue researching
For the full reconstitution mechanics across any peptide vial, the peptide reconstitution complete guide is the reference. For the GLP-1 head-to-head comparison, the retatrutide vs tirzepatide vs semaglutide 2026 guide covers the trial data side by side. For what happens when treatment stops, the stopping GLP-1s withdrawal trials covers the regain dynamics. The live reconstitution calculator handles non-standard math. To verify a specific 5mg semaglutide vial before purchase, the semaglutide lab-tests page lists every COA on file.
This article is for educational and research purposes only. Semaglutide is FDA-approved as Wegovy (chronic weight management) and Ozempic (type 2 diabetes); the doses, reconstitution math, and protocols discussed here reflect what the published clinical trials used and the standard research-peptide practice for vial reconstitution. Nothing in this article is medical advice. Off-label use, use outside the trial-defined inclusion criteria, and use without medical supervision are not supported by the data reviewed here. Consult a qualified clinician for individual medical questions.
