Dosage Units Guide
U-100 vs U-40 syringes, sterile vs bacteriostatic water, and how peptide reconstitution actually works — in plain English.
U-100 vs U-40: what the numbers mean
The U number is the number of units per milliliter the syringe barrel is calibrated for. A U-100 syringe reads 100 units per 1 mL. A U-40 syringe reads 40 units per 1 mL. Same volume of liquid, different unit scale.
| Syringe | Units per 1 mL | 0.1 mL equals | Common use |
|---|---|---|---|
| U-100 | 100 units | 10 units | Human insulin, most peptides, GLP-1s |
| U-40 | 40 units | 4 units | Veterinary insulin (older protocols) |
Mixing them up under-doses by 60%. Drawing 20 units on a U-40 syringe from a U-100 concentration delivers only about 8 units of medicine. Always confirm the syringe label before every injection.
Sterile water vs bacteriostatic water
Bacteriostatic water (BAC water)
Contains 0.9% benzyl alcohol, which inhibits bacterial growth. Standard choice for multi-dose peptide vials. Reconstituted vials are typically usable for around 28 days refrigerated.
Sterile water for injection
No preservative. Use only for single-dose reconstitution or when benzyl alcohol is contraindicated (e.g. neonates). A reconstituted single-dose vial should be used within 24 hours.
Never use tap, distilled, or spring water. They are not sterile and can introduce contaminants directly into your bloodstream.
Reconstitution basics
Reconstitution means mixing freeze-dried peptide powder with a sterile diluent so it becomes injectable. The amount of water you add sets the concentration, which determines how many units you draw per dose.
- 1. Choose a BAC water volume
Common: 2 mL of BAC water for a 5 mg vial → 2.5 mg/mL. More water = weaker concentration = more units per dose (easier to measure).
- 2. Add water slowly down the vial wall
Do not spray directly onto the powder — peptides are fragile. Let it dissolve; swirl gently, never shake.
- 3. Calculate units per dose
dose (mg) ÷ concentration (mg/mL) × 100 = units on a U-100 syringe. Or skip the math: paste your numbers into the peptide dosage calculator.
- 4. Store correctly
Refrigerate at 2–8°C, upright, out of light. Label the vial with the date mixed.
Worked example: 5 mg vial, 250 mcg dose
- Vial: 5 mg peptide powder
- BAC water added: 2 mL
- Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL (2500 mcg/mL)
- Target dose: 250 mcg (0.25 mg)
- Volume: 0.25 mg ÷ 2.5 mg/mL = 0.1 mL
- On a U-100 syringe: 0.1 mL × 100 = 10 units
- Doses per vial: 5 mg ÷ 0.25 mg = 20 doses
mg, mcg and IU: converting between them
Milligrams and micrograms are pure weight, so they convert with arithmetic alone: 1 mg is 1000 mcg, and a 250 mcg dose is 0.25 mg. International units are different. An IU is a measure of biological activity, defined separately for each substance, so there is no universal IU-to-milligram factor. HCG, vitamin D and growth hormone each have their own conversion, and applying one substance's factor to another produces a dose that is wrong by an order of magnitude rather than a rounding error.
| You have | You want | Conversion |
|---|---|---|
| Milligrams (mg) | Micrograms (mcg) | Multiply by 1000 |
| Milliliters (mL) | U-100 syringe units | Multiply by 100 |
| Milliliters (mL) | U-40 syringe units | Multiply by 40 |
| Dose (mg) + concentration (mg/mL) | Volume to draw (mL) | Divide dose by concentration |
| International units (IU) | Milligrams (mg) | Substance-specific — check the label, never assume |
One habit prevents most measurement mistakes: write the concentration on the vial in marker the moment it is mixed. Every later conversion on this page needs that one number, and it cannot be recovered by looking at the vial afterwards.
Frequently asked questions
What is the difference between a U-100 and U-40 insulin syringe?
A U-100 syringe holds 100 units per 1 mL. A U-40 syringe holds 40 units per 1 mL. Using the wrong syringe changes your dose by up to 60%. Nearly all human insulin and peptide protocols in the US use U-100. Check the barrel label before every injection.
Can I use sterile water instead of bacteriostatic water?
Sterile water contains no preservative, so a reconstituted vial must be used within 24 hours. Bacteriostatic water contains 0.9% benzyl alcohol which inhibits bacterial growth and extends the usable window to roughly 28 days refrigerated. For multi-dose peptide vials, bacteriostatic water is the standard choice.
How do I convert milligrams to insulin syringe units?
Find your concentration in mg/mL (vial mg divided by BAC water mL added). Divide your dose in mg by that concentration to get mL, then multiply by 100 for a U-100 syringe or by 40 for a U-40 syringe.
How long does a reconstituted peptide last?
Most reconstituted peptides are considered stable for about 28 days when refrigerated at 2–8°C (36–46°F). Keep vials upright, protected from light, and never freeze after reconstitution unless the manufacturer states it is safe.
What size syringe should I buy for peptides?
For most peptide protocols, a 0.5 mL or 1 mL U-100 insulin syringe with a 29–31 gauge, 8 mm (5/16") needle works well for subcutaneous injection. A 0.3 mL U-100 syringe gives finer control for very small doses under 30 units.
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