How to reconstitute peptides: the arithmetic, step by step

The short answer

• Three formulas do the work. Concentration = mg in the vial divided by mL of diluent. Volume to draw = dose divided by concentration. U-100 syringe units = volume in mL times 100.
• Convert everything to one unit first. 1 mg = 1,000 mcg. The most dangerous error in this practice is a factor of 1,000 or 10 at that boundary.
• Syringe “units” are volume, not dose. On a U-100 insulin syringe, 1 unit is 0.01 mL, whatever is in the barrel.
• A stated post-reconstitution concentration beats your arithmetic. Approved labels measure the real concentration, which can differ from naive division by around 10%.
• Always back-check. Convert your syringe units back to milligrams. If you do not land on the dose you intended, stop.
• 28 days is a diluent rule, not a stability period for the reconstituted peptide.
• This is measurement arithmetic, not a dosing recommendation. Nothing here tells anyone what to take.

Why injection is the route at all

Peptides are large, polar and often charged. They cross gut and other epithelia poorly and are digested on the way, which is why injection is the default. The numbers are unforgiving: unformulated sublingual desmopressin produced no detectable plasma drug in healthy volunteers, and intranasal desmopressin reached about 3.4% of the subcutaneous exposure. Oral semaglutide, with a purpose-built absorption enhancer, still manages only about 0.4% to 2%. That is why vials of powder exist, and why the arithmetic below matters.

The three formulas, with a worked example

Formula 1. Concentration (mg/mL) = peptide in the vial (mg) / diluent added (mL).

Formula 2. Volume to draw (mL) = dose (mg) / concentration (mg/mL).

Formula 3. U-100 units = volume (mL) x 100.

Worked through, with a 5 mg vial reconstituted in 2.0 mL of bacteriostatic water and a 250 mcg dose:

1. Concentration: 5 / 2.0 = 2.5 mg/mL.
2. Convert the dose: 250 mcg = 0.25 mg. Volume: 0.25 / 2.5 = 0.10 mL.
3. Units: 0.10 x 100 = 10 units on a U-100 syringe.
4. Back-check: (10 / 100) x 2.5 = 0.25 mg. Correct.

That fourth step is the whole discipline. Run it every time.

Choose the diluent volume on purpose

The diluent volume is a decision, not a default, because it determines how readable the draw is. Draws under about 5 units are hard to measure accurately, so the target is usually a dose that lands somewhere in the 10 to 50 unit range.

Consider a 10 mg vial and a 250 mcg dose. With 3.0 mL of diluent the reading is 7.5 units, with a repeating decimal behind it. Making 250 mcg equal exactly 10 units would need 4.0 mL, which many small vials cannot hold, so check capacity before committing: plenty hold about 3 mL or less.

Our reconstitution calculator runs these combinations, and how much water to add to a peptide vial covers the volume decision in more depth.

When the label overrides the arithmetic

This is the single most useful thing in this article, and it is invisible unless someone shows you an example.

EGRIFTA WR (tesamorelin) comes as an 11.6 mg vial reconstituted with 1.3 mL of the supplied bacteriostatic water. Naive division gives 11.6 / 1.3 = 8.92 mg/mL. The label states the resulting concentration is 8 mg/mL, with each 1.28 mg dose contained in 0.16 mL, and each vial giving 7 daily doses.

Anyone relying on the naive figure would draw 0.143 mL rather than 0.16 mL, roughly 10% short on every injection.

The reason is displacement volume. The lyophilized cake is not just drug; it contains excipients such as mannitol, and all of it adds volume when dissolved. Vials are also filled with a margin for withdrawal losses. The manufacturer measures the real post-reconstitution concentration, and you cannot derive it.

The corollary is uncomfortable. An unlabelled grey-market vial states no concentration, so the true value is unknown by at least that margin, before any question of what the vial actually contains.

Bacteriostatic versus sterile water

Bacteriostatic Water for Injection is water with a benzyl alcohol preservative, typically 0.9%, supplied in multiple-dose containers. Date the vial at first puncture and discard within 28 days unless the manufacturer says otherwise.

Sterile Water for Injection is water only, usually single-dose, and should be treated as single-use once opened.

Compatibility is product-specific and not interchangeable: EGRIFTA WR must be reconstituted with its supplied bacteriostatic water, while EGRIFTA SV must use its supplied sterile water only. Benzyl alcohol carries a label warning about toxicity in neonates, so preservative-free diluent is required there, and either water given intravenously without a solute can cause haemolysis because it is hypotonic. Bacteriostatic vs sterile water goes through the comparison in full.

Some community protocols use 0.6% acetic acid or bacteriostatic saline for particular peptides. Those are conventions without published stability data for those products.

Handling: mixing, storing, inspecting

• Prepare cleanly. Hand hygiene, clean surface, swab the septum with 70% alcohol and let it dry. Use a new sterile needle and syringe for every entry, never re-enter a vial with a used needle, and never share pens, needles or vials.
• Run the diluent down the vial wall and mix gently. EGRIFTA SV, for example, is rolled for 30 seconds and not shaken. Agitation and air-water interfaces promote aggregation.
• Label the vial at reconstitution: date, diluent, volume, concentration in mg/mL, discard date, initials. An unrecorded concentration makes every later dose a guess.
• Refrigerate reconstituted solution at 2 to 8°C unless the label says otherwise, protect from light, and do not freeze unless the label allows it. Freeze-thaw promotes aggregation.
• Inspect before every dose. Clear, colourless or as labelled, no particles, no gel, no colour change. Discard if any of those appear. Clear does not mean potent, but persistent particles, a colour change or late-onset haze mean the vial is not used.

On timing: the 28-day figure applies to a punctured multi-dose diluent container. Reconstituted peptide stability is product-specific and ranges from “use immediately” (glucagon, discard if a gel forms) to days (EGRIFTA WR, 7 days at room temperature). The “2 to 4 weeks in the fridge” windows circulating for grey-market peptides are unvalidated convention, not stability data.

One more planning point. Withdrawal losses from the needle hub, vial dead volume and bubbles mean the last dose in a vial is often short. A 10 mg vial at 5.0 mg/mL with a 0.5 mg dose theoretically holds 20 doses; plan for 18 to 19 and do not scrape the vial to finish it.

The errors that actually cause harm

FDA warned in July 2024 that confusion among milligrams, millilitres and “units”, combined with inappropriate syringe sizes, led patients using compounded semaglutide to inject 5 to 20 times the intended dose, with hospitalizations for severe nausea, vomiting and hypoglycaemia. A poison-centre case series documented 10-fold self-administration errors with vials dispensed without counselling.

The specific patterns, using a compounded semaglutide vial at 2.5 mg/mL with a 0.25 mg dose:

• Correct: 0.25 / 2.5 = 0.10 mL = 10 units on a U-100 syringe.
• Error A: the patient hears “draw 1” and draws 1 mL, which is 2.5 mg. A 10-fold overdose.
• Error B: the patient uses a U-40 veterinary insulin syringe, where 10 units is 0.25 mL, delivering 0.625 mg. A 2.5-fold overdose.

U-500 syringes are the other trap: 1 unit there is 0.002 mL, and the Humulin R U-500 label warns that using any other syringe risks dangerous overdose. A plain tuberculin or 1 mL syringe has no “units” at all, only mL graduations, so the dose must be written in mL.

Other repeat offenders: decimal errors (always a leading zero, 0.5, never a trailing one), daily versus weekly confusion (write the frequency in words), rounding without recording it, and “blends” of two peptides in one vial, where stability is unknown and each component’s dose shifts with every draw. Our mg to mL and syringe conversion guide sets out the unit maths on its own.

The single best safeguard: for any new product or dose change, a second person, or the person themselves with a written worksheet, recalculates concentration, volume and units from scratch before the first dose.

The bottom line

Reconstitution is three formulas, one back-check and a short list of handling rules. Write the dose in milligrams, millilitres and syringe units, and name the syringe type in writing, because unit confusion is the documented cause of the worst errors in this space. Where a manufacturer states a post-reconstitution concentration, that number governs, and where no manufacturer exists to state one, the uncertainty is yours to carry.

Education only. This page is educational. Most compounds referenced are not approved for human use, and a “research use only” label carries no legal status of its own. Nothing here constitutes medical advice, diagnosis, or treatment.

Sources

• Lambson JE, et al. Administration errors of compounded semaglutide reported to a poison control center-Case series. Journal of the American Pharmacists Association, 2023.
• Fjellestad-Paulsen A, et al. Pharmacokinetics of 1-deamino-8-D-arginine vasopressin after various routes of administration in healthy volunteers. Clinical Endocrinology, 1993.
• Manning MC, et al. Stability of protein pharmaceuticals: an update. Pharmaceutical Research, 2010.
• Overgaard RV, et al. Clinical Pharmacokinetics of Oral Semaglutide: Analyses of Data from Clinical Pharmacology Trials. Clinical Pharmacokinetics, 2021.
• Frid AH, et al. New Insulin Delivery Recommendations. Mayo Clinic Proceedings, 2016.
• US FDA. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products, 26 July 2024.
• EGRIFTA WR and EGRIFTA SV prescribing information (effective July 2026); Humulin R U-500 prescribing information; Bacteriostatic Water for Injection, USP, label.
• CDC. Multi-dose vial safety reminders (28-day rule for opened multi-dose vials unless the manufacturer specifies otherwise).