Reconstituting peptides: bacteriostatic water explained
In short. Peptides are often sold as a dry powder because they keep better dry than in solution. Before injection the powder is dissolved, usually in bacteriostatic water. This page explains the concepts and the risks. It is not a how-to, and we do not recommend mixing.
Fimea, the Finnish medicines agency, advises against using injectable peptides bought online [9]. We explain this because people ask about it and mistakes are dangerous.
Why powder and not a ready solution
The powder is freeze-dried, or lyophilised. A dry peptide can keep for months, but in solution it may change. In one study the freeze-dried peptide drug teriparatide was stable for months when dry, but after reconstitution some samples precipitated within two to four weeks, depending on concentration and temperature [3]. Every peptide behaves differently, so one substance's result cannot be transferred to another.
What bacteriostatic water is
Bacteriostatic water is sterile, nonpyrogenic water for injection with 0.9% benzyl alcohol as a preservative, supplied in a multiple-dose container [1]. The preservative slows bacterial growth, but it does not make a contaminated product clean.
- Not for newborns. Benzyl alcohol has been linked to poisoning in neonates [1][2].
- Not on its own into a vein. The label says bacteriostatic water alone given intravenously may cause haemolysis [1].
- Aseptic technique. The label requires aseptic technique for every entry into the container [1].
In August 2026 the FDA held that bacteriostatic water sold alongside peptides is a drug, because it is intended for preparing an injectable product [8].
Concentration and syringe units
This is where most mistakes happen. Three different measures:
- Milligram (mg) and microgram (mcg) measure the amount of substance. 1 mg = 1000 mcg.
- Millilitre (ml) measures liquid.
- Unit is the syringe scale. On a U-100 insulin syringe, 1 ml is 100 units [5]. A unit says nothing about the amount of substance unless you know the concentration.
Worked example (numbers chosen to show the arithmetic, not a dose): if a vial holds 10 mg of powder and 2 ml of liquid is added, the concentration is 10 / 2 = 5 mg/ml. Then 10 units on a U-100 syringe is 0.1 ml and contains 0.5 mg. If 1 ml is added instead, the same 10 units contain 1 mg. The volume of liquid changes everything.
In a poison centre case series, two of three patients took a tenfold dose of semaglutide. The semaglutide came from compounding pharmacies or an aesthetic spa. One patient measured the dose in millilitres and units instead of milligrams [6]. All had symptoms lasting days. The calculator does the conversions, but it cannot know whether the vial really holds what the label says.
Sterile means more than "clean"
An injection bypasses the skin and gut defences. Fimea says contaminants from manufacturing, such as endotoxins, can cause serious harm in injectable products [9]. In a study of three semaglutide vials bought online without a prescription, no viable microbes were found, but endotoxin was found in every sample [4]. Sterile and endotoxin-free are different things.
In the US in 2001-2013, pharmacy-compounded sterile preparations were linked to 19 infection outbreaks and at least 1,000 cases, including deaths. Recurring causes were lapses in aseptic processing and in sterility and endotoxin testing [7]. Those were products made by professionals.
Cloudy solutions and storage
The label says to inspect reconstituted drugs for clarity and precipitation, and not to use a solution unless it is clear [1]. It also says not to store reconstituted solutions unless the manufacturer directs otherwise [1]. Research peptides have no manufacturer directions, so their shelf life is unknown.
Approved peptide medicines come with storage instructions from the manufacturer. An opened teriparatide pen (Forsteo), for example, is kept in a refrigerator at 2-8 °C for up to 28 days and must not freeze [10]. That instruction applies to this tested product only. No such data exist for research peptides.
Where next
- How to read a COA: how to tell whether the vial holds what is promised.
- Side effects: injection site reactions and when to seek care.
- Legal status in Finland.
FAQ
What is bacteriostatic water?
Bacteriostatic water is sterile water for injection with 0.9% (9 mg/ml) benzyl alcohol added as a preservative. The preservative allows repeated withdrawals from the same vial [1].
Can I use ordinary water or saline?
Tap water is not sterile. The difference between sterile water and bacteriostatic water is the preservative. The label says to follow the instructions of the manufacturer of the drug being dissolved [1], and research peptides have no such instructions.
Why is the unit maths so confusing?
Because the same thing is measured three ways: milligrams (substance), millilitres (liquid) and syringe units. On a U-100 syringe, 1 ml is 100 units [5]. In a poison centre case series, confusion between units led to tenfold errors [6].
The solution is cloudy. Can it be used?
The label says not to use the solution unless it is clear, and to inspect reconstituted drugs for precipitation and discoloration [1]. Cloudiness can point to precipitation or a quality problem. See the COA guide.
How long does a reconstituted peptide keep?
Nobody knows. The label says not to store reconstituted solutions unless the manufacturer directs otherwise [1]. Research peptides come with no manufacturer stability data.
Sources
- DailyMed: Bacteriostatic Water for Injection, USP (Hospira), prescribing information, published 8 Jul 2026. dailymed.nlm.nih.gov
- Gershanik J, et al. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med. 1982;307(22):1384-8. PMID 7133084
- Merutka G, et al. Stability of lyophilized teriparatide, PTH(1-34), after reconstitution. Eur J Pharm Biopharm. 2016;99:84-93. PMID 26620825
- Ashraf AR, et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription. J Med Internet Res. 2024;26:e65440. PMID 39509151
- Reid TS, et al. Higher concentration insulins: an overview of clinical considerations. Postgrad Med. 2017. PMID 28475455
- Lambson JE, et al. Administration errors of compounded semaglutide reported to a poison control center: case series. J Am Pharm Assoc. 2023;63(5):1643-1645. PMID 37392810
- Shehab N, et al. U.S. Compounding Pharmacy-Related Outbreaks, 2001-2013. J Patient Saf. 2018;14(3):164-173. PMID 26001553
- FDA: Warning letter, NuScience Peptides LLC, 733652, 24 Aug 2026. fda.gov
- Fimea: Fimea warns: do not use injectable peptides purchased online. 29 Jun 2026. fimea.fi
- EMA: Forsteo (teriparatide), summary of product characteristics and package leaflet, section 6.3. ema.europa.eu (PDF)
Sources checked in PubMed and DailyMed on 6 Oct 2026, the EMA product information on 13 Oct 2026.
Content is general information about research, not medical advice. If you have symptoms, contact a healthcare professional.